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Using Ventilated Envelopes to Improve the ThermalPerformance of Buildings in Hot-Humid Climate

Item Type text; Electronic Thesis

Authors Bakri, Miassar Mohammed

Publisher The University of Arizona.

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Link to Item http://hdl.handle.net/10150/603493

http://hdl.handle.net/10150/603493

USING VENTILATED ENVELOPES TO IMPROVE

THE PERFORMANCE OF BUILDINGS IN HOT-

HUMID CLIMATE

By

Miassar Mohammed Bakri

____________________________

Copyright © Miassar Bakri 2015

A Thesis Submitted to the Faculty of the

SCHOOL OF ARCHITECTURE

In Partial Fulfillment of the Requirements

For the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

2015

1 December 14, 2015

STATEMENT BY AUTHOR

The thesis titled “Using ventilated envelopes to improve the thermal performance of buildings in

Hot-Humid Climate” prepared by Miassar Bakri has been submitted in partial fulfillment of

requirements for a master’s degree at the University of Arizona and is deposited in the University

Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that an

accurate acknowledgement of the source is made. Requests for permission for extended quotation

from or reproduction of this manuscript in whole or in part may be granted by the head of the

major department or the Dean of the Graduate College when in his or her judgment the proposed

use of the material is in the interests of scholarship. In all other instances, however, permission

must be obtained from the author.

SIGNED: Miassar Bakri

APPROVAL BY THESIS DIRECTOR

Ray Barnes

This thesis has been approved on the date shown below:

Defense date

Thesis Director Ray Barnes December 14th, 2015

Professor of Architecture

2 December 14, 2015

Using ventilated envelopes to improve the

thermal performance of buildings in Hot-

Humid Climate

By

Miassar Bakri

Supervisors:

Ray Barnes Nader Chalfoun Colby Moeller

3 December 14, 2015

Table of content

Subject Page

number

Introduction 5

Background information of Jeddah, Saudi Arabia 6

Problem statement 15

Hypothesis 16

Experiment description 17

Anticipating the data 18

The significance of the experiment 19

Understanding thermal mass 20

Building envelope characteristics in hot humid climate 30

Understanding ventilated envelopes 37

Case studies 66

The experiment assembly 72

Pictures from the test location 79

The data acquisition instruments 83

The results 86

Discussion 99

Conclusion 99

References 100

4 December 14, 2015

Abstract:

Many attempts have been made to design buildings that reduce the heat gain inside the building. In hot-humid region, architects deal with many forces of nature. These forces might be Rain, Humidity, and solar heat gain. In order to deal with the excessive heat gain, builders have used thermal mass materials to mitigate the heat gain inside the building. Hot arid region was known to be the best climate region to apply thermal mass due to the diurnal temperature swing. However, there are some architects who agree that thermal mass materials could be used in hot-humid climate. This thesis project suggests using ventilated envelope that incorporates thermal mass in the design of the ventilated envelope. The result of the experiment shows that using ventilated envelopes with thermal mass would allow the heat gained in the cladding and in the thermal mass to be released to the air cavity and; therefore, releasing the heat from the building to the exterior atmosphere. The ventilated facade could be improved by adding thermal insulation and by using reflective materials on the cladding

5 December 14, 2015

Chapter 1: Introduction:

The building sector consumes an extravagant amount of energy. According to the department

of energy’s website “In the United States, the buildings sector accounted for about 41% of primary energy

consumption in 2010, 44% more than the transportation sector and 36% more than the industrial sector”

(DOE, 2015). Heating and cooling loads have the most profound impact on the energy consumption in

most buildings. In the past buildings were not using as much energy as they are using today. Nowadays,

seldom would someone find a building that does not rely on energy. There are many aspects that led to

the typical consumption of modern day buildings. One of these reasons is the transition from the old type

buildings to the new modern style buildings. Old buildings have no electricity running through them to

power anything inside the house. Most old building occupants relied on what is called active usage, the

engagement of the occupants with the building in order to adapt the building to the users’ needs. This will

include activities like, for example, opening the window to allow for natural ventilation, wetting the floor

to reduce the ambient air temperature by evaporative cooling, and, in some places, sleeping on the rooftop

of the house to take advantage of the cool breeze of the night. These activities were exercised inside the

building naturally. People at that time were engaging with the building and the climate around them in a

very effective way according to their intrinsic feelings of what might lead them to feel comfortable.

In addition to the active usage of the building, vernacular buildings have utilized the passive

strategies fully. Passive strategies are the methods and implementation of environmental strategies that

would use the forces of nature to create an acceptable microclimate inside the building space. Some of

these passive strategies are thermal mass and natural ventilation. Thermal mass is essential especially in

hot-aired regions where there is a wide range of diurnal temperature swing. The most appealing thing

about thermal mass is the simplicity and the availability of the material. It is so simple that even the

poorest person in the community could build his/her own house by backing the molded mud in the sun

and then stack them to construct the walls. Thermal mass has proven to be one of the best passive

strategies that architects could apply to their building due to the affordability and ease of construction.

Architects could save energy and choose a natural way of constructing buildings.

Each place has its own microclimate and hot humid is no exception. Many describe the

hot-humid regions as challenging. There are many reasons to believe so. Some of these reasons are high

levels of humidity, and, in many tropical regions, heavy rain. All these elements make the hot-humid

region very difficult to deal with. Some of the most renown methods that have been used in hot-humid

climate are cross ventilation and reflective light surface. High levels of humidity could be as discomfort

as high air temperature. Natural ventilation is utilized to mitigate the effect of high humidity levels. Many

other strategies are employed to condition the building as passively as possible without relying on

mechanical systems to condition the occupied space. What is very important is to have the right

environmental strategy at the right location. Combining two or more strategies would not be necessarily

better, but if they were chosen carefully, they will make strong contribution to energy savings.

6 December 14, 2015

1.2 An overview of the region of Jeddah, Saudi Arabia:The location:

This thesis project is concentrated on the study of ventilated envelope in hot-humid region. The

representative city of this project will be the city Jeddah which is located in Saudi Arabia. Jeddah is a

coastal city located at the west side of the Kingdom of Saudi Arabia with the latitude of 29.21 north, and

39.7 east (Jeddah Municipality, 2015). It is considered as the second largest city in Saudi Arabia. It has a

very long history dating back centuries even before the beginning of the Islamic era. As most cities in the

middle east, Jeddah was surrounded by barricades that surround the perimeter of the city. After the

beginning of the establishment of the third Saudi Kingdom, the city has undergone several changes at the

building scale and the urban scale. The city has been transformed into one of the main cities in the

Kingdom of Saudi Arabia. The city has an international airport, seaport, and many governmental

buildings and embassies that are not located in most cities in the country.

Figure 1 the location of Jeddah in the Kingdom Saudi Arabia (CDM Smith.com, 2015)

7 December 14, 2015

1.2.2 The climate:

According to the municipality of Jeddah, the climate is very hot during the summer with an

average of 40° C and high humidity due to the rise of sea temperature. In the winter, the temperature is

moderate with low levels of humidity due to the impact of moderate winds that blows through the city

that is associated with high air pressure (Jeddah Municipality, 2015).

Table 1 in this table the climate data for the city of Jeddah (Climatemp.com, 2015):

Climate

variables

January February March April May June July August September October November December

Average

maximum

air

temperature

(°C)

32 32 33 36 39 41 40 39 39 37 35 32

Average

temperature

(°C)

24 24 25 27 32 32 32 32 31 28 29 25

Average

Minimum

temperature

(°C)

16 16 17 18 21 23 25 26 24 22 20 18

Average

precipitation

level mm

14 6 1 5 2 0 0 0 0 2 12 12

Number of

wet-days

(probability

of rain)

1 0.5 0.5 1 0.5 0.5 0.5 0.5 0 0 2 1

Average

Daylight

hours (H)

10 11 11 12 13 13 13 12 12 11 11 10

Sun

Altitude at

solar noon

on the 21st

day of the

month(°)

48.3 57.6 68.5 80.2 88.3 88.2 88 80.3 68.9 57.4 48.2 44.9

8 December 14, 2015

Figure 2 the average temperature, humidity, precipitation, sea temperature, days with frost, wet days, and Day length. (Climatemp.com, 2015)

9 December 14, 2015

1.2.3 The psychrometric chart:

Figure 3 the temperature and humidity levels in Jeddah plotted on the Psychrometric chart (House energy Doctor, 1986)

10 December 14, 2015

1.2.4 The vernacular architecture in Jeddah:

According to an online journal written by Mohammad Arif Kamal (2014), our ancestors have had

a better understanding of the conditions of local environment and they have built their building

accordingly. Jeddah is one of those cities that have been established to deal with excessive heat and

humidity in the summer. The art of construction was a great collaboration of many experiences that

influenced the way these buildings in Jeddah were built. For example, the Mashrabi’ah (the protruded

perforated windows) were taken from the Egyptian style of architecture.

The tall lavish buildings that have been made for the wealthy merchants have lasted two to three

hundred years. The buildings in Jeddah have utilized amazing passive strategies that helped to achieve the

thermal comfort of people at that time. The buildings in the old city are clustered in a place known to the

natives as “Al-Balad.” Unlike many vernacular cities in the hot Middle Eastern region, the houses were

separated from each other to allow the air to pass by the buildings freely. This gives the indication of how

vigilant the natives were about their surrounding environment. The buildings were separated by streets

that vary in their size. The street with the least width (secondary street) ranges from (2-4) meters in width,

the second widest street (primary street) ranges from (4-10) meters, and the widest street (the main street)

ranges from (10-20) meters. The main street is on the north –south access so that it will block the sun and

shade the buildings (Mohammed Kamal, 2014).

There are some open spaces that will allow the sun to shine down on them. When these spaces

are heated, the air becomes less dense. Therefore, the much heavier cold air that is in the shade will be

driven to these hot spaces allowing the air movement to occur in the city streets (Mohammed Kamal,

2014).

Figure 4 a simple equation that shows the relationship between the Egyptian style and the style of Jeddah (Mohammad A. Kamal, 2014).


Figure 6 street pattern in Al-Balad region of Jeddah (Mohammed Kamal, 2014).

Figure 5 settlement pattern and layout of Al-Balad region (Mohammed Kamal, 2014)


1.2.5 House form:

The size of the house Is Dependent on the socio-economic status of the residents. The rich houses

range from 15-18 meters in height. The design of the house is dependent on the privacy, and the

maximization of the natural ventilation. The first floor is allotted to the male guest room; the other floors

are for the family bedrooms. The ventilation occurs at the extended windows “Mashrabiyya”, and the

vertical opening “Al-Menwar” that brings the fresh air to the inside of the house (Mohammed Kamal,

2014).

The structure of the building is similar in techniques like those in Egypt. The walls are made out

of thick coral rocks that are been brought from the reefs of the red sea. The stones have lasted for a very

long time, almost two hundred years in many cases. The floors are constructed by using heavy hardwood

that is imported from either India or Burma, and some times from Indonesia (Llewelly-Jones, 1995).

Finally, the gypsum has played an important role in the construction of the house. It had been used for

plastering the walls, and for a bonding agent for the walls.

