Almond Tree Effect Thermal Design 061231b

8/22/2019 Almond Tree Effect Thermal Design 061231b

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Chapter Five

Thermal Design

Chapter FiveThermal Design

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1. Introduction.2. Thermal quantities3. Thermal properties of building materials and elements4. Heat flow through buildings5. Periodic heat flow6. Required thermal performance for building elements7. Condensation8. Tests and Exercises.9. References.

1. Introduction

Thermal design of buildings for thermal comfort requires a knowledge of thermal quantities involvedin heat flow, especially as a preliminary to understanding the basic thermal properties of buildingmaterials and elements: air-to-air transmittance, solar gain factor, time lag and admittance. Thermalbalance of buildings requires that heat flow be equal to heat gain. Periodic heat flow calculationsassume non-steady state conditions as opposed to steady state conditions. The required thermalperformance of walls and roofs are established by codes for different climatic zones - hot dry

climates, warm humid climates, cold climates and composite climates. The design of floors in thevarious climates requires special attention both for moderation of indoor temperatures and footcomfort. The problem of surface and interstitial condensation can be solved by good thermal design.

Thermal designof buildings for thermal comfort requires a knowledgeof thermal quantities involvedin heat flow, especially as a preliminary to understanding the basic thermal properties of buildingmaterials and elements: air-to-air transmittance, solar gain factor, time lag and admittance. Thermalbalance of buildings requires that heat flow be equal to heat gain. Periodic heat flow calculationsassume non-steady state conditions as opposed to steady state conditions. The required thermalperformance of walls and roofs are established by codes for different climatic zones - hot dryclimates, warm humid climates, cold climates and composite climates. The design of floors in thevarious climates requires special attention both for moderation of indoor temperatures and footcomfort.Theproblemof surface andinterstitialcondensation canbesolvedbygood thermal design.


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2. Thermal Quantities

There are several thermal quantities used indiscussions about heat flow through buildings.An understanding of these quantities and theirunits of measurement is necessary for thermal

design which aims at utilizing thermal propertiesof building elements to achieve and maintaincomfort in buildings.

This is an indication of the thermal state of a

body and it is measured in degrees Celsius ( C)

or degrees Kelvin ( K).

This is a form of energy measured in Joules (J).

Specific heat of a substance is the amount ofheat energy necessary to cause unit temperatureincrease of a unit mass of the substance. It ismeasured in J/kg deg C.

Thermal capacity of a body is the amount of heatrequired to raise the temperature of the body byone unit. It is measured in J/deg C.

This is the ability to carry out a certain work inunit time -measured in Watts (W), that is J/S.

Thermal conductivity of a material is the rate ofheat flow through a unit area of unit thickness ofthe material for a unit temperature differenceacross the material. It is also known as the K-value and is measured in W/m deg C. Goodinsulators have lower thermal conductivities.

This is the rate of heat flow through a unit area ofa body when the temperature differencebetween the two surfaces is one degree Celsius.It is measured in W/m2 deg C.

This is the reciprocal of thermal conductivity. It ismeasured in m deg C/W. Good insulators havehigh thermal resistivities.

Thermal resistance: is the reciprocal of thermalconductance.

Temperature:

Heat:

Specific heat:

Thermal Capacity:

Power:

Thermal conductivity

Thermal Conductance:

Thermal resistivity:

Thermal resistance

There are several thermal quantities used indiscussions about heat flow through buildings.An understanding of these quantities and theirunits of measurement is necessary for thermaldesignwhich aims at utilizing thermal propertiesof building elements to achieve and maintaincomfort in buildings.This is an indication of the thermal state of abody and it is measured in degrees Celsius ( C)ordegrees Kelvin ( K).This is a formofenergymeasured in Joules (J).Specific heat of a substance is the amount ofheat energynecessaryto cause unit temperatureincrease of a unit mass of the substance. It ismeasured in J/kg degC.Thermal capacityofa body is the amountofheatrequired to raise the temperature of the bodybyone unit. It ismeasured in J/degC.This is the ability to carry out a certain work inunit time -measured inWatts (W), that is J/S.Thermal conductivity of a material is the rate ofheat flow through a unit area of unit thickness ofthe material for a unit temperature differenceacross the material. It is also known as the K-value and is measured in W/m deg C. Goodinsulators have lower thermal conductivities.This is the rateof heat flow througha unit areaofa body when the temperature differencebetween the two surfaces is one degree Celsius.It ismeasured inW/m2degC.This is the reciprocal of thermal conductivity. It ismeasured in m deg C/W. Good insulators havehigh thermal resistivities.Thermal resistance: is the reciprocal of thermalconductance.

Temperature:

Heat:Specific heat:

Thermal Capacity:

Power:

Thermal conductivity

Thermal Conductance:

Thermal resistivity:

Thermal resistance

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Surface resistance and conductance:

Air-to-air resistance

Cavity resistance and conductance:

Absorptivity:

Sol-air temperature:

Surface resistance refers to the resistanceoffered to heat flow by the surface of a body, asdifferent from the resistance offered by the bodyitself. The surface conductance is the reciprocalof surface resistance. The units are the same as

for thermal resistance and conductance.

