Week 04 Building Fabric Losses 100215

8/6/2019 Week 04 Building Fabric Losses 100215

1/25

A4112Environmental Systems/MEP

Columbia University

School of Architecture, Planning and Preservation

Week 4: Building Heat Loss

Overview

Loss and Gain Balance

Conduction

Material Properties

Moisture Transfer

Thermal Mass

Infiltration

Total Heat Loss



2/25

Thermal Balance

Thermal balance occurs when thesum of all heat gains and losses iszero.

Qc + Qv + Qr + Qi + Qe = 0

Qc = Conductive Loss

Qv = Ventilation Heat LossQr = Radiant Loss


=

Qe = Evaporative Loss

Thermal balance is REQUIRED fora stable indoor condition

Building Envelope

The building envelope serves as an

environmental mediator and affects the.

It needs to balance often contradictinggoals to optimize the overall

performance of the faade.

Examples of such contradictions are:

Solar Heat Gain vs. Daylighting

Solar Heat Gain vs. Thermal


Storage

Thermal Storage vs. Daylighting

Natural Ventilation vs. Convection

Heat Gain/ Natural Lighting vs.Conduction

Floor Losses


3/25

Building Envelope

TWO COMPONENTS

Thermal ResistanceRoof

Window

Losses

Infiltration /

Ventilation

Losses

The rate of heat loss

from a room = the

sum of the heat

losses through the


Wall Losses

Floor Losses

individual

components.

Envelope Heat Transfer

When heat passes out of a building through a structure, all three modes of

heat transfer are used and a composite heat transfer calculation can be.

CONDUCTION

RADIATION


CONVECTION

inside outside


4/25

Conduction

Heat transferred directly through a material (e.g. wood,concrete)

Function of conductivity (k), thickness (L), area (A), temperature

difference (T)

TR

ATUATA

L

kQ


,

Conductance: (aka the U-value): specific material quality (of

specific makeup or thickness)

Resistance: (aka the R-value): reciprocal of the conductance(i.e. R=1/U)

Conductivity (k)

material specific value anddepends partly on the densityo e ma er a s ee as a

high conductivity whilst woodhas a low conductivity and

gases even less.

The lower the k value, thebetter its insulating effect.


Note: 1in. Polystyrene hasthe same conductance as

5in. glass


5/25

The U-value

The U-value of a construction is a composite calculation whichadds the effect of heat transfer through each surface and makes

an a owance or e con uc ve an convec ve ea rans er nthe form of surface factors and resistances.

The overall thermal conductance/transmittance is described as:

overallU

11


n

n

...321

Unventilated Cavity

Resistance (Rc) = 1.02 h-ft2-F/Btu

(>3/4 wide and unventilated)

Heat transfer through a wall

inside outside

Resistance of a layer of giventhickness [in] =

ThicknessThickness

kk


External Surface Resistance(Rso) = 0.34 h-ft2-F/Btu

(Sheltered Location =0.45

Severe Location =0.17)Internal Surface Resistance

(Rsi) = 0.69 h-ft2-F/Btu


6/25

U-Value Calculation

Property or Materia l Thickness Conductivity Resistance(in) (btu-in/h-ft-F) (h-ft-F/btu)

External Surface resistance 0.34

Brick outer skin 6 5 1.20

Unventilated Cavity 1.02

Insulation 2 0.2 10.00

Gyp. Board 0.375 1.2 0.31

Internal Surface resistance 0.69


Total Resistance 13.56

U-Value (btu/h-ft--F) 0.07

U-Value Calculation

The effect of added insulation

Property or Material Thickness Conductivity Resistance(in) (btu-in/h-ft-F) (h-ft-F/btu)





Gyp. Board 0.375 1.2 0.31




U-Value (btu/h-ft-F) 0.03


7/25

U-Value Calculation

Note that you cannot add together U Values of differentelements juxtaposed you must add the Resistancestogether and take the inverse. ie

U + U = 0.07 + 0.07 =

0.14 Btu/h-ft-F

Property or Materia l Thickness Conductiv ity Resistance

(in) (btu-in/h-ft-F) (h-ft-F/btu)





Brick inner skin 0.375 1.2 0.31





Brick inner skin 0.375 1.2 0.31




U-Value Calculation

FOR A CAVITY TO BE EFFECTIVE AS AN

INSULATOR, IT IS IMPORTANT THATCONVECTION CURRENTS WITHIN THE

VOID ARE AVOIDED. NORMALLY THIS

REQUIRES THE CAVITY TO BE BETWEEN

and 3 WIDE


X


8/25

Temperature Gradient

The ratio of temperature changes inside astructure is proportional to the ratio of the thermal

res s ances.

INSIDETEMPERATURE

70F 50%RHOUTSIDETEMPERATURE

T = R

Tt RtR

T


14F, 75% RH

Tt

ToTi Rt


RQTTQA

TTR

AQ

oi

T

T)(

For steady state conditions,the heat flow that passes

through each material is thesame as through the

assembly.