Figure 7 ground floor plan, elevation, and section of a typical rich house in Jeddah (Mohammed Kamal, 2014)


Figure 9 ocean rocks and coral stone that are used for the load bearing walls (Mohammed Kamal, 2014)

Figure 8 the timber roofing system of a typical house (Mohammed Kamal, 2014)


Figure 10 picture of the protruded-perforated windows (Mohammed Kamal, 2014)

Figure 11 picture of the protruded-perforated windows (Mohammed Kamal, 2014)


1.3.1 Problem statement:

Hot humid climate is one of the most challenging climates. Architects and building engineers

have to deal with heat, humidity, and, in some occasions, heavy rain. Using thermal mass would not be

very efficient. The reason is that in order for a material with good thermal mass criteria to work well, it

needs to be located in a location where there is wide diurnal temperature swing. This phenomenon does

not happen in hot-humid regions. The temperature in hot-humid region is mostly hot during the summer

and spring, then the temperature will be mild in winter with a little temperature rise in the fall. The

diurnal swing is not really profound with the exception of sometimes during the winter season. Only in

the winter when the temperature might drop to 59° F. When the temperature drops significantly, then

thermal mass would work as a good strategy. The essence of thermal mass is that it stores the heat during

the day and releases it during the night. Assuming that in hot-humid regions occupants would need the

heat during the winter night, and then thermal mass could be used. However, many building engineers

would argue that savings from thermal mass in hot-humid regions are very limited, and it is hard to rely

on it as an environmental system that does not work efficiently most of the time.

In order for a thermal mass material to function well, it should release most of the heat that has

been stored during the day in order to have the chance to store heat in the next day. If the temperature in

hot-humid climate does not drop to a certain level, the material would not release that heat during the

night. The reason why there is no temperature swing is that the humidity in the air is preventing heat from

being released to sky. According to Meteorologist Jeff Haby high moisture content in the air cause

“greenhouse gas” effect that will trap the longwave radiation from being emitted to the atmosphere

(2015). For this reason, thermal mass would not work well in hot humid climate. In this thesis, the focus

is to optimize the use of thermal mass in hot-humid region by placing the thermal mass material in a place

where it is shaded from the sun.

The benefits of using a ventilated envelope (Fig 12) are many. The first benefit is the creation of

an interstitial area where the thermal mass could release the stored heat due to the low air temperature

inside the cavity. The reason why the temperature inside the air cavity is low is because the air cavity is

protected from the sun rays during the day. The protection from sun rays during the day will eliminate the

conductive heat transfer through the air cavity. The second benefit is the use of natural ventilation inside

the occupant space and air cavity. Air ventilation is key in this concept. Heat is transferred by three main

methods (conduction, convection, and radiation). Heat loss by convection is one of the three main ways of

heat transfer. The air that passes through the air cavity could bolster the process of heat loss from the

thermal mass material by convection, then the absorbed hot air will carry the heat by natural buoyancy to

the outlet slit and then after that to the atmosphere. This method of using ventilated walls or roof is not

new. Many architected in cold climate have used the ventilated roof to eliminate ice dam effect. Many

other practices have used the air cavity in masonry walls since the mid-twentieth century. Ventilated

envelopes might contribute in the mitigation of heat transfer from the outside of the building to the

occupant space by utilizing the effect of both thermal mass and natural ventilation.


1.3.2 The hypothesis:

The strategy that is going to be used is a ventilated envelope (Fig 12) that consists of a basic

envelope which will serve as the base envelope for the building. After that, there will be another envelope

that will cover the base envelope with an air gap cavity between the base envelope and the other

envelope. The hypothesis behind this experiment is to prove that the building envelope thermal

performance will improve by using air ventilation within the interstitial space of the envelope itself. The

ventilated envelope is assumed to improve the thermal performance of the building envelope by

eliminating the conductive heat transfer of the outer skin of the ventilated envelope. Secondly, the air that

passes through the air gap will help in the removal of the heat from the inner envelope and exhausts it

outside the building. In addition to the double envelope system, the inner envelope is proposed to be

made out of thick thermal mass material that will absorb the heat from the inside of the occupant space

and then it will be released through the air cavity. Furthermore, the design includes a tilted roof which

will allow for the hot air to be removed from the occupant space by creeping below the tilted ceiling. This

tilt position of the roof and ceiling will direct the hot air to the ceiling’s opening. The air that is coming

from the outside through the air cavity will induce this process, allowing a difference in air pressure to

speed up the exhaustion of hot air from inside the occupant space.

Figure 12 Conceptual drawing of the Idea and how the air flows through the air cavity. The room is supplied by air coming through the window which is shown on the façade of the section, and then it is exhausted by the small opening on the top of the ceiling.


1.3.3 Experiment description:

In order to conduct the research, a testing model would have to be built in order to verify the

hypothesis. To test the thermal behavior of this model, a half-scale test chamber is recommended. The

half scale model size is more accurate then smaller size model. The test chamber will be four feet wide,

four feet long, and four feet high resembling a cubic shape. In addition, there will be an additional one

feet that is added on the top south side of the test chamber to create the tilted roof. The material that is

chosen for this test has to resemble the specification of a brick material. Concrete board has been chosen

to resemble the specification of the brick material, and it has the ability to act as a thermal mass. It will be

used for the inner envelope of the system. The outer envelope will be constructed of a simple plywood.

Due to weather condition in Tucson Arizona, the outer envelope has to be protected from the rain with a

waterproof paint. This paint will not alter the results since it is translucent. On the outer surface of the

outer envelope, a bottom slit will be made on both the south side and the north side of the test chamber.

These slits will allow the air to enter through them so that the ventilation process will take place. On the

top of the tilted roof, another slit will be made so that the air inside the air cavity could exit through it.

Figure 13 3-D view of the proposed model


1.3.4 Anticipating the data:

It is believed by the author that the temperature inside the air cavity will be lower than that of the

outer atmosphere ambient temperature.

The thermal mass will help in the absorption of heat from the interior space, and it will release the

heat to the air cavity during nighttime.

The overall temperature of the interior of the test chamber will be lower than the atmospheric

temperature.


1.3.5 The significance of this experiment:

The importance of this experiment stems from the fact that designing an effective passive

building envelope in hot humid climate is very difficult. Some of the basic strategies that are

implied in hot/ or warm humid climate will be explained in the following chapters. Most of these

strategies involve using a thin (low mass) layer of building envelope with another thin layer of

envelope that serves as a shading element for the lower layer.

This thesis is suggesting the use of a higher thermal mass in hot humid climate to utilize

the benefit of heat storage in internal surfaces of the building envelope. The most important

element in the experiment is to prove that thermal mass can function in hot-humid climate if an

area adjacent to the thermal mass is lower in temperature. The lower temperature created next to

the thermal mass element will allow the thermal mass to absorb more heat and then releases it to

the atmosphere. There are many benefits are gained if the system has proven to work properly.

The first benefit is the reduction of solar heat gain. With this design, the solar radiation

could drop significantly since the outer envelope shades the inner envelope from the sun. In

addition, the conductive heat transfer is being eliminated by creating the air gap that only allow

for the convective heat and radiative heat to pass through the air gap. Heat is one of the most

influential element that affects the performance of the building envelope. Therefore, it has to be

taken into consideration when designing buildings in hot arid or hot hot-humid climate.

The second benefit is that this system uses a passive strategy that relies on the forces of

wind and air movement to cool off the internal layer of thermal mass. If this experiment was

successful, it will allow architects to design a better productive buildings with very little reliance

on active systems to condition the spaces. Furthermore, this system will allow the occupants to

drag the heat from the room using the upper opening to the air cavity. This allows the room to

exhaust the hot air that is trapped inside the room and to be released to the outside. Not only that

the internal hot air is driven outside, it also allows the room to have better cross ventilation and

stack ventilation working in both horizontally and vertically.

Lastly is the energy savings. The main objective is to save as much energy as possible.

With the reduction in heat gain inside the building, Occupants would not need to consume an

excessive amount of energy to reach thermal comfort. Energy savings will reduce the operational

cost of the building, reduce CO₂ emissions, and saves the energy needed to fulfill the needs of

the future generation. This research experiment will contribute to the thousands of research

papers on energy consumption.


Chapter 2: Understanding thermal mass

There are many ways to use passive strategies that would reduce the heat gain inside the building,

and would reduce the energy consumption of the building. A material that has been used for millennials

which is thermal mass. Many builders in the past used thermal mass for its natural capability to store heat

for a long period of time. It allowed builders to reduce the heat gain inside the building and stabilize the

temperature swing inside the building. Thermal mass could be beneficial in many parts of the world

where hot climate exists. However, not all regions could benefit from the attributes of thermal mass

material. There are some places that would not benefit from thermal mass the same way as other regions.

Hot humid climate is one of the regions that thermal mass would not work very effectively.

Although that might be true; however, not all researchers agree that thermal mass is not beneficial

in hot humid climate. In this chapter, the general understanding of thermal mass will be described. The

chapter will encounter the basic characteristics of thermal mass, where it is best to be applied, and the

possibilities of using thermal mass in hot humid climate.

2.1 How thermal mass works:

According to (Martin Holladay, 2013) thermal mass is a solid or liquid material that can store

heat. He also mentioned that many objects are considered thermal mass. However, they are not designed

specifically to store significant quantities of thermal mass. What distinguishes a material as a thermal

mass is the specific heat of that material. The reason why specific heat is important in a material with high

thermal mass is the fact that the denser the material is, the more heat is needed to raise its temperature.

That is why heavy concrete materials are preferred in hot regions.

The mechanism in which thermal mass work is that a special material suitable to be used as a

thermal is chosen. In the past, adobe and stones are the most well-known sources of thermal mass.

Thermal mass works in a cycle which starts from the early morning when the wall is cool. When the sun

starts to rise, it heats up the wall. Because the wall is thick and it has a high specific heat, it will take time

before the heat reaches the other side of the wall. At noon around 1:00 P.M., the wall is still cool. After

sunset, the heat had already reached the other side of the wall. Due to the temperature difference between

the recently heated wall and the cooler atmosphere, the heat inside the wall will start to emit. The wall

should lose all its heat so that it could store more heat the next day (Martin Holladay, 2013).

One very important aspect about thermal must be noted in order to use it well which is the

location of usage. The important thing to know about thermal mass is that this wall system works best

when it is subjected to wide diurnal temperature swing. If the temperature is constant throughout the day,

then this system will not work as efficient as planned. That is why thermal mass works best in hot arid

regions. According to Alex Wilson, the editor of Environmental Building News, “Nearly all areas with

significant cooling loads can benefit from thermal mass in exterior walls.” In an article called, “Mass

Confusion,” Charles Wardell reported, “Greg Kallio, a professor of mechanical engineering at California

State University in Chico who specializes in heat transfer, recently … model[ed] ‘the whole gamut’ of

wall systems, from stick-built to SIPs to insulated concrete, using industry standard energy analysis

programs like EnergyPlus, as well as his own custom software. His conclusion? ‘The effectiveness of

http://www.buildinggreen.com/auth/article.cfm/1998/4/1/Thermal-Mass-and-R-value-Making-Sense-of-a-Confusing-Issue/

http://www.greenbuildermag.com/News/Green-Building/Mass-Confusion#.UXWKlaMfFGM

http://www.greenbuildermag.com/News/Green-Building/Mass-Confusion#.UXWKlaMfFGM


thermal mass is very dependent on diurnal temperature variation. You want nighttime temperatures that

get at least 10 degrees cooler than the thermostat set point” (Charles Wardell, 2011).