Air-to-air resistance is the sum of the resistance ofthe body and the internal and external surfaceresistances.

Where:

Ra = air-to-air resistance.

Rsi = internal surface resistance.R = body resistance.

R = external surface resistance.

The unit of measurement is the same as forresistance.

Cavity resistance is the resistance offered to heatflow by a cavity enclosed within a body. Thereciprocal is cavity conductance.

This is the property of a surface whichdetermines what proportion of incidentradiation it absorbs.

Combines the heating effect of radiationincident on a building with the effect of warm air.It is measured in degrees Celsius.

Ts = To + (I x a)/ Fo

Where:

Ts = sol-air temperature.To = outside air temperature.I = radiation intensity.a = absorbance of the surface.Fo = outside surface conductance

Heat transmission and absorption by buildingmaterials is affected by the absorptivity, theconductivity and thermal capacity of the

Ra = Rsi + Rb + Rso

b

so

.3. Thermal Properties of Building

Materials and Elements

Surface resistance and conductance:

Air-to-air resistance

Cavity resistance and conductance:

Absorptivity:

Sol-air temperature:

Surface resistance refers to the resistanceoffered to heat flow by the surface of a body, asdifferent from the resistanceoffered by the bodyitself. The surface conductance is the reciprocalof surface resistance. The units are the same asfor thermal resistance andconductance.Air-to-air resistance is thesumof the resistance ofthe body and the internal and external surfaceresistances.

Where:Ra = air-to-air resistance.Rsi = internal surface resistance.R = body resistance.R =external surface resistance.The unit of measurement is the same as forresistance.

Cavity resistance is the resistanceoffered to heatflow by a cavity enclosed within a body. Thereciprocal is cavity conductance.This is the property of a surface whichdetermines what proportion of incidentradiationit absorbs.Combines the heating effect of radiationincidentona buildingwith the effectofwarmair.It ismeasured indegreesCelsius.

Ts = To + (I x a)/ FoWhere:

Ts = sol-air temperature.To=outside air temperature.I = radiationintensity.a = absorbanceof the surface.Fo=outside surface conductance

Heat transmission and absorption by buildingmaterials is affected by the absorptivity, theconductivity and thermal capacity of the

Ra = Rsi + Rb + Rso

bso

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materials. These properties of materialsdetermine the characteristics of wall and roofelements and therefore the way they will modifythe thermal environment.

Building elements possess four characteristics

which affect the internal conditions -the air-to-airtransmittance (U-value), the solar gain factor, thetime lag and the admittance.

This is the reciprocal of air-to-air resistance. It iscommonly known as U-value and measured inthe same unit as conductance. It is defined as therate at which heat is transmitted from the air onone side of a wall or roof to the air on the otherside.

This is the rate of heat flow through aconstruction due to solar radiation expressed asa fraction of the incident solar radiation.

This is the time delay between the impact of thediurnal variation of temperature and radiation onthe external surface, and the resultanttemperature variation on the internal surface.See figure 1.

Admittance of a surface is the rate at which thesurface absorbs or emits heat from or to the airwhen the air temperature is different from thetemperature of the surface.

The total heat gained by a building must be lostin order to maintain a thermal balance. Anexcess heat gain will result in a constant rise intemperature of the building while an excess heatloss will cause a fall in temperature.

Buildings gain heat by conduction through thewalls, by insolation through windows, internallyfrom occupants and appliances, by naturalventilation and from heating equipment.

Buildings lose heat by conduction, evaporation,natural ventilation and through mechanicalcooling aids.

Air-To-Air Transmittance:

Solar Gain Factor:

Time Lag:

Admittance:

Heat Gains:

Heat Losses:

4. Heat Flow through Buildings

materials. These properties of materialsdetermine the characteristics of wall and roofelements and therefore the way theywill modifythe thermal environment.Building elements possess four characteristicswhich affect the internal conditions -the air-to-airtransmittance (U-value), the solar gain factor, thetime lagandtheadmittance.This is the reciprocal of air-to-air resistance. It iscommonly known as U-value and measured inthe sameunit as conductance. It is defined as therate at which heat is transmitted from the air onone side of a wall or roof to the air on the otherside.This is the rate of heat flow through aconstruction due to solar radiation expressed asa fractionof the incident solar radiation.This is the time delay between the impact of thediurnal variationof temperature andradiationonthe external surface, and the resultanttemperature variation on the internal surface.See figure1.Admittance of a surface is the rate at which thesurface absorbs or emits heat from or to the airwhen the air temperature is different from thetemperatureof thesurface.

The total heat gained by a building must be lostin order to maintain a thermal balance. Anexcess heat gain will result in a constant rise intemperature of the buildingwhile an excess heatlosswill cause a fall in temperature.Buildings gain heat by conduction through thewalls, by insolation through windows, internallyfrom occupants and appliances, by naturalventilation andfromheatingequipment.