Ti=70oF, 50%RH

T

T4T5

A

RQTT

A

RQTT

A

RQTT

A

RQTT

A

RQTT

ARAR

T

i

TTTT

o

o

oT

65

545

434

323

212

1

1

1

1

,,,,


To=14oF, 75%RH

T12


9/25


Temperature Gradient Calculation

Assembly U-Value (btu/h-ft-F) 0.07

knowing: Qdot=U*A*T

Qdot/area 4.13

Property or Materia l Thickness Conductiv ity Res is tance Temperature( in) (btu- in/h-ft-F) (h-ft -F/btu) (F)

To 14.0

T1 (ext. surf) 0.69 16.8

T2 (brick) 6 5 1.20 21.8

T3 (air) 1.02 26.0

T4 (insulation) 2 0.2 10.00 67.3


T5 (gyp. board) 0.375 1.2 0.31 68.6

Ti 0.34 70.0



Air at 70oF and 50% RH has a dew point temperature of 50oF



10/25

Moisture Movement in Structures

INSIDE OUTSIDE

Warm, moist

air

@

High Vapor

Pressure

Cool air,

possibly

saturated but

@

Low Vapor

Pressure


n ers a

Condensation in

Structure

Dewpoint

Location of Vapor Barriers



11/25

Cold Bridges


Cold Bridges*>12.4C

10.0

12.0

*>14.7C

10.0

12.0

14.0

*


12/25

Cold Bridges


Cold Bridges



13/25

Detailing to avoidCold Bridges

In every envelope detail theoal is to be able to draw a

continuous non-crossing linesfor:

Thermal separation

Vapor separation

Rule of thumb:


The vapor barrier is always

located on the warm side of theinsulation.

Ground Heat Losses



14/25

Insulation against Ground Heat Losses


Transparent Elements Single Glazing

GLASS IS A POOR INSULATOR.

FOR A SINGLE GLAZING THEMAIN RESISTANCE ISDETERMINED BY THE HEATTRANSFER COEFFICIENTSINSIDE AND OUTSIDE

Ri = 0.69 h-ft2-F/btu

Rglas = 0.05 h-ft2-F/btu

Ro = 0.34 h-ft2-F/btu


Glazing U-Value =0.9 1.0 Btu/h-ft2-F

Inside Outside


15/25

Transparent Elements Double Glazing

There are a number ofways to improve theerformance and

decrease the heat lossthrough the glazing

One way is to use double-glazing, with an air space

between the panes.

Rcav = 1.02 h-ft2-F/btu

50%


Glazing U-Value =0.49-0.52 btu/h-ft2-F

50%

Inside Outside

Transparent Elements LowE Glazing

Adding a low-e coating

to the inner surface ofthe outer an reduces

the radiation transferbetween the two glass

surfaces and reducesthe heat loss.


Glazing U-Value = 0.29-0.35 btu/h-ft2-F

Inside Outside


16/25

Transparent Elements LowE Glazing

Replacing the cavity airwith an inert as such as

Argon, Krypton or Xenonfurther reduces the heat

loss by reducing theconvection and conduction

through the cavity


Glazing U-Value = 0.22-0.32 btu/h-ft2-F

Inside Outside

Thermal Mass: time lag, decrement factor

The time lag of a construction relates to the delay of the peak amplituderea ve o e me a e pea s experence .

The decrement factoris the reduction in peak amplitude that the materialachieves.



17/25

Thermal Mass

outside


Thermal Mass



18/25

Construction Comparison


Construction Comparison



19/25


20/25

Thermal Mass


Overview

INFILTRATION

Infiltration = uncontrolled by cracks or openings

Ventilation = controlled for fresh air demand



21/25

Heat Loss due to Infiltration

The infiltration of air constitutes a significant proportion of the heat loss from abuilding.

The heat lossdue to the infil tration throu h windows and a s in the buildin structure is

Infiltration / Ventilation


calculated from the equation:

V ... Volume of air flowing in/out (ft/h) cp .. Density (lbs/ft

3)* specific heat (btu/lbs*F)

T .. Inside/outside temperature difference (F)

At 70F:

Density of air= 0.075lbs/ft3

Specific Heat =0.24btu/lbs*F

TcVQ p .

Air Change Rate (ACH)

Infiltration / Ventilation

Definition: air change rate = volume flow rate / room volume [ac/h or 1/h]

(number of air volumes exchanged per hour)

typical values:

air ti ht buildin : 0.1 - 0.2 ac/h


TVh

acQ

room 018.0

low leakage 0.5 - 1.0 ac/hbasic ventilation 1.0 - 2.0 ac/hopen sloped window 3.0 - 5.0 ac/h

With normal properties for air applied this heat loss can be approximated as:


22/25

Detailing

So how to stop Infiltration?

Detailing!

Sealing gaps aroundwindow and door frames

Closing junctions andsealing vapor control

membranes

Using bars rather than just

sealants.


4. Infiltration: Air Change