In cold regions, thermal mass has very little benefits. According to (Martin Holladay, 2013) the

reason is that the heat is flowing from one direction, from inside out. Alex Wilson has written. “In

northern climates, when the temperature during a 24-hour period in winter is always well below the

indoor temperature, the mass effect offers almost no benefit, and the mass-enhanced R-value is nearly

identical to the steady-state R-value.”

2.1.1 Passive solar heating with thermal mass:

Although thermal mass is not very beneficial in cold climates. However, there is a way to

incorporate the benefits of thermal mass with passive solar heating. According to (Martin Holladay, 2013)

this could be achieved if a south facing wall has a large glazing area to allow the sun rays to penetrate into

the building. This sun radiation should hit a concrete floor that would store the heat from the sun

radiation. The floor could later reradiate the heat to the interior space at night.

2.1.2 Using insulation with thermal mass:

Thermal mass does not provide the thermal comfort needed inside the space. The temperature

could perpetually be too cold or too hot that do not allow the thermal insulation to perform effectively.

Therefore, thermal insulation might be incorporated. To install the thermal insulation in the right

direction, it should be mounted behind the thermal mass so that the thermal mass would face the interior

of the house. The thermal mass should not be blocked by any object that might hinder the absorption or

the release of heat from the thermal mass material (Martin Holladay, 2013).

Not all walls have the same effect in dealing with heat transfer. Alex Wilson wrote. “High-mass

walls really can significantly outperform low-mass walls of comparable steady-state R-value — i.e., they

can achieve a higher ‘mass-enhanced R-value, but this mass-enhanced R-value is only significant when

the outdoor temperatures cycle above and below indoor temperatures within a 24-hour period.” Therefore,

even if there are two wall systems that have the same R-value, the effect of each of them is different. R-

value is simply the amount of BTUs or Watts being reduced in an area of feet or meters in an hour. This

definition of R-value could not explain the dynamic movement of heat inside the material.

http://www.buildinggreen.com/auth/article.cfm/1998/4/1/Thermal-Mass-and-R-value-Making-Sense-of-a-Confusing-Issue/


2.2.1 The areas where thermal mass works best:

Thermal mass has special features that make it not practical to implement everywhere. As

mention earlier in the previous section, places, where large diurnal temperature swing occurs, are the best

places to implement thermal mass material. A study done by (Randa Ghattas, Franz-Joseph Ulm, Alison

Ledwith; 2013) highlighted the areas where thermal mass could produce potential savings. They made

computer simulation on the same building with different locations in the United States. In their study,

they gave a percentage of where the most savings occur in the different Parts of the United States.

One of the parameters that are measured in this research article is the diffusivity of the materials.

According to (Randa Ghattas et al; 2013), diffusivity is the measurement of heat flow through the

material or the ratio of heat transmittance to heat storage. For a given thickness, diffusivity captures both

thermal mass of the wall and the heat flow through the wall due to the temperature difference between the

inside and the outside. This phenomenon occur due to the fact that for a given conductivity, the higher the

density of the material and the specific heat, the lower the diffusivity. i.e. if we have a material that is

very low in diffusivity, we could determine along with the specific heat that this material will be a great

source of thermal mass. This parameter is very important in the mapping of the benefits of thermal mass

in different parts of the country. Here are some of the materials in regards to the thermal mass properties

of each material

The results:

The authors have the same wall but they have tried to change the location of the wall to determine the

benefit of the walls with high values of specific heat and low diffusivity (Randa Ghattas et al; 2013). Here

are some the outlines from the simulation results:

1- At all conductivities, walls with higher specific heat and densities will contribute in the energy

savings of the building.

2- The climate is a key factor in determining the range of benefits from low diffusivity walls.

3- Reducing the conductivity of an equivalent wall is a key factor in reducing energy consumption.

4- Thermal mass benefit has the most impact when daily outdoor temperature variations are above

and below the balance point of a building. Hence, cold climates benefit most in the summer

season and hot climates in the winter season.

5- For walls with the same density and specific heat and different conductivities, there is less

thermal mass benefit at lower conductivities.

Table 2.1 thermal properties of typical building material (Randa Ghattas et al; 2013)


Mild, Marine Climate

Mild climates benefit most from the use of walls with low diffusivity. Annually, the potential

savings is in the range of 22% annually for a typical wall. In addition, a wall with a higher conductivity

and low diffusivity can be exchanged with a wall with low conductivity and high diffusivity as a tool to

achieve comparable energy consumption. Hence, there is a double benefit of using a wall with low

diffusivity at lower conductivities. Seasonally, there are greater energy savings in the summer (Randa

Ghattas et al; 2013).

Hot, Dry Climate

Thermal mass benefit is in the range of 4.9% annually for a typical wall in a hot, dry climate. In

addition, lowering conductivities is a key factor in reducing energy consumption. However, lower

conductivities reduce the impact of thermal mass benefit. Seasonally, there are greater energy savings in

the winter (Randa Ghattas et al; 2013).

Hot, Humid Climate

Hot, humid climates exhibit similar characteristics to a hot, dry climate, but the impact of thermal

mass is lower. Thermal mass benefit is in the range of 3.1% annually for a typical wall (Randa Ghattas et

al; 2013).

Cold Climate

The conductivity of an equivalent wall is the primary driving force in reducing energy

consumption in a cold climate. When considered annually, thermal mass benefits are minimal in a cold

climate. For a typical wall, the annual benefit is in the range of 1.5% annually. The amount of thermal

mass benefits are dependent on the seasons, with greater impact in the summer than winter (Randa

Ghattas et al; 2013).

Table 3 prioritizing strategies to improve the energy efficiency of a typical wall (Randa Ghattas et al; 2013)


Figure 15 percentage annual energy savings vs diffusivity for a typical wall (Randa Ghattas et al; 2013)

Figure 14 percentage summer energy savings vs diffusivity for a typical wall (Randa Ghattas et al; 2013)


2.3.1 Studies on the implementation of thermal mass in hot humid region:

Some studies have been conducted to assess the implementation of thermal mass in regions where

heat and humidity is a major aspect regarding these areas. On one hand, there are some studies which

shows that using thermal mass could reduce the energy consumption in hot humid climate claiming that

thermal mass would reduce the peak maximum temperature gain in the building. However, other studies

are not suggesting using thermal mass due to limited savings from thermal mass.

A research that has been conducted in the University of Texas by (Mina Akhavan, Sara

Motamedi; 2012) studied the effect of thermal mass on a hot humid climate like Austin. The goal of this

study was to estimate the effectiveness of thermal mass in hot-humid climate even if there is no

fluctuation of diurnal temperature as seen in hot-aired regions. There have been some studies that

simulated the effect of thermal mass in hot humid regions; however, most of them concluded that thermal

mass does save energy in hot humid climate.

In their study, they simulated two models that reside in Austin Texas. The first model was a low-

mass office building, and the second model was the same office building with a high-mass envelope.

They kept all the other parameters the same such as the lighting, the R-value of the exterior walls, and the

electric consumption. The typical office building consumption in Austin was 13 KWh/ft². However, the

baseline model has a consumption of 11 KWh/ft², because it complies with ASHRAE 90.1 standards

(Mina Akhavan, Sara Motamedi; 2012).

The following chart shows the comparison between the energy consumption of both models. The

cooling load on the high mass model consumes 1015.8 KWh less than the low-mass model. For heating,

the high-mass model consumes 370.31 KWh less than the low mass model.

Figure 16 percentage winter energy savings vs diffusivity for a typical wall (Randa Ghattas et al; 2013)


By analyzing the results from the savings. It turned out that thermal mass has a very low energy

savings. There are energy savings from using higher mass, but the results are not promising. In all four

orientations, the south wall seemed to have the highest savings among the other orientations (Mina

Akhavan, Sara Motamedi; 2012).

Figure 17 annual fan load (Mina Akhavan, Sara Motamedi; 2012)

Figure 18 annual cooling load (Mina Akhavan, Sara Motamedi; 2012)

Figure 20 annual heating load (Mina Akhavan, Sara Motamedi; 2012)

Figure 19 total cost (Mina Akhavan, Sara Motamedi; 2012)


2.4.1 Other studies that indicate the benefit of thermal mass in hot humid climate:

Some architects do think that buildings with high thermal mass in hot humid climate is effective.

Larry Speck was visiting an old building that was built during the roman ruling in Turkey and says, “I

became interested in using high thermal mass as an alternative while traveling in Turkey with my son

Sloan, eight years ago. He and I visited remote Roman ruins on the south coast and the interior, where the

sites are in raw states and are not much frequented by tourists. The summer climate in Turkey is very hot

and humid, not unlike Texas. But it was strikingly comfortable inside the stone ruins with their high

thermal mass.” (L. Speck, 2012). He also noted the same effect in Ping Yao in china where he lived in a

home that has high thermal mass material. The bed was made out of stone and according to him even

when the temperature was 100° F outside, it was remarkably cold from the inside. Convinced by the

results, he decided to build a simple single story office building in Austin Texas for a firm called “Wiss,

Janney, Elstner Associates (WJE).” During the final stages of the construction, he and the rest of the

construction crew noticed that even without using the air condition, it was surprisingly cold inside the

building (L. Speck, 2012).

There is another study, which takes place in Ghana. The study was focusing on the effect of

thermal mass, and night ventilation on the buildings in the city Kumasi. The climate condition in Kumasi,

the second largest city in Ghana, is considered hot and humid. The office building’s design adopted the

international style which focus on the use of large parts of glazing. The use of glazing has caused some

problems in the interior temperature of the office building which raises the indoor temperature due to the

admittance of sun radiation into the building (S. Amos-Abanyie, F.O.Akuffo, V. Kutin-Sanwu; in 2013).

The typical building material that is used in Ghana is sandcrete. This material is known to be low

in thermal mass and according to the author of this article, it does not benefit from the night relatively low

temperature. In addition, it does not take advantage of the use of night ventilation due to the fact that the

windows are always closed in office buildings during the night (S. Amos-Abanyie, et al; in 2013).

The research was carried out by computer simulation and by the experimental use of test

chambers that were built for the research.

The temperature difference ratio TDR = (𝑇max out –𝑇max in) / (𝑇max out –𝑇min out)


This equation was proposed by Givoni which was described by the authors as “…been used with

good results to compare passive cooling systems with different configurations” (S. Amos-Abanyie et al;

2013). The numerator calculates the difference between the indoor maximum temperature and the outdoor

maximum temperature, while the denominator calculates the temperature swing of the outdoor

temperature. The higher the number, which is the temperature difference, the better in delaying the peak

energy demand for the indoor space.

The results of this study have shown that using materials with heavier thermal mass would delay

the peak indoor maximum temperature. This reduction in the peak maximum temperature would reduce

the energy consumption of HVAC systems. This table will illustrate the time lag of each material (S.

Amos-Abanyie, et al; in 2013).

Figure 21 schematic drawing of the experiment setup (S. Amos-Abanyie et al; 2013)

Table 4 description of models (S. Amos-Abanyie et al; 2013)


In this study, the nighttime ventilation was analyzed and it was noticed that among the nine-test

model that they had, the one with the most thermal mass, air change per hour (ACH), and no windows has

the best performance. The researchers have noticed that increasing the ACH from 10 to 20 or even 30 did

not produce significant results. However, the researchers do confirm that night ventilation in the baked

bricks and the concrete model is beneficial when the windows are open to remove the heat that is

observed by the thermal mass. In addition, it allows the material to become a “heatsink” so that it could

absorb heat from the following day. Another finding in the study is that in Kumasi the wall temperature

difference between the low mass and high mass material is negligible. According to (S. Amos-Abanyie et

al; 2013) this might be the result of the limited diurnal temperature difference in the region.