Buildings lose heat by conduction, evaporation,natural ventilation and through mechanicalcoolingaids.

Air-To-Air Transmittance:

Solar Gain Factor:

Time Lag:

Admittance:

Heat Gains:

Heat Losses:

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Figure 1: Time lag and decrement factor.


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In a state of equilibrium therefore, the heat loss isequal to the heat gain. See figure 2. We cancalculate one of these loads given the othersfrom .

We can calculate, for example, the amount ofmechanical cooling or heating required in anexisting or freshly designed building. We mayalso find out how much insolation is needed toheat a solar house with no auxiliary heatingfrom this equation. In heat gain or heat losscalculations the various sources of loading areconsidered individually.

This is usually calculated for walls of a given areaand is the product of the surface area, thetransmittance value and the temperaturedifference between the exterior and the interior.

This refers to heat loss or heat gain through theexchange of air between the building and theopen air and it covers infiltration as well asnatural and forced ventilation. The rate ofventilation heat flow is the product of thevolumetric specific heat of air, the ventilationrate and the temperature difference. See the

equation below.Qv = 1300 x U x T

Conduction:

Convection:

D

the equation below

Qi + Qs + Qc + Qv + Qm + Qe

Where:

Qi = internal heat gain rate (W).Qs = solar heat gain rate (W).Qc = rate of heat loss or heat gain by

conduction (W).Qv = rate of heat loss or heat gain by

ventilation (W).Qm = mechanical heat gain or loss rate

(W).Qe = rate of heat loss by evaporation (W).

See the equation below.

Qc = A x U x T

Where:

Qc = rate of heat loss or heat gain byconduction (W).

A = surface area (m2).

U = transmitance value (W/m2 degC).T = temperature difference.

D

D

In a state of equilibrium therefore, the heat loss isequal to the heat gain. See figure 2. We cancalculate one of these loads given the othersfrom .

We can calculate, for example, the amount ofmechanical cooling or heating required in anexisting or freshly designed building. We mayalso find out how much insolation is needed toheat a solar house with no auxiliary heatingfrom this equation. In heat gain or heat losscalculations the various sources of loading areconsidered individually.This is usually calculated for walls of a given areaand is the product of the surface area, thetransmittance value and the temperaturedifference between the exterior and the interior.

This refers to heat loss or heat gain through theexchange of air between the building and theopen air and it covers infiltration as well asnatural and forced ventilation. The rate ofventilation heat flow is the product of thevolumetric specific heat of air, the ventilationrate and the temperature difference. See theequationbelow.Qv = 1300 x U x T

theequationbelowQi + Qs + Qc + Qv + Qm + Qe

Where:Qi = internal heat gain rate (W).Qs = solar heat gain rate (W).Qc = rate of heat loss or heat gain byconduction (W).Qv = rate of heat loss or heat gain byventilation (W).Qm = mechanical heat gain or loss rate(W).Qe = rate of heat loss by evaporation (W).

Seetheequationbelow.Qc = A x U x T

Where:Qc = rate of heat loss or heat gain byconduction (W).A = surface area (m2).U = transmitance value (W/m2 degC).T = temperature difference.

Conduction:

Convection:

D

D

D5

Figure 2: Thermal balance of buildings.


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Where:

Qv = rate of heat loss or heat gain byventilation (W).

U = transmitance value (W/m2 degC).

T = temperature difference.

The glass in windows acts as a filter and its typeand quality reduces the solar heat gain by thesolar gain factor. The actual solar gain will thenbe a product of the area of the window, theintensity of solar radiation and the solar gainfactor. See the equation below.

Qs = A x I x

Where:

Short-wave radiation incident on glass is partlyr e fl e cte d, pa rt l y a bs or be d b ut m ai nl ytransmitted. This is because glass is "transparent"to short-wave radiation. Other materials, such asconcrete or mud however absorb the larger

portion of short-wave radiation. The absorbedenergy causes a rise in their temperature and thisenergy is emitted in the form of long-waveradiation. Glass is "opaque" to long-waveradiation and if it encloses the emitter the heat istrapped within the enclosure. This leads to a risein temperature within the enclosure known asthe greenhouse effect. See figure 4.

The internal heat gains are made up of the heat

output of the occupants, and the lamps andmotors, if any, in the enclosure. The human bodyproduces about 70 W while sleeping and amaximum of about 1100 W. Sedentary activityproduces about 140 W. The lamps and motorsusually have their wattage marked. The totalinternal heat gain is the sum of heat productionby the individual persons or equipment. See theequation below.

Where:

D

F

Solar Gains:

The Greenhouse Effect:

Internal Heat Gains:

n1xq1+ n2xq2 + ..... nnxq

Qs = solar heat gain rate (W).A = surface area (m2).I = radiation heat flow density (W/m2).

= solar gain factor of window glass.

Qi =

Qi = internal heat gain rate (W).

F

Where:Qv = rate of heat loss or heat gain byventilation (W).U = transmitance value (W/m2 degC).T = temperature difference.

The glass in windows acts as a filter and its typeand quality reduces the solar heat gain by thesolar gain factor. The actual solar gain will thenbe a product of the area of the window, theintensity of solar radiation and the solar gainfactor. Seetheequationbelow.Qs = A x I x

Where:

Short-wave radiation incident on glass is partlyreflected, partly absorbed but mainlytransmitted. This is because glass is "transparent"to short-wave radiation. Other materials, such asconcrete or mud however absorb the largerportion of short-wave radiation. The absorbedenergycauses a rise in their temperature and thisenergy is emitted in the form of long-waveradiation. Glass is "opaque" to long-waveradiation and if it encloses the emitter the heat istrapped within the enclosure. This leads to a risein temperature within the enclosure known asthegreenhouse effect. Seefigure4.The internal heat gains are made up of the heatoutput of the occupants, and the lamps andmotors, if any, in the enclosure. The human bodyproduces about 70 W while sleeping and amaximum of about 1100 W. Sedentary activityproduces about 140 W. The lamps and motorsusually have their wattage marked. The totalinternal heat gain is the sum of heat productionby the individual persons or equipment. See theequationbelow.

Where:

D

F

Solar Gains:

The Greenhouse Effect:

Internal Heat Gains:

Qs = solar heat gain rate (W).A = surface area (m2).I = radiation heat flow density (W/m2).= solar gain factor of window glass.

Qi = n1xq1+ n2xq2 + ..... nnxq

Qi = internal heat gain rate (W).

F

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Figure 4: The greenhouse effect. The shortwave radiation istransmitted by the glass, absorbed by the concrete massand emitted as longwave radiation.


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N1xq1 = number of persons or equipment.n2xq2 = heat output rate of persons or

equipment.

Mechanical Devices:

Evaporation:

Periodic Heat Flow Calculation

These are usually heaters or air-conditioners andtheir rating is usually indicated.

This refers to heat loss from the interior orexterior of the building by evaporation, forexample from roof ponds or fountains. It isusually ignored except for detailed studies of air-conditioning.

Simple calculations of the thermal balance of abuilding assume that both the indoor andoutdoor temperatures are constant. Thisassumption of steady state conditions areunreliable when there are large fluctuations intemperatures. In such a case, non-steady stateconditions are assumed.

In climates with large diurnal temperature rangesthe variations in temperature are clearly marked.There is a 24-hour cycle of increasing and

decreasing temperatures with the minimumtemperature around 6.00 am and the maximumaround 2.00 pm. The fabric of the buildingabsorbs and stores some of the heat during theday and releases this heat to the interior duringthe night. The variation of temperatures insidethe building also has a fixed cycle but the peakoccurs at a different time depending on the timelag. The cycle is further characterised by thedecrement factor which is the ratio of themaximum outer and inner surface temperature

amplitudes taken from the mean. See figure 1.

Knowledge of the time lag and decrement factorfor different materials can be used to designbuildings capable of reducing heat flow intobuildings during the hot afternoons and usingthe stored energy to heat the building during thecold nights. Such a building will be very usefulduring the cold harmattan but may proveuninhabitable during the hot season. It is

necessary to make calculations of the periodicheat flow through the building for differentseasons of the year to obtain the optimal time lag

5. Periodic Heat Flow

N1xq1 = number of persons or equipment.n2xq2 = heat output rate of persons orequipment.Mechanical Devices:

Evaporation:

Periodic Heat Flow Calculation

These are usually heaters or air-conditioners andtheir rating isusually indicated.This refers to heat loss from the interior orexterior of the building by evaporation, forexample from roof ponds or fountains. It isusually ignored except for detailed studies of air-conditioning.

Simple calculations of the thermal balance of abuilding assume that both the indoor andoutdoor temperatures are constant. Thisassumption of steady state conditions areunreliable when there are large fluctuations intemperatures. In such a case, non-steady stateconditions areassumed.In climateswith largediurnal temperature rangesthe variations in temperature are clearly marked.There is a 24-hour cycle of increasing anddecreasing temperatures with the minimumtemperature around 6.00 am and the maximumaround 2.00 pm. The fabric of the buildingabsorbs and stores some of the heat during theday and releases this heat to the interior duringthe night. The variation of temperatures insidethe building also has a fixed cycle but the peakoccurs at a different timedepending on the timelag. The cycle is further characterised by thedecrement factor which is the ratio of themaximum outer and inner surface temperatureamplitudes taken fromthemean. See figure1.Knowledge of the time lag and decrement factorfor different materials can be used to designbuildings capable of reducing heat flow intobuildings during the hot afternoons and usingthe stored energy to heat the building during thecold nights. Such a building will be very usefulduring the cold harmattan but may proveuninhabitable during the hot season. It isnecessary to make calculations of the periodicheat flow through the building for differentseasons of the year toobtain the optimal time lag

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and decrement factor. The momentary heatflow rate is calculated using the equation below.