Table 5 outdoor and indoor air temperature of model 9 and the controlled model (S. Amos-Abanyie et al; 2013)

Table 7 indoor air temperature models with varied windows pattern (S. Amos-Abanyie et al; 2013)

Table 6 maximum temperature differences, temperature difference ratios, and percentage of overheated hours (S. Amos-Abanyie et al; 2013)


Chapter 3: The basic design principles for hot humid climate region:

Building in tropical region is very challenging. The temperature is high most of the year that

ranges from 30° – 35° C almost all the year. The wind is almost non-existent due to the stability of the

temperature; also, the high levels of humidity make it very hard for wind to pass through the tropical

regions. The sun radiation in the tropical region is very high. Many designers recommend using large

amount of shading instrument on the building to protect the building from the direct solar radiation and

from the diffused solar radiation (Paul Gut, Fislisbach, Dieter Ackerknecht, Zollikon; 1993).

One of the benefit of living in the tropical region is the abundance of vegetation throughout the

land areas. The presence of vegetation will reduce the ambient temperature because of the

evapotranspiration. Two things must be taken into consideration when using vegetation for evaporative

cooling. The first is the exacerbated amount of humidity that might be generated from vegetation.

Secondly, the vegetation cover should not restrict the movement of air to the building. According to (Paul

Gut, et al; 1993), the use of thermal mass is highly not recommended. Instead, the use of low mass with

highly reflective surfaces and / or the use of double structure is much more appropriate in hot-humid

climate. Like in many other locations with hot-humid region, the use of natural ventilation is the key.

3.1.1 The main point:

Tropical location with maximum natural ventilation and shading.

Orient the building so that lowest amount of solar radiation would impact the building.

Scattered pattern of building.

Avoid locating the building in places of hazard such floods and hurricanes.

3.1.2 Sun orientation:

If the building would to be located on a mountain, then east and west slope better to be avoid for

they have the most impact of sun radiation than the other orientation (Paul Gut, et al; 1993).

3.1.3 Wind orientation:

The windward direction should be on main façade of the building. The use of vegetation could

assist in the orienting the wind toward the building (Paul Gut, et al; 1993).


Buildings should not block each other from receiving wind.

Therefore, a scattered layout of neighborhood is preferred. The

pavement should not be left without shading. If the pavements are

not shaded, the air that passes by the pavement will heat up by

convection and then it will hit the building envelope causing the

building to heat up even more (Paul Gut, et al; 1993).

3.1.4 Building design considerations:

The building rooms should face south and north so that the

cross ventilation movement could take place. To avoid the solar

radiation from the east and from the west, the majority façade of the

building should be oriented to face the south elevation. It is hard to

fulfill the need for sun protection and providing the best orientation

for the building. That is why some designers advice to rotate the

building so that it could use the benefit of both air ventilation and sun

protection. As a rule of thumb, in low-rise building when the sun

radiation would not hit the building too often, orientation should be

according to the wind direction. When a high-rise building is

considered, protection from sun radiation should be the decisive

factor (Paul Gut, et al; 1993).


3.1.5 Room arrangements:

Some architects suggest that the arrangement of rooms should be dependent depends on their

function. Since the thermal load is related to the orientation, rooms on the east side are warm in the

morning. However, they will cool down in the afternoon as soon as the sun moves toward the south

elevation. Rooms on the west side are cooler in the morning and heat up in the afternoon. Rooms facing

north and south remain relatively cool if provided with adequate shading. Thus, the rooms can be

arranged according to their functions and according to the time of the day, they are in use (Paul Gut, et al;

1993).

3.1.6 Room arrangement according to climatic preferences:

It may not always be possible to arrange all the main rooms in an ideal manner. In this case,

special care must be taken for the disadvantaged rooms (Paul Gut, et al; 1993).

3.1.7 Bedrooms:

Bedrooms can be adequately located on the east side, where it is coolest in the evening. Good

cross-ventilation is especially important for these rooms because, at rest, the human body is more

sensitive to climate. On the other hand, stores and other auxiliary spaces can be located on the west side

(Paul Gut, et al; 1993).

3.1.8 Kitchen:

Provided the kitchen is mainly used during morning and midday hours, it can be located on the

west side as well (Paul Gut, et al; 1993).

Figure 22 the arrangement of room as suggested by (Paul Gut, et al; 1993).


3.1.9 Main room:

The main rooms which are in use most times of the day, such as living rooms, should not be

located on the east or west side (Paul Gut, et al; 1993).

3.1.10 Rooms with internal heat load:

Rooms where internal heat occurs, such as kitchens, might be detached from the main building,

although they can be connected by a common roof (Paul Gut, et al; 1993).

3.1.11 Building components:

The main points

• Heat storage and time lag should be minimal.

• Thermal insulation is not effective except on surfaces exposed to direct radiation.

• Materials should be permeable to air.

• Reflectivity and emissivity are important.

According to (Paul Gut, et al; 1993), the main three strategies in hot humid climate. First, avoid

heat storage as much as possible to take advantage of the night cooling. Second, use natural ventilation to

take advantage of cooling the building by perspiration. Third, avoid the direct and indirect solar radiation.

However, in some cases, high thermal mass with a time lag of five hours would prove to be beneficially

in some cases. This might happen in early morning when the temperature of the wall surface is below the

ambient air temperature. This is also dependent on the diurnal temperature difference. There is one thing

that must be bore in mind which is the condensation of the cold surfaces during the early morning hours.

Due to high level of humidity, condensation could occur very often in humid climates (Paul Gut, et al;

1993).

According to (Paul Gut, et al; 1993), the use of thermal insulation has little effect in improving

the thermodynamic features of the building envelope. The reason is because of the use of natural

ventilation which make the inside temperature almost equal to the outside temperature. Insulating the roof

however, could help in reducing the heat transfer of solar radiation through the roof. The use of reflective

color on the roof and walls could improve the thermal performance of the building envelope.

Direct contact with the ground does not necessarily help, because if the ground is shaded from the

sun, the temperature of the shaded part of the ground will be equal to the ambient temperature. Therefore,

Paul Gut suggests that the ground floor be lifted to cool of underneath the floor by ventilation (Paul Gut, et

al; 1993).


3.1.12 Using double ventilated roof:

The use of double skin roof has proven to be very effective. It will perform better if the lower

skin is insulated with at least a 1.5 W/m² U-value insulation. The surface temperature of the inner ceiling

should not be higher than 4° C than the ambient temperature (Paul Gut, et al; 1993).

A book written by (Patti Stouter; 2008) give very important guidelines into the basic designs

principles of hot humid climate. In places where people live the traditional life style, buildings are

designed to meet the need of the people. They use simple treatment to adjust the building to their need.

For example, the kitchen and washing room are kept separate from the main building to reduce the source

of moisture. In addition, they are allocated in a way to remove the moisture by the breeze.

3.2.1 1- Ventilation:

The house interior should be ventilated for removing the heat from the inside of the building.

Ventilation could also reduce moisture levels by having the air dehumidified so that mold would not

grow. The speed of wind could be increased with the use of wing walls (Patti Stouter; 2008)

Figure 23 layer suggested by (Paul Gut, et al; 1993) to improve the thermal performance 1) reflective metal membrane; 2) air gap; 3) wood cover; 5) thermal insulation; 6) interior finish.


3.2.2 2- Shading:

The building in hot humid climate should be

shaded from the sun. The orientation could help in

reducing the heat gain. East-west orientation is the

most beneficial orientation. The east and west

façades produce harmful effect on the building. They

could increase the building’s temperature during the

early morning and during the afternoon. Therefore,

the use of windows at these orientations should be

kept at its minimum. In addition to orientation, the

color of the building can reduce the heat gain of the

building significantly (Patti Stouter; 2008)

.

3.2.3 3- Planting:

According to (Patti Stouter; 2008)

heavy walls should not trap air inside a

pavement area that might increase the air temperature by convection. Plants could reduce the

temperature of the air by the effect of evapotranspiration. In addition, they could funnel the wind

in order to increase the air speed within the building. Some of the benefits of using plant is that

they reduce the ambient air temperature. The temperature in the city could be higher than places

near green places. In hot humid climate wet land are used to trap the rain underneath them. With

the growth of the city, floods may occur because there is no place for the rain to go. These

damped lands are also beneficial in reducing the ambient air temperature. It happens when air get

in close contact with the damped soil therefore reducing the air temperature above it.

3.2.4 4- Insulation:

Two ways to deal with overheated periods in hot humid climate. The first way is to vent

the roof so that the hot air will raise and exists the space from the upper vents. The second way is

to insulate the roof. According to (Patti Stouter; 2008) however, thermal insulation should not

soak up the humidity.

Figure 24 the use of wing walls to drag the wind inside the house (Patti Stouter; 2008)

Figure 25 this figure illustrate the best orientation of building in hot humid climate (Patti Stouter; 2008)


3.2.5 Light weight construction:

Building in the hot humid regions are not the same as in hot dry regions. The buildings are light

and do not trap the heat so that it might reradiate into the inside of the house. Indigenous people in many

tropical regions have used trees bamboos and other materials instead of concrete because they are much

cooler than concrete. However, they may not use them some times because insects might eat them.

Therefore, many of them use concrete on the first floor and bamboo on the second floor.

Some materials are used as insulators. An example of these materials is reed-thatched roof.

These materials are natural and do not transfer heat but are flammable and must be treated to create a

much safer roof. Another inexpensive material is earth materials. This material is available everywhere

and is much better insulator than concrete or brick. There are many ways to build walls with earth

materials like Cob walls, compressed earth block (CEBs). However, these materials should not be

exposed to the rain and therefore a good roof protection and strong dry foundation is needed. In (table 8) a

list of materials and a comparison between them and concrete in terms of insulation and heat storage.

Table 8 some of examples of materials and the amount of heat and insulation each materials could stand compared to concrete (Patti Stouter; 2008)


Chapter 4: Understanding Ventilated Envelope:

Vented cavities are not new to the building industry sector. The idea of having an air gap between

two layers of wall assemblies have been in use for decades. It was the method commonly adopted in

building brick buildings especially in the 1940s (department of labor – Australia, 1946) at first it was not

considered as ventilated façade due to the fact that the bricks would only vent the walls so that moisture

would not accumulate. These walls had no openings from the top and bottom to vent the wall. However,

there are some blocks that are pierced to vent the walls. In this chapter, the focus is on the performance

and the applicability of vented façade in different part of the world. Researches have studied the effect of

ventilated façade on the energy performance of the building during the cooling season and the heating

season. This chapter also discusses the difference between sealed cavity facades and open joint cavity

façades. These studies will help in the understanding of the thermal behavior of these types of wall

system.

There is no consensus among engineers as to whether the use of ventilated façade is beneficial or

not. Therefore, it is important to understand the benefits behind using ventilated façade and the

disadvantages from using ventilated façade. here are some of the benefits from using ventilated façade: 1)

Provide a capillary break form water penetration; 2) Reduce direct moisture bridge; 3) Allow the removal

of moisture that might have penetrated the cladding; 4) It can permit pressure equalizer (Mikael

Salonvarra, Achilles N. Karagiozis, Marcin Pazera, William Miller; 2007).