= A x U x [(Tm - Ti) + (T - Tm)]

Where:

The required thermal performance of walls androofs are established by codes for differentclimatic zones. The aim of this is to reduceheating costs and reduce the discomfort ofoccupants in case of inadequate heating. Theserecommendations are based on economicanalysis involving the cost of heating and the

cost of building materials. The optimal valuesvary from country to country and are influencedby the climate and living standards.

In Nigeria heating is required only for a fewweeks during the harmattan, especially in thenorthern parts of the country where theharmattan can be very severe. In practice,heating appliances are very rarely installed andpeople resort to other means of keeping warm.Peasants sometimes keep a fire burning in their

rooms and block all apertures to reduceinfiltration. The design of walls and roofs in thisclimate should ensure adequate insulation andthermal capacity.

The major problem in Nigeria is however that ofoverheating. In the warm humid climates foundnear the coast, conditions are uncomfortablyhot during most of the year. In these conditionsthermal storage should be avoided and highinsulation provided. The choice of either air-

conditioning or fans will influence the size andtype of windows used. We shall now considerfour climates and the performance of walls and

m f

6. Required Thermal Performance of

Building Elements

Q

Q = momentaryheatflow rate (W).A = area of wall or roof (m2).U = transmittance(W/m2degC).Tm = daily mean outdoor sol-air

temperature ( C).

Ti = constant indoor temperature ( C).

T = outdoor sol-air temperature hours

earlier( C).

= decrement factor.

= time lag.

f fmf

and decrement factor. The momentary heatflow rate is calculatedusing theequationbelow.= A x U x [(Tm - Ti) + (T - Tm)]

Where:

The required thermal performance of walls androofs are established by codes for differentclimatic zones. The aim of this is to reduceheating costs and reduce the discomfort ofoccupants in case of inadequate heating. Theserecommendations are based on economicanalysis involving the cost of heating and thecost of building materials. The optimal valuesvary fromcountry to country and are influencedby theclimate andlivingstandards.In Nigeria heating is required only for a fewweeks during the harmattan, especially in thenorthern parts of the country where theharmattan can be very severe. In practice,heating appliances are very rarely installed andpeople resort to other means of keeping warm.Peasants sometimes keep a fire burning in theirrooms and block all apertures to reduceinfiltration. The design of walls and roofs in thisclimate should ensure adequate insulation andthermal capacity.The major problem inNigeria is however that ofoverheating. In the warm humid climates foundnear the coast, conditions are uncomfortablyhot duringmost of the year. In these conditionsthermal storage should be avoided and highinsulation provided. The choice of either air-conditioning or fans will influence the size andtype of windows used. We shall now considerfour climates and the performance of walls and

Q

Q= momentary heat flow rate (W).A= areaofwallor roof (m2).U= transmittance (W/m2degC).Tm = daily mean outdoor sol-airtemperature ( C).Ti = constant indoor temperature ( C).T = outdoor sol-air temperature hoursearlier( C).= decrementfactor.= time lag.

m f

f fmf

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roofs required for them. See Table 1.

These climates are characterised by a highdiurnal temperature range and low humiditywith discomfort caused by either high or lowtemperatures. The design of walls and roofsshould therefore moderate temperaturefluctuations. This is achieved by a long time lag of8 to 14 hours for both internal and external walls.See figure 5.

These climates are characterised by a low diurnaltemperature range, high humidity and generallyhigh temperatures. Comfort is achieved by

Hot Dry Climates:

Warm Humid Climates

roofs required for them. See Table 1.These climates are characterised by a highdiurnal temperature range and low humiditywith discomfort caused by either high or lowtemperatures. The design of walls and roofsshould therefore moderate temperaturefluctuations. This is achievedbya longtime lag of8 to14hours for both internal and externalwalls.See figure5.These climates arecharacterised bya lowdiurnaltemperature range, high humidity and generallyhigh temperatures. Comfort is achieved by

Hot Dry Climates:

Warm Humid Climates

Climate Element Condition Max U value Time lag

Rooms for night use: diurnal range 10 C 0.90 8 - 14Rooms for night use: diurnal range 12.5 C 0.85 8 - 14Rooms for night use: diurnal range 15 C 0.80 8 - 14Rooms for night use: diurnal range 17.5 C 0.75 8 - 14Rooms for night use: diurnal range 20 C 0.70 8 - 14Rooms for day use only - 8 - 14

Roofs

Schools and offices - 4 - 7

East wall 1.2 10 - 16

West wall 1.2 6 - 14

North and south walls 1.2 8 - 14

Hot dry

Heavywalls

Walls shaded from direct solar radiation 2.0 8 - 14

Rooms for day and night use 1.1 0 - 3

Roof Rooms for day use only 1.1 0 - 14

West wall 2.0 0 - 5

East, south and north walls 2.0 0 - 5

Warmhumid

Walls

Walls shaded from direct solar radiation 2.8 0 - 14

Rooms for day and night use 0.85 0 - 4Light roofs

Rooms for day use only 0.85 0 - 14

West walls 1.2 0 - 5

East, north and south walls 1.5 0 - 5

Composite

WallsWalls shaded from direct solar radiation 2.0 0 - 14

Table 1: Recommended thermal properties for walls and roofs.


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ventilation and by restricting the flow of heat intothe building. To achieve this a short time lag, lowthermal capacity, high insulation and reflectiveroofs are used. See figure 6.

Cold climates are distinguished by low airtemperatures. The design of walls and roofsshould therefore prevent loss of heat. Anadditional problem is condensation which canform on internal surfaces if temperatures areallowed to drop too low. The preservation ofheat is achieved through high insulation andauxiliary heating in severe cases. The insulationprovided must prevent condensation on theinternal surfaces.

Cold Climates

ventilation andby restricting the flowofheat intothe building. Toachieve this a short time lag, lowthermal capacity, high insulation and reflectiveroofs are used. See figure6.Cold climates are distinguished by low airtemperatures. The design of walls and roofsshould therefore prevent loss of heat. Anadditional problem is condensation which canform on internal surfaces if temperatures areallowed to drop too low. The preservation ofheat is achieved through high insulation andauxiliary heating in severe cases. The insulationprovided must prevent condensation on theinternal surfaces.

Cold Climates

10

Figure 6: Wall treatment - insulation.Figure 5: Wall treatment - thermal capacity.


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Composite Climates

Floors

Composite climates are characterised byalternating hot dry, cold dry and warm humidseasons. The design of roofs and walls dependson the relative duration of the seasons. Light,insulated walls and roofs are used when the dry

season is up to two months while heavy wallsand light roofs are used when the dry seasonlasts for more than three months.

Floors also influence the thermal environmentwithin buildings and their design should beconsidered along with that of walls and roofs. Inhot dry climates the floor should help moderatetemperatures. This is achieved by heavy floorslaid in direct contact with the ground thereby

utilizing the high thermal capacity of the soil. Insevere cases buildings should be partly or totallysubmerged. In warm humid climates the floorshould help in cooling down the building atnight. Light floors raised well above the groundimprove the cooling rate but where this is notarchitecturally feasible a heavy floor in directcontact with the ground is used.

In cold climates floors should be well insulated toprevent heat loss and probable condensation. In

composite climates heavy floors are used.

Foot comfort is very important in the design offloors. The thermal sensation experienced by thebare foot on a floor is however dependent noton the subfloor but on the finish. The choice offinishes should therefore depend on the climateand the average air temperature in dwellings. Inhot climates PVC tiles or terrazzo may beadequate while softwood or carpets may beneeded in colder climates. See Table 2.

The amount of vapour air can hold depends onthe air temperature. Warm air can hold morevapour than air at a lower temperature as isillustrated in table 3. When warm air is cooledtherefore, there comes a time when the vapourin the air is sufficient to saturate the air mass. Thevapour pressure at this temperature is called the

s at ur at io n v ap ou r p re ss ur e w hi l e t hetemperature is the dew point of air for the givenvapour content. When the air is cooled further, it

7. Condensation

Composite Climates

Floors

Composite climates are characterised byalternating hot dry, cold dry and warm humidseasons. The design of roofs and walls dependson the relative duration of the seasons. Light,insulated walls and roofs are used when the dryseason is up to two months while heavy wallsand light roofs are used when the dry seasonlasts formore than threemonths.Floors also influence the thermal environmentwithin buildings and their design should beconsidered along with that of walls and roofs. Inhot dry climates the floor should help moderatetemperatures. This is achieved by heavy floorslaid in direct contact with the ground therebyutilizing the high thermal capacity of the soil. Insevere cases buildings should be partly or totallysubmerged. In warm humid climates the floorshould help in cooling down the building atnight. Light floors raised well above the groundimprove the cooling rate but where this is notarchitecturally feasible a heavy floor in directcontactwith the ground isused.Incoldclimates floors shouldbewell insulated toprevent heat loss andprobable condensation. Incomposite climatesheavy floorsareused.Foot comfort is very important in the design offloors. The thermal sensationexperienced by thebare foot on a floor is however dependent noton the subfloor but on the finish. The choice offinishes should therefore depend on the climateand the average air temperature in dwellings. Inhot climates PVC tiles or terrazzo may beadequate while softwood or carpets may beneeded incolderclimates. See Table 2.

The amount of vapour air can hold depends onthe air temperature. Warm air can hold morevapour than air at a lower temperature as isillustrated in table 3. When warm air is cooledtherefore, there comes a time when the vapourin the air is sufficient to saturate the air mass. Thevapour pressure at this temperature is called thesaturation vapour pressure while thetemperature is the dew point of air for the givenvapour content.When the air is cooled further, it

MaterialComfortable floor

temperature for bare feet CIron/steel 29.5

Gravel concrete 25.5 to 26.5

Granolithic 26

Terrazzo tiles 26

Quarry tiles 25

Foam slagconcrete

24

Brick 24 to 25

Linoleum 22

Rubber floor tiles 22

Hard wood 21 to 22

P.V.C. tiles 22

Plaster 19

Softwood 17 to 19

Cork floor tiles 15 to 16

Insulation board below 5

Carpet below 5

Cork below 5

Table 2: Comfortable floor temperatures.