The ventilated cladding provides the opportunity for moisture removal through convective and

diffused air transportation.

4.1.1 The walls:

Numerous researches have been done to assess the performance of the cladding system that

contradicts each other. For example, a study at Belgium have determined that the ventilated cladding have

no effect on heat transmission within the air gap space. A study done by Guy and Stathopoulus (1982) has

shown that a reduction of 35% of cooling load was achieved when using the vented area that is 100% of

the cross sectional area of the cavity. In addition, they included that if the area is reduced the saving will

also be reduced. They have also demonstrated that by reducing the emissivity within the cavity from (0.90

- 0.40) with a simultaneous 25% reduction of the cavity size, a reduction of 50 % of cooling load was

achieved (Mikael Salonvarra, et al; 2007).


4.1.2 Roof:

Many techniques are used on the roof to increase the performance of the roof. The roof

construction has utilized ventilated roof to reduce the cooling load. In a study done by Beal and Chandra

(1995) who discovered that daytime heat flux reduction by 45% is obtained in a counter batten concreate

tile compared to direct-nailed shingles. Another study done by Miller (2006) decided to test the effect of

roof ventilation and the tile color of the roof and found that when they used darker color, the buoyancy

movement was faster than in light colors.

In addition, the effects of the air cavities extend beyond heat reduction. It could be used to dry out

the inner surfaces of the wall and block the moisture migration into the inner wall of the ventilated roof.

In order to improve the desiccation of the cavity, semi-permeable vapor barrier materials (0.4 -0.5 perm)

could be used to increase the desiccation of the cavity. The reason is that semi permeable vapor barrier

materials would hold moisture for too long. Thus allowing the air that passes through the cavity to carry

out the water from inside the cavity (Mikael Salonvarra, et al; 2007).

Figure 26 the assembly of counter batten (radiant Guards, 2015)


4.1.3 Comparing vented roof to unvented roofs:

Vented roof:

Some studies have investigated the effect of vented roofs and unvented roof as the online article

posted on the (Building envelope theory, 2015) which discusses the basic principles and benefits using ventilated roofs. In their article, they discuss the traditionally four main reasons for venting a roof:

Removing moisture from roof cavities, structural members, sheathing and insulation.

Controlling ice damming by keeping the roof cold.

Enhancing roofing material life span by reducing sheathing temperature.

Reducing cooling loads and increased occupant comfort during the cooling season.

In their study, they highlighted the reason for applying ventilated roofs in cold climate. The first

reason is to remove excessive moisture from the roof assembly. The desiccation of moisture will

occur if the air passes though the ventilated roof cavity without any obstructions. The second reason

for ventilating the roof is to reduce the ice dam effect. This phenomenon occurs when the heated air

inside the space rises up and gets in contact with the unvented roof. The heat from the room will melt

the snow that is located on the roof. However, when the snow melts it will leave room for the rest of

the snow on the roof to accumulate on top of the melted ice (Building envelope theory, 2015).

Moisture can enter a roof assembly in several ways:

Roof leaks (which may be caused by ice dams), flashing problems, roofing failure, and wind-driven rain and snow.

An air leak in the building envelope which transports water vapor into the roof cavity.

From inside the building via water vapor diffusing through the interior sheathing.

From inside the building through holes in the vapor retarders that allow water vapor suspended in the air to bypass the vapor retarder.

In a study done by Bill Rose of the University of Illinois stated in an unpublished report that,

“Researchers have compared the shingle temperature of both vented and un-vented roof systems. It has

been shown that ventilation has a great effect on the attic air temperature, but much less on shingle

temperature. The exterior surface of the shingle is practically unaffected by the presence or absence of

ventilation in the attic.” (Bill Rose, 2002). Some researchers have found that ventilated roofs do not

reduce the cooling load of the building. The National Bureau of Standards published an article “Summer

Attic and Whole House Ventilation” (Dutt and Harrji, 1979), in which the authors stated that “with

recommended levels of insulation, the attic air temperature had little effect on the cooling load.” They

also observed an increase in cooling costs because venting the roof caused pressure differences within the

building envelope. These pressure differences caused cool air from the interior of the house to escape into

the attic and out via the ventilation system.


4.2.1 The pattern of heat transfer through the vented wall system:

A research that has been done by (Griffith, 2006) highlights the method of heat transfer and types

of heat transfer through the ventilated wall system. This research represents a whole building numerical

model system that will analyze the use of ventilated envelopes. According to (Griffith, 2006) the wall is

made out of the outside layer that faces the atmosphere which is called the baffles, the air cavity layer,

outside plane and the underlying layer. The author assumes that the outer baffles are restricting the

transfer of convection and radiation heat transfer.

The test was run in Chicago Illinois at July 8. The result is comparing the dry bulb temperature of

the surfaces of the two configurations.

In the results, the baffles are heated much faster than the integrated wall system (which is a wall

system made of a typical from wall with solar panels attached to it), and cools of much faster than the

integrated wall system. The reason is that the baffles are made of highly conductive materials that absorb

the heat and emits it very fast. The result shows that the interior surface of the ventilated wall

Figure 27 the configuration of the wall assembly (Griffith, 2006)

Figure 28 exterior temperature results (Griffith, 2006)


configuration is cooler by 1.6° C. The integrated system is warmer during the night due to the fact that it

losses the heat in a slower rate than the ventilated wall system (Griffith, 2006).

According the author (Griffith, 2006) the underlying surface temperature is much higher than the

cavity air temperature, which indicates that the predominant source of heat transfer is radiation. It also

shows that the convection heat transfer is very low. This also tells that that ventilated-driven cooling is

limited inside the cavity.

Figure 29 Photovoltaic roof configuration, (a) with ventilation cavity, and (b) with integrated surface mounting (Griffith, 2006)

Figure 30 interior surface temperature results (Griffith, 2006)


4.2.2 using phase change materials with ventilated façade system:

A research done by (Alvaro de Gracia, Lidia Navarro, Albert Castell, Álvaro Ruiz-Pardo,

Servando Álvarez, Luisa F. Cabeza ; 2012) focuses on the usage of double ventilated façade system with

the use of phase change materials (PCM). The test has been done in Spain in the winter season. According

to some research on the topic of phase change materials, the benefit of using phase change materials is to

improve the performance of the building façade by storing the heat in the PCMs. However little research

has been done to prove the effectiveness of using PCMs with ventilated façade systems. Moreover, the

PCMs could provide a storage of cool air that is trapped inside the cavity when needed.

Figure 31 heat transfer coefficient results (Griffith, 2006)


The experiment is been done on two test chambers. One is made with a simple cubic shape room

with (2.4 * 2.4 * 5.1 m). The other was a chamber that has a ventilated system on the south wall of the

chamber. The dimension of the second chamber is the same as the first with the exception of the

ventilated system attached to it. Inside the ventilated chamber, there are 112 panels of PCM that are made

out of salted hydrate SP-22 from Rubitherm ©. These 112 panels create a 14-channel airflow through the

cavity. Additionally, the openings have fans that are controlled by sensors (Alvaro de Gracia et al; 2012).

Figure 32 the two models that have been studied in the research (Alvaro de Gracia et al; 2012)


4.2.3 The method:

The test chamber has sensors that will indicate that if the PCM materials are melted and if the

cavity air is hot enough to introduce it to the inner space. Then the discharge operation take place

replacing the heated air with inside air of the space.

4.2.4 The results:

The test was run under two separate conditions (sever winter and mild winter). The researchers

have used the free-floating method, not using the HVAC system during this period. As it could be seen

from figure 33, the inner temperature of the ventilated façade kept the room temperature around 9 - 18 °C.

However, the base case model has an inner temperature that is oscillating synchronously with outer

temperature. These temperatures however, are taken during the sever winter season. Figure 33, also

shows the temperature obtained during the mild winter season (Alvaro de Gracia et al; 2012).

Figure 33 inner and outer temperature results during sever winter and mild winter season (Alvaro de Gracia et al; 2012)

Figure 34 thermal performance of PCMs, airflow, and inner and outer temperature during March 17 Summarizing the article (Alvaro de Garcia et al; 2012)


4.3.1 using ventilated façade with double skin façade:

This study is an experimental research that has been conducted in Serbia to analyze the merits of

the double façade system by (Aleksandar S. Andelkovic, Branka Gvozdenac-Urosevic, Miroslav Kljaji;

October 2014). In this research, researchers show the times and the climate when this system could

achieve energy savings and other times when this system could cause overheating. There are many things

that are tested in this experiment. The internal air temperature of the air cavity, cavity air velocity, and

the enthalpy energy.

Figure 35 the experimental setup for the NE side of the building and shows the placement of sensors inside the air-cavity (Aleksandar S Et al; 2014).

Table 9 the material properties of the opaque wall (Aleksandar S et al; 2014)


4.3.2 The metrological data:

The winter temperatures are around 0-(-8) °C. With high air speed reaching as high as 20 m/s.,

the summer conditions are very different, and lastly the solar radiation is as low as 560 W/m². The air

temperature is around 5-33 °C. the wind speed is typically low in the summer ranging from 1.5 – 2 m/s.

the solar radiation is very high in the summer which could get as high as 1000 W/m² (Aleksandar S et al;

2014).

4.3.3 Air temperature results:

The air temperature inside the cavity is always higher than the exterior temperature in the

daytime. This happens because the shortwave radiation is trapped inside the walls in the southwest side of

the building. However in the nighttime, the temperature drop dramatically. In the winter for example, the

air cavity’s temperature is 3.2 °C higher than the outside temperature. In summer, the number is even

higher. Raising to up to 43 °C. This overheating is certainly damaging the envelope performance

(Aleksandar S Et al; 2014).

In addition to the variation in the horizontal cross section of the cavity, the vertical cross section

also has variation of temperature difference from the bottom to the top. This phenomenon is caused by the

natural buoyancy of hot air to the upper part of the cavity.

Figure 36 Fig. the cavity temperatures, and solar radiation on the outer surface during the three selected periods.


Figure 38 vertical temperature gradient among the air cavity for typical cold winter (sunny/cloudy) day (31/1 and 25/1/2014)

Figure 37 vertical temperature along the air cavity for a typical warm winter (sunny/cloudy) day (25/2 and 6/3/2014)


In the figure (37-38), the picture shows the loggings of the different air temperatures in

different times of the day along the vertical cross sectional area of the air cavity. In the winter, the

double skin façade is essential and works very well because it increases the temperature inside the air

cavity. This will reduce the heat gradient between the inside and the outside. What is also special about

the air cavity is that it eliminates the conductive heat transfer and only the convective heat transfer

occurs. In summer, the cavity temperature also increases but in this building, there are shading louvers

that will tilt to block the excessive heat of the sun.