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will no longer be able to hold some of the vapourand this excess vapour is converted to a liquid ina process called condensation.

There are two types of condensation - surfacea n d i n te r st i t ia l c o nd e ns a ti o n.

occurs when air comes intocontact with a surface at a temperature below itsdew point. A layer of moisture is formed on thesurface of the wall or roof as may be observed insome kitchens, bathrooms or rooms. This leadsto damp interiors and mould growth.

is condensation withinwalls or roofs. This is a result of temperature andvapour pressure gradients across the wall. It mayalso be caused by surface condensation beingabsorbed into the wall. This may cause damageto organic building materials and increase heatloss through a reduction in resistance of thebuilding material.

The factors affecting condensation are theindoor vapour pressure level, the temperatureand absorptivity of the internal surfaces and thevapour transmission of walls. Interstitialcondensation is prevented by predicting dewpoint temperatures at different points within thewall and checking if these do not cause

condensation. Adequate insulation should beprovided and cold bridges avoided. It issometimes necessary to restrict vapours tobathrooms, washing rooms and kitchens. Goodventilation will reduce the risk of surfacecondensation.

1. Describe how a building maintains its thermal

balance.

2. With the aid of sketches, analyze how heat isgained by buildings.

3. List ten thermal quantities used in discussionson heat flow through buildings.

4. What are the effects of the environment onthe following building materials:

a. Steel

b. Concrete

c. Timber

Surface

condensation

Interstitial condensation

8. Tests and Exercises

will no longerbeable tohold someof the vapourand this excess vapour is converted to a liquid ina process called condensation.There are two types of condensation - surfaceand inter st it ial condensat ion .occurs when air comes intocontact with a surface at a temperature below itsdew point. A layer of moisture is formed on thesurface of the wall or roof asmaybe observed insome kitchens, bathrooms or rooms. This leadstodamp interiorsandmouldgrowth.

is condensationwithinwalls or roofs. This is a result of temperature andvapour pressure gradients across the wall. Itmayalso be caused by surface condensation beingabsorbed into the wall. This may cause damageto organic building materials and increase heatloss through a reduction in resistance of thebuildingmaterial.The factors affecting condensation are theindoor vapour pressure level, the temperatureand absorptivity of the internal surfaces and thevapour transmission of walls. Interstitialcondensation is prevented by predicting dewpoint temperatures at different points within thewall and checking if these do not causecondensation. Adequate insulation should beprovided and cold bridges avoided. It issometimes necessary to restrict vapours tobathrooms, washing rooms and kitchens. Goodventilation will reduce the risk of surfacecondensation.

1. Describe how a buildingmaintains its thermalbalance.2.With the aid of sketches, analyze how heat isgainedbybuildings.3. List ten thermal quantities used in discussionsonheat flow throughbuildings.4. What are the effects of the environment onthe followingbuildingmaterials:

a. Steelb.Concretec.Timber

Surfacecondensation

Interstitial condensation

12

Saturation vapour pressure

Air temp. C KN/m2 mmHg g/m3 g/kg ofdry air0 0.61 4.6 4.8 3.8

5 0.87 6.5 6.8 5.4

10 1.23 9.2 9.4 7.6

15 1.71 12.8 12.8 10.5

20 2.33 17.5 17.3 14.4

25 3.17 23.8 23.0 19.4

30 4.23 31.7 30.4 26.2

35 5.60 42.0 39.6 34.7

Table 3: Saturation vapour pressure as a function of airtemperature.


13/13

d. Plastic

5 What is meant by thermal conductivity of amaterial?

6. Materials like coke, slag wool and glass woolare known for their excellent insulating qualities.

Explain why?

7. What happens to materials like coke, slagwool and glass wool when they become wet?Explain.

8. Discuss the various ways dampness gets intothe interior of a building. How can you preventthis as a designer?

9. Differentiate between surface and interstitialcondensation explaining methods of their

prevention in buildings.

10. List four characteristics of building elementswhich affect the interior comfort conditions.

11. Describe the greenhouse effect.

12. Briefly define the following:

(a). Non-steady state conditions

(b). Foot comfort

(c). Dew point

(d). interstitial condensation.

ASHRAE (1971).Ed. Richard,

C.J. and Benjamin, Y.H. ASHRAE, New York.

ASHRAE (1977).ASHRAE, New York.

Burberry, P. (1971). "Method for DeterminingCondensation Risk". In: , 26May 1971 and 2 June 1971. pp 1201-1208 and1265-1269.

Callender, J.H. (1974).McGraw-Hill Book

Company.

Down, P.G. (1969).Pergamon Press, Oxford.

Evans, M. (1980).The Architectural Press, London.

9. References

Applications of Solar Energy forHeating and Cooling of Buildings.

Handbook of Fundamentals.

Architects' Journal

Time-Saver Standards forArchitectural Design Data.

Heating And Cooling LoadCalculations.

Housing, Climate and Comfort.

d.Plastic5 What is meant by thermal conductivity of amaterial?6. Materials like coke, slag wool and glass woolare known for their excellent insulating qualities.Explainwhy?7. What happens to materials like coke, slagwool and glass wool when they become wet?Explain.8. Discuss the various ways dampness gets intothe interior of a building. How can you preventthis asa designer?9. Differentiate between surface and interstitialcondensation explaining methods of theirprevention in buildings.10. List four characteristics of building elementswhichaffect the interior comfort conditions.11. Describe thegreenhouseeffect.12. Briefly definethe following:

(a).Non-steady stateconditions(b). Foot comfort(c). Dewpoint(d). interstitial condensation.

ASHRAE (1971). Ed. Richard,C.J. andBenjamin,Y.H. ASHRAE,NewYork.ASHRAE (1977).ASHRAE,NewYork.Burberry, P. (1971). "Method for DeterminingCondensation Risk". In: , 26May 1971 and 2 June 1971. pp 1201-1208 and1265-1269.Callender, J.H. (1974). McGraw-Hill BookCompany.Down, P.G. (1969).PergamonPress,Oxford.Evans,M. (1980).TheArchitecturalPress, London.

Applications of Solar Energy forHeating and Cooling of Buildings.Handbook of Fundamentals.

Architects' Journal

Time-Saver Standards forArchitectural Design Data.Heating And Cooling LoadCalculations.

Housing, Climate andComfort.

13

Fisk, D.J. (1981).Applied Science Publishers, London.

G iv on i, B . ( 19 76 ).Second Edition. Applied Science

Publishers Ltd., London.

Jain, S.P. (1969). "Thermal Performance ofPerforated Brick, Hollow and Lightweight BlockConstruction In The Tropics". In:

March 1969, pp 1-7.

Jarmul, S. (1980).

McGraw-Hill. New York.

Jennins, B.H. (1978).Harper and Row,

New York.

Koenigsberger, O. and Lynn, R. (1965).Lund Humphries,

London.

Koenigsberger, O.H., Ingersoll, T.G., Mayhew,A. and Szokolay, S.V. (1974).

Longman, London.

Markus, T.A. and Morris, E.N. (1980).Pitman International,

London.

Olgyay, V. (1963).

Princeton University Press,Princeton, New Jersey.

Ove Arup Partnership (1980).The Construction Press,

Lancaster.

Szokolay, S.V. (1975).

The Architectural Press, London. John Wiley andSons.

United Nations Centre For Human Settlements -HABITAT (1984).

UNCHS-HABITAT. Nairobi, Kenya.

Thermal Control of Buildings.

M an , C li ma te A nd Architecture.

ArchitecturalScience Review,

The Architect's Guide to EnergyConservation: Realistic Energy Planning forBuildings.

The Thermal Environment:Conditioning And Control.

Roofs Inthe Warm Humid Tropics.

Manual of TropicalHousing And Building, Part I, Climatic Design.

Buildings,Climate and Energy.

Design With Climate -B io c li ma t ic A pp ro a ch t o A rc h it e ct u ra l Regionalism.

Building Designfor Energy Economy.

Solar Energy And Building.

Energy Conservation In TheConstruction And Maintenance of Buildings.Volume One: Use of Solar Energy and SolarCooling In The Design of Buildings In DevelopingCountries.

Fisk, D.J. (1981).AppliedSciencePublishers, London.Givoni, B. (1976).Second Edition. Applied SciencePublishers Ltd., London.Jain, S.P. (1969). "Thermal Performance ofPerforated Brick, Hollow and Lightweight BlockConstruction In The Tropics". In:March1969, pp1-7.Jarmul, S. (1980).

McGraw-Hill.NewYork.Jennins, B.H. (1978). Harper and Row,NewYork.Koenigsberger, O. and Lynn, R. (1965).Lund Humphries,London.Koenigsberger, O.H., Ingersoll, T.G., Mayhew,A. and Szokolay, S.V. (1974).Longman,London.Markus, T.A. and Morris, E.N. (1980).Pitman International,London.Olgyay, V. (1963).

Princeton University Press,Princeton,NewJersey.Ove Arup Partnership (1980).The Construction Press,Lancaster.Szokolay, S.V. (1975).TheArchitectural Press, London. JohnWiley andSons.UnitedNations Centre For Human Settlements -HABITAT (1984).

UNCHS-HABITAT.Nairobi,Kenya.

Thermal Control of Buildings.Man, Climate AndArchitecture.

ArchitecturalScienceReview,The Architect'sGuide to EnergyConservation: Realistic Energy Planning forBuildings.

The Thermal Environment:Conditioning And Control.Roofs Inthe Warm Humid Tropics.

Manual of TropicalHousing And Building, Part I, Climatic Design.Buildings,Climate and Energy.

Design With Climate -Biocl imatic Approach to ArchitecturalRegionalism.Building Designfor Energy Economy.

Solar EnergyAndBuilding.

Energy Conservation In TheConstruction And Maintenance of Buildings.Volume One: Use of Solar Energy and SolarCooling In The Design of Buildings InDevelopingCountries.

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