Figure 39 vertical temperature gradient along the air cavity for a typical sunny (spring/summer) day (28/5 and 21/7/2014)


Figure 40 horizontal trend of temperature and air cavity velocity values during a typical (cold/warm) day

Figure 41 horizontal trend of temperature and air cavity velocity values during a typical sunny (spring/summer) day


4.4.1 Open joint and closed joint ventilated façade:

This paper will discuss the importance of ventilated façade for the use in cooling seasons. The use

of ventilated façade was essentially being used to remove the excessive amount of moisture that might

penetrate into the wall and into the interior space. However, the use of ventilated façade might help in the

reduction of heat gain inside the occupied space. According to (Michael D. Gibson, 2015) 40 % of heat

transfer could be reduced when using ventilated façade. Ventilated façade works in similar manner to rain

screen protection in walls. The analogy is that the air cavity eliminates the transfer of heat by conduction

the same way it eliminates the penetration of moisture.

The mechanism of heat transfer could be understood with this equation.

q net = q gain – q loss

Conduction between the outside of the wall and the inside of the wall happens because of the

temperature difference between the two surfaces. When the surface temperature is much higher than that

of the interior surface, the heat is transferred from the outside to the inside. The reason for this is that the

more the object is heated, the more heat it will lose to next lowest object in temperature until it reaches

equilibrium. Also important is the equivalent exterior temperature, and the aggregating heat flow from the

exterior air with the impact of solar radiation. These could be measured through the following equation

(Michael D. Gibson, 2015).

T Sol-air = Tao + R so (α ·G)

T Sol-Air = the sol-air temperature, used to calculate heat transfer by conduction

Tao = temperature of air outside

R so = resistance of outside (cladding) surface, ℓ /k

α = absorptivity of outside (cladding) surface

G = incident solar radiation

In this research, the author used an example of a regular four-story building located in Houston

Texas. According to the author (Michael D. Gibson, 2015) 15.5 % of the all heat gain in the building

comes from opaque part of the envelope. If the ceiling is not counted, then 7.1 % of the heat gain comes

from the walls. According to (Michael D. Gibson, 2015), in unventilated claddings, the heat could only be

released to the outside by convection and radiation and it could be released to the interior of the wall by

conduction. However, in ventilated wall cladding, heat could be released to the air cavity and the outside

atmosphere by convection. In this scenario, the heat loss (q loss) from the cladding will result in the heat

flow equilibrium with the heat gain from the sun (q gain). It must be bore in mind that the drainage plan is

the most important temperature. Because, it source of the heat that will enter the interior wall if it reaches

high levels of temperatures.

In the ventilated wall system, the most important temperature that should be monitored is the

drainage plane temperature. Because the difference between the drainage temperature and the controlled

interior space will determine the intensity of heat flow through the interior wall (Michael D. Gibson,

2015).


In another study by the same author (Michael D. Gibson, 2013); the author assessed the

performance of three test chambers. The firs model was a typical non-ventilated cladding; the second

model was a ventilated cladding; the third model was a ventilated cladding with open joints between the

cladding tiles. In this experiment, the two ventilated claddings performed better than the closed cladding.

However, the open joint cladding performed even better than the close joint by having a cooler drainage

plan and cooler cladding temperatures. Open joint ventilated façade was performing better than the closed

joint due to many reasons. One of them is that open joint cladding system allows more air to enter the

wall system in different levels. Thus, allowing more cooling to the cavity. Second reason is that, open

joint ventilated façade losses heat much more than in closed joint ventilated façade system.


The results of these experiments gave different surface temperatures in which the non-ventilated

wall surface temperature was 119° F. The closed joint ventilated façade surface temperature was 109° F.

lastly; the open joint ventilated façade surface temperature was 107° F. These temperatures were

measured on a typical sunny day in September at 94 – 102° F dry bulb temperature (Michael D. Gibson,

2013).

These results have prompted the author (Michael D. Gibson, 2013) to investigate further the heat

transfer mechanism but with different materials base on the assumption that diffusivity could increase the

performance of ventilated façades. According to a hypothesis made by (Michael D. Gibson, 2013), using

materials with high diffusivity will increase the performance of the ventilated wall system by allowing the

cladding to lose the heat that has been gained rapidly. This is a very different way of dealing with heat

gain than the traditional way of thinking. Usually the focus is on the reduction of heat gain by the use of

thermal insulation that would reduce the (q gain). However, having a material that is very high in heat

gain and loss such as aluminum would redirect the heat that is absorbed by the solar radiation through the

air cavity by radiation and convection. This strategy will not work well if the high diffusive material is in

contact with wall. However, because there is an air gap between the cladding and the wall, the material

could lose most of the heat through the air cavity without transferring that heat to the drainage plane.

Figure 42 a constrained boundary condition using computational fluid dynamics (CFD) to simulate the effect of

different material performance (Michael D. Gibson, 2013)


The materials that have been used for the test were: a typical wall with extruded polystyrene

insulation with thickness of 25, 50, 75 mm. the ventilated wall was a wall that has a 1 cm high-pressure

laminate (HPL) cladding. Thermal insulation was added to the HPL wall to assess the combined effect of

ventilated façade with thermal insulation.

According to (Michael D. Gibson, 2013) the heat transfer reduction in ventilated facade reached

52% with 25 mm insulation, 50% with 50 mm of insulation, and 50% with 75 mm of thermal insulation.

According to (Michael D. Gibson, 2013), thermal insulation might reduce the heat flow from the outside

of the wall to the inside. However, it also increases the cladding surface temperature by having the heat

being trapped between the thermal insulation and the cladding surface.

Figure 43 insulation variation and resultant equilibrium temperatures

Figure 44 insulation variation and resultant heat transfer rates - BTU/SF


The next test will compare three types of materials which are aluminum, high-pressure laminate,

and granite (stone). The test is intended to measure the effect of diffusivity with the effect of ventilated

façade. The result shows that aluminum.

Some of the key findings in this are as follows. First, aluminum exhibits the lowest temperature

on both sides of the cladding giving the fact that aluminum propagates heat much faster than HPL and

granite. HPL had the highest outside exterior temperature. However, the temperature on the other side is 8

F lower. This result shows that HPL with ist physical properties, have a continuous conduction operation

that is transferring most of the heat from one side to the other. This might result in a higher air cavity

temperature compared with the aluminum cladding (Michael D. Gibson, 2013).

Figure 45 material variation: temperature comparison


Table 10: Ventilated Cladding Temperatures, Exterior Side Vs. Cavity Side

Material Temperature

of cladding

exterior

Temperature

of cladding

air cavity

Temperature

difference

2mm aluminum – vented

cladding

89.2 89.2 0.00

10 mm HPL cladding – vented 115.9 107.7 8.2

30 mm granite – vented 110.6 109.6 1.0

By comparing the drainage plane temperature with interior cladding surface temperature, the

result of the drainage surface temperature should be predicted based upon the cladding interior surface

temperature. The result shows that aluminum has the lowest drainage plane temperature followed by HPL

and granite. Granite cladding have benefited the most from the air cavity resulting in a 47% reduction in

heat transfer compared to aluminum that shows a reduction in 21% reduction in heat transfer (Michael D.

Gibson, 2013).

Table 11: Heat transfer rates from material simulations

Material Q (heat gain) un-

vented cladding,

Btu/h*ft2

Q (heat gain)

vented cladding*,

Btu/h*ft2

2mm aluminum – vented cladding 1.38 1.09

10 mm HPL cladding – vented 2.58 1.52

30 mm granite – vented 3.04 1.60


Figure 46 comparison: incident solar radiation and resultant temperatures


4.5.1 Open-joint versus sealed cavity façade:

A study conducted by (Cristina Sanjuan, Maria Jose Suarez, Marcos Gonzalez, Jorge Pistono,

Eduardo Blanco; 2011) that address the benefit of using ventilated façade with open joints or having a

wall system with sealed cavity. According to some companies that have been using this system, open

joint ventilated façade could reduce moisture and it could reduce the heat gain of the building.

The design of a regular building has a brick wall with gypsum board on the interior and thermal

insulation on the exterior of the inner wall. After that, an air gap between the final layer and the interior

wall is allowed. However, the researchers wanted to take this design and improve it by making the

cladding to have open joints as it is shown in (figure 49) the tiles are separated by metal brackets that will

give space between the tiles. Usually if the cladding does not have open joints, the slits are made at the

bottom and at the top of the wall system. In this case however, the space between the cladding is

sufficient enough to allow for the air passes through (Cristina Sanjuan, et al; 2011).

To explain how the open joint system could outperform the sealed joint system, the mechanism of

heat transfer between the two must be analyzed. In the example of the sealed cavity, the air inters the

cavity from the bottom opening and passes through the air cavity going upwards. The exterior layer will

transfer the heat that has been absorbed by the solar radiation and from hot air outside. Eventually the

higher portion of the air cavity will become high in air temperature because all the heat that is being

released by the two layers is accumulating upward. Nevertheless, the hot air will create a convective loop

inside the air cavity. According to (Cristina Sanjuan, et al; 2011) this might add convection to the

conduction and radiation heat transfer to the building interior. On the other hand, with the use of open-

joint ventilated cavity, the convection loop could be eliminated because the heat could escape in any part

of the cladding through the open joints (Cristina Sanjuan, et al; 2011).

Figure 47 sketch design of the ventilated facade system


According to (Cristina Sanjuan, et al; 2011) in their literature review, the movement of air inside

the sealed ventilated façade is homogenous and continuous, while it is more complex in open joint

ventilated façade. The heat flow through the sealed joints and the open joints is different and in order to

analyze it, it is important to understand the heat transfer mode in each layer separately: the exterior layer,

air gap layer, and the inner wall.

In the exterior layer, the solar radiation hit the exterior cladding causing its surface temperature to

raise. The cladding will reflect some of the solar radiation depending on the type of material. Some the

radiation might be reradiated to an adjacent surface that has a lower surface temperature. The cladding

might also lose some of the heat by convection to the atmosphere of there is a temperature difference

(Cristina Sanjuan, et al; 2011).

In the interior of the air gap, heat transfer by radiation from one end to the other due to the

temperature difference between the exterior layer and the interior layer. In addition, convection heat

transfer also occurs when the excessive heat from the inner layer of the exterior layer release the heat to

the air cavity. The air passes through the air cavity could influence this temperature. If the air passes

through the inlet is positive, its temperature increases (i.e. intensifies as the airflow reaches the outlet). If

the air that goes through the inlet is negative, the air that exits the outlet will lower in the globe air

temperature (i.e. decreasing the intensity of the outlet airflow). A steady state is reached when the inlet

airflow heat is equal to the outlet (Cristina Sanjuan, et al; 2011).

Figure 48 cross section of the sealed joint cavity wall and the open-joint ventilated facade


Q Solar + Q Refl + Q Rad eL-env + Q Conv eL-env + Q Rad eL-iW + Q Conv eL-gap = 0

Where

Q Solar = solar radiation.

Q Refl = direct reflected solar radiation.

Q Rad eL-env = radiation of the exterior layer to the environment

Q Conv eL-env = convection heat transfer between the external layer and the

environment.

Q Rad eL-iW = radiant heat transfer between the external layer and the inner gap.

Q Conv eL-gap = convective heat transfer between the external layer and the inner gap.

The steady state equation will calculate the heat flow through the external layer and the

environment, and the heat transfer between the external layer and the inner gap (air cavity) (Cristina

Sanjuan, et al; 2011).

The inner wall has also heat transfer phenomenon that are similar to the external layer that is

written in this formula:

Figure 49 the difference in heat transfer process


Q Rad iW -eL + Q Conv iW -gap + Q Rad iW -room + Q Conv iW -room = 0

Q Rad iW –eL = radiative heat transfer between the inner wall and the external layer

Q Conv iW –gap = convective heat transfer between the inner wall and the air gap

Q Rad iW –room = radiative heat transfer between the inner wall and the room

Q Conv iW –room = convective heat transfer between the inner wall and the room

As been said earlier that the main difference between the sealed and open –joint cavity is that in the

sealed joint system the air inside the cavity will develop a convective loop inside the air gap. According

to (Cristina Sanjuan, et al; 2011) the convective heat transfer between the external layer and the air gap

has the same impact compared to the convective heat transfer between the inner wall and the air gap with

opposite signs. It is illustrated in the equation:

- Q Conv eL-gap = Q Conv iW –gap

The mean air temperature in the cavity will be somewhere between the ambient temperature of the

exterior outside air temperature and the inner surface temperatures of both side (Cristina Sanjuan, et al;

2011).

The difference between the sealed joint and open joint is very prominent in two conditions. The first

condition in the summer when the outside radiation and temperature is far higher than the room

temperature, so the heat transfer will be more intense especially the radiative heat transfer. In the situation

is reversed in the winter where the room temperature is higher than the outside air temperature. This will

let the temperature inside the room to transfer to the outside by a much faster rate. Thus, using open-joint

ventilated façade will be very beneficial in the summer, whereas the sealed-joint façade is better at the

winter time when the the heat could be trapped inside the air gap (Cristina Sanjuan, et al; 2011).

The results:

The results were obtained by the mathematical and numerical methods equations and simulation

methods. Two different results were generated from the simulation and calculation. The first was the fluid

dynamics of the air movement inside the air cavity. The second was the thermodynamics behavior that

occurred between the external layer, the air gap, and the internal wall (Cristina Sanjuan, et al; 2011).


The fluid dynamic behavior:

The fluid dynamics inside the open-joint slabs is effected by the solar radiation that hits the

external layer of the slabs. The air velocity increases as the air rises and then it reaches its peak around the

middle portion of the cavity because it escapes through the open joints. According to , the increase in air

velocity in the middle portion of the air cavity is explained by the fact that an intake of air is produced in

the middle part of the wall system (Cristina Sanjuan, et al; 2011).

In (figure 52) it shows the difference of air speed in the section profile between the open-joint

ventilated façade and the sealed-joint ventilated façade. It could be noticed that the air speed in the open-

joint façade has higher air velocity than the sealed-joint façade. In addition the ascension of air in the

open joint façade is in the whole width of air cavity and it does not form the convective loop that occurs

in the sealed-joint façade (Cristina Sanjuan, et al; 2011).

Figure 50 horizontal average velocity magnitude and z velocity component, at the middle vertical plane in the open-joint ventilated facade, at summer condition (24 C room, 30 C exterior, 400 W/m absorbed solar radiation)


Figure 51 air flow velocity at mid-height comparison between open-joint ventilated facade and sealed cavity facade


The thermal behavior analysis:

The results have shown that when the heat enters the open-joint system, it temperature is lower

than the two layer of the wall system. Thus, the cooler air will absorb the heat from both sides of the

cavity. The process could be reversed if the air entering the cavity was higher than the two layers. It will

losses the heat to the adjacent layers and exits the cavity with cooler air. In the sealed joint however, the

convection loop increase the convective heat transfer between the external layer and the inner wall.

(Figure 54) shows the temperature of air inside the cavity for both systems. Even though the numbers

might seem very for the sealed joint, it represent the heat that is been trapped inside the cavity due to the

solar radiation (Cristina Sanjuan, et al; 2011).

In (figure 55) it shows the heat flux that goes through the room from the inner wall. In the open-

joint case, the heat flux increases as the air ascend through the wall. However, in the sealed-joint façade,

the heat flux is almost uniformed through the height of the wall (Cristina Sanjuan, et al; 2011).

Figure 53 heat flux to room comparison

Figure 52 cavity temperature comparison


The evaluation of the energy consumption of open-joint ventilated façade compared to the

sealed joint ventilated façade:

The performance of open-joint ventilated façade is very prominent in the south façade at summer

times. It performs 26% better than the sealed joint faced in cooling load needed. However, in winter

times, the open joint façade system behaves very poorly. The heat loss during a typical winter is 50%

more than the sealed joint façade which make the open-joint ventilated faced performs better in the

summer and not so much in the winter because it main strategy is to loss heat and not restore heat.

Figure 54 energy performance of the two wall system


Chapter 5: Case study:

This article written by professor Chalfoun explains the benefit of using ventilated cavity to reduce

the heat gain on the building envelope. Some of the benefits that are addressed in this paper is the shading

effect on the inner layer of the ventilated cavity. The second benefit addressed in this paper is the

blackbody radiation. This research takes place in southern Arizona. This region is a hot arid region that is

hot during the summer times and with mild cold winters. In that region, building envelopes experience a

phenomenon known as the blackbody radiation, which is the process of releasing the stored heat during

the day and release it during nighttime. Third benefit of using ventilated cavity system is the loss of heat

from these building by convection (Nader Chalfoun, 2012).

Project description:

The project started in 2005. It is a joint effort between the University of Arizona college of

Architecture and Landscape Architecture through the Drachman institute, and the City of Tucson. The

project was aim to study the possibility of building five residential building and incorporating energy

efficient strategies into these buildings. Professor Chalfoun was designing the DDBC2 building that

included the ventilated cavity on the south side of the house (Nader Chalfoun, 2012).

The house is a three-bedroom home with 1,072 ft². Three of the five home were built with light-

gauge steel framing, concrete masonry unit (CMU), and mud adobe construction respectively. The subject

building is a wood frame house. The house has a long east west axis, which is the best orientation for

building in hot climate (Nader Chalfoun, 2012).

Figure 55 Barrio San Antonio site of the project in Tucson (Chalfoun, 2012)


The roof is design as a heat regulator and it consist of a ventilated space and with a tilt angel in

order to ease the heat out of the house. In addition, the south facing wall has an air cavity of 5’’.

Figure 56 floor plan of the DDBC2 wood farm house (Chalfoun, 2012)

Figure 57 west-east section longitudinal (Chalfoun, 2012)

Figure 58 picture of the cavity wall from the south west corner (Chalfoun, 2012)


To analyze the climate condition in and out of the residence, special instruments were used to

analyze the thermal heat loss, heat gain, and the climate condition. A central weather station was used and

mounted on the highest point in the residence (Nader Chalfoun, 2012).

The central weather station is important in collecting eight different parameters in the location

which are dry-bulb temperature, wet-bulb temperature, wind speed, wind direction, relative humidity,

gust, and global solar radiation. The inside temperature were monitored using the HOBO U30 GSM

cellular Data Logger. The devise support 15 channels of data loggers sensors that were distributed

through the interior of the residence. The sensors where measuring:

Bedroom and living room air and surface temperatures

• Living room mean radiant temperature

• Roof surface temperature

• Cavity wall surface temperature

• Cavity inlet and outlet air temperature

• Cavity air‐movement

Figure 59 position of sensors inside the house (Chalfoun, 2012)


Data analysis:

The weather station has provided the needed information about the thermodynamics features of

the moist air. The data collected from the instruments have confirmed the anticipated results of the

experiment.

1- The air temperature inside the air cavity was reduced by 40 % of from the outside

temperature (Nader Chalfoun, 2012).

2- The exterior layer of the cladding had a higher Sol-air temperature which triggers the effect

of the convective loop inside the cavity. This could result in more heat loos (Nader Chalfoun,

2012).

The author (Nader Chalfoun, 2012) has noticed the difference of the temperature that has been

registered on the inlet and the outlet of the cavity wall. In August 21, the inlet temperature was noticed to

be in the average of 12° F lower than the outlet. This suggest that while the air inters through the inlet of

the cavity wall, the air pick up heat from the surfaces adjacent to the air cavity on both sides and from the

hot already residing from the heat gained from increased Sol-air temperature of the exterior layer that has

been mentioned previously. This along with the black body radiation effect during the night will improve

the conditions of the wall significantly (Nader Chalfoun, 2012).

Figure 60 thermodynamics properties of the moist air (Chalfoun, 2012)


To further explain the phenomenon that occurs inside the air cavity, a chart is represented by

(figure00) illustrate the changes in air speed and sun radiation effect on the air cavity temperature.

According to Professor Nader Chalfoun, when the sun starts to hot the façade of the building, it raises the

temperature of that surface. This rise in temperature will induce a convective loop inside the cavity. Thus,

when the sunsets and the heat of the sun radiation is gone, the temperature of exterior layer of the cavity

starts to drop causing the air inside the cavity to be still.

Figure 61 performance of the cavity wall (Chalfoun, 2012)

Figure 62 cavity convection loop as it is triggered by the solar radiation (Chalfoun, 2012)


Figure 63 energy simulation results by using Energy10 software (Chalfoun, 2012)


Third section of the thesis:

Chapter 6: The experiment assembly

The experiment took place in Tucson Arizona at the heart of the University of Arizona. The

location of the test chamber took place at the rooftop of the College of Architecture Planning and

Landscape architecture (CAPLA). The experiment started at October 20th of the year 2015. The air

temperature at that time was not considered as high as in the summer. The dry-bulb temperature ranges

from as low as 8° C to as high as 32° C. At that time, it does not represent the same condition as in the

summer. However, the presence of the cool roof has increased the solar radiation dramatically. This might

add to the original radiation from the sun.

Figure 64 CAPLA building at the University of Arizona where the test took place, the university of Arizona website.


Test location

Figure 66 the top view of the CAPLA building, Google earth image

Figure 65 isometric view of the test location, Goggle earth image


The model is consist of two layers of envelopes. The first layer is made out of ¼ of an inch of

concrete board. The reason for chosen concrete board was to test the effect of thermal mass in the inner

layer of the model. The second layer is made of 5/8 plywood. The plywood is installed after the concrete

board. Between the concrete board and the plywood, there is an air gap of two inch wide. The gap

between the plywood and the concrete is made with the use of a stud wood with a thickness of two inch.


The surface temperatures where measured with the use of a thermocouple instrument that has a

four channel, K-type thermocouples. The sensors were arranged to measure the inlet air temperature and

the outlet air temperature. The temperature that is taken from the inlet and the temperature that will be

taken from the out let will explain the heat gain of the air that passes through the cavity. The other two

sensors are placed on both sides of the concrete board to assess the effect of thermal mass. Secondly is

the HOBO onset dry-bulb and relative humidity sensors and data loggers. The HOBO sensor will be used

to measure the ambient dry-bulb temperature and relative humidity inside the test chamber. The reason

for measuring the ambient air temperature and humidity inside the test chamber is to calibrate the heat

gain inside the test chamber in order to the performance of the test chamber. Lastly, is the climate station

that is placed on top of the test chamber. This climate station will record the hourly ambient air

temperature and humidity of the air outside the test chamber.


3-D views of the model:


Pictures from the test location and position of the sensors:


The instruments used in the experiment:

1- AZ Instruments Digital 4 Channels K Type Thermocouple SD Card Temperature

Logger

Thin instrument is used to measure the surface temoerature it has four channles to measure four

different surfaces, and it is also equiped with data logger that will register the information on the CD card.

The first probe will be installed on the ouer surface, the second will be installed on onner surface of the

cavity wall, the third will be installed on the inner most surface of the chmaber, and the fourth will be

installed of the inside surface of the tilted roof.


2- Onset HOBO UX100-003 Temperature/Relative Humidity Data Logger.

This instrument is suitable to measure the ambient temperature and the ambient humidity it is

equipped with a data logger to register the diurnal temperature and humidity information. Two of these

instruments needed to be installed. The first one will be installed outside to measure the ambient

temperature and humidity of the outside air, and the other one will be installed inside the test chamber to

measure the ambient temperature and humidity.


3- AcuRite 00589 Pro Color Weather Station with Wind Speed,

Temperature and Humidity.

This instrument is called the weather station. It is very similar to the one that is used in the environmental

laboratory. It will be used to measure the temperature, humidity, and wind speed.


Chapter 7: The result of the experiment:

The result for the test chamber is plotted on the subsequent charts. The starting day was

November 9th 2015. Although the ambient air temperature is not the same as in the summer period;

however, the reflected roof (cool roof have made an impact on the microclimate temperature of the roof.

In order to understand the results, five days have been chosen in this thesis to represent the final findings.

The ambient air temperature during the five days of testing shows temperatures that range from

35- 4 °C. from the general perspective, the temperature on the 9th, 12th, and 13th are relatively higher than

the 10th and the 11th.

The ambient air temperature (°C) (11/9/2015 – 11/13/2015):

0

5

10

15

20

25

30

35

Tem

pe

ratu

re (

°C)

Time (Hours)

the dry builb temperature (°C) on november 9, 2015


0

5

10

15

20

25

30

Tem

pe

ratu

r (°

C)

Time (hours)


0

5

10

15

20

25

30

Tem

per

atu

re (

°C)

Time (hours)



The combined result

0

5

10

15

20

25

30

35

40

12

:00

AM

4:0

0 A

M

8:0

0 A

M

12

:00

PM

4:0

0 P

M

8:0

0 P

M

12

:00

AM

4:0

0 A

M

8:0

0 A

M

12

:00

PM

4:0

0 P

M

8:0

0 P

M

12

:00

AM

4:0

0 A

M

8:0

0 A

M

12

:00

PM

4:0

0 P

M

8:0

0 P

M

12

:00

AM

4:0

0 A

M

8:0

0 A

M

12

:00

PM

4:0

0 P

M

8:0

0 P

M

12

:00

AM

4:0

0 A

M

8:0

0 A

M

12

:00

PM

4:0

0 P

M

8:0

0 P

M

Tem

per

atu

re (

°C)

Time (hours)

combine result from novenber 9- 13, 2015


Surface temperatures from (11/9/2015 – 11/13/2015):

Surface temperature are close to each other in magnitude. The reason behind the close range of

temperature is the thickness of the material that is thin in which it makes the material gain and loss heat

more rapidly than thick materials. However, even with close approximation of temperature range, it

possible to discern some characteristics of the material assembly. It is noticed that the temperature of

sensor 1 is lower than sensor 2. Sensor 1 represent the inlet air temperature. Sensor 2 represent the

temperature that higher in the air cavity (midway). Sensors 3 and 4 are attached on both sides of the

concrete board. Sensor 3 and 4 are close to each other. Sensor 3 is slightly higher than sensor 4 due to

close proximity of sensor 3 to the air cavity.

0

5

10

15

20

25

30

35

Tem

per

atu

re (

°C)

Time (hours)

surface temperatures on november 9, 2015

ambient air temperature 1 ambient air temperature 2

surface temperature 1 surface temperature 2


0

5

10

15

20

25

30

35

40

Tem

per

atu

re (

°C)

Time (hours)




0

5

10

15

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25

30

35

40

Tem

per

atu

re (

°C)

Time (hours)





0

5

10

15

20

25

30

Tem

per

atu

re (

°C)

time (hours)




0

5

10

15

20

25

30

Tem

per

atu

re (

°C)

Time (hours)


ambient temperature 1 ambient temperature 2



Combined surface temperature result from November 9 – 13, 2015:

0

5

10

15

20

25

30

35

40

0:1

1:5

3

4:1

1:5

3

8:1

1:5

3

12

:11

:53

4:1

1:5

3

8:1

1:5

3

0:1

1:5

3

4:1

1:5

3

8:1

1:5

3

12

:11

:53

4:1

1:5

3

8:1

1:5

3

0:1

1:5

3

4:1

1:5

3

8:1

1:5

3

12

:11

:53

4:1

1:5

3

8:1

1:5

3

12

:11

:53

4:1

1:5

3

8:1

1:5

3

12

:11

:53

4:1

1:5

3

8:1

1:5

3

12

:11

:53

4:1

1:5

3

8:1

1:5

3

12

:11

:53

4:1

1:5

3

8:1

1:5

3

12

:11

:53

Tem

per

atu

re (

°C)

Time(hours)

combine surface temperatures from november 9 - 13, 2015

ambient temperature intering the caviy ambient temperature in the middle of the cavity



Interior and exterior ambient temperature from 11/9/2015 – 11/13/2015:

The difference between the outer temperature is lower than the interior temperature during time

period from 12:00 AM until 8:00 AM. After that from 9:00 am until 1:00 PM it noticed that the interior

temperature is lower than the exterior temperature. Lastly from 8:00 PM until 12:00 AM the temperature

inside the chamber get higher than the exterior temperature.

0

5

10

15

20

25

30

35

tem

per

atu

res

in (

°C)

Time(hours)

exterior and interior temperature 11/9/2015

exterior temperature interior temperature


0

5

10

15

20

25

30

12

:00

AM

1:0

0 A

M

2:0

0 A

M

3:0

0 A

M

4:0

0 A

M

5:0

0 A

M

6:0

0 A

M

7:0

0 A

M

8:0

0 A

M

9:0

0 A

M

10

:00

AM

11

:00

AM

12

:00

PM

1:0

0 P

M

2:0

0 P

M

3:0

0 P

M

4:0

0 P

M

5:0

0 P

M

6:0

0 P

M

7:0

0 P

M

8:0

0 P

M

9:0

0 P

M

10

:00

PM

11

:00

PM

12

:00

AM

tem

per

atu

res

in (

°C)

Time (hours)



0

5

10

15

20

25

301

2:0

0 A

M

1:0

0 A

M

2:0

0 A

M

3:0

0 A

M

4:0

0 A

M

5:0

0 A

M

6:0

0 A

M

7:0

0 A

M

8:0

0 A

M

9:0

0 A

M

10

:00

AM

11

:00

AM

12

:00

PM

1:0

0 P

M

2:0

0 P

M

3:0

0 P

M

4:0

0 P

M

5:0

0 P

M

6:0

0 P

M

7:0

0 P

M

8:0

0 P

M

9:0

0 P

M

10

:00

PM

11

:00

PM

12

:00

AM

tem

per

atu

res

in (

°C)

Time (hours)




0

5

10

15

20

25

30

35

12

:00

AM

1:0

0 A

M

2:0

0 A

M

3:0

0 A

M

4:0

0 A

M

5:0

0 A

M

6:0

0 A

M

7:0

0 A

M

8:0

0 A

M

9:0

0 A

M

10

:00

AM

11

:00

AM

12

:00

PM

1:0

0 P

M

2:0

0 P

M

3:0

0 P

M

4:0

0 P

M

5:0

0 P

M

6:0

0 P

M

7:0

0 P

M

8:0

0 P

M

9:0

0 P

M

10

:00

PM

11

:00

PM

12

:00

AM

1:0

0 A

M

tem

per

atu

res

in (

°C)

Time (hours)



0

5

10

15

20

25

30

35

40

tem

per

atu

res

in (

°C)

Time (hours)




Most of the readings in the five days have the same pattern except November 9th. The most

prevalent phenomenon is that at the beginning of the day (12 O’clock AM) the interior temperature is

slightly higher than the outside temperature. The temperature inside the test chamber is slightly higher

because of the thermal mass which emits the heat that has been absorbed during the day. At the peak hour

(3 O’clock PM), the temperature outside the test chamber is higher than inside the test chamber.

0

5

10

15

20

25

30

35

401 5 9

13

17

21

25

29

33

37

41

45

49

53

57

61

65

69

73

77

81

85

89

93

97

10

1

10

5

10

9

11

3

11

7

12

1

tem

per

atu

res

in (

°C)

Time (Hours)

combained results (exterior & interior ) from november 9 to 13 2015

exterior dry buld temperature interior dry bulb temperature


Discussion:

The design of the ventilated envelope was intended to mitigate the solar radiation by creating a

second layer that encloses an air gap which could be used to restrict the passage of conductive heat

through the air gap. While this might be true, it was noticed that air cavity temperature might increase.

There are many elements that have contributed to the increase in the air gap temperature. The first

element was the release of heat from both sides of the air cavity to the air cavity. When the air passes

through the inlet that is located at the lower part of the cladding, its temperature is colder than outside

temperature due to the fact that the air pressure difference between the air cavity and the outside drives

the air inside the cavity in a fast rate. The solar radiation hits the cladding during the morning hours which

raises its temperature. When the cladding is fully heated, it releases the heat to the air cavity by

conduction. Additionally, when the air rises up in the air cavity it absorbs the heat from the adjacent

surfaces by convection. This rise in temperature is very concerning due to the fact that if the air gap

temperature is much higher than the inner envelope, it transfers the heat from the air cavity to the interior

space. In order to insure that the temperature inside the air is kept at a lower temperature, it is very

important to insure that the air inside the cavity is constantly evacuated from the cavity. According to

previous studies that have been cited previously, it is possible to reduce the air temperature inside the air

cavity by adding open joints system to the cladding.

The concrete board was intentionally used for its thermal mass capabilities. The thickness of the

concrete board was only a ¼ of an inch. This thickness might not be as effective as it should be due to the

size of the model. Thus when applying the thermal mass layer (the inner envelope), it advised to use

thicker material that is proportionately appropriate to the actual scale. The thickness of the materials

should be carefully measure for their time lag, because any additional thickness is not necessarily better.

According to a previous study cited, the effective thickness of thermal mass material is 4’’ thick. Any

more than 4’’ did not perform significantly better .During the morning hours the inner envelope is

relatively cooler than the plywood. During the night, the heat absorbed by the concrete board is released

to the air cavity. This shows that the thermal mass was working effectively during the nighttime.

Conclusion:

From the result of this experiment, it shows that the ventilated envelope succeeded in achieving

lower levels of heat gain inside the test chamber. The air temperature inside the air cavity was not as it

was hypothesized. The air cavity layer reached higher temperatures than the two layers adjacent. This

strategy has proven to be very effective in hot climates because it allows the heat that is absorbed by the

material from the solar radiation to be released to the air cavity. Otherwise, the only way to combat the

heat is by storing it inside the building material either by thermal mass or by thermal insulation. The use

of thermal insulation might be very effective in buildings that use HVAC systems as the main source of

thermal comfort. However, when considering a building that is primarily passive, it is better to have a

design that is flexible with constant changes in the climate. Avoiding heat storage is one of the main

benefits of using ventilated envelope because it releases the heat as soon as it is absorbed. There are some

solutions to deal with excess of heat gain inside the cavity. Some of these strategies involve having a

thicker cladding with thermal insulation. A second strategy is to make more openings on the outer

envelope so that the heated air could be released to the outside instead of remaining inside the cavity for a

long time.

10

0 December 14, 2015

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1 December 14, 2015

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2 December 14, 2015

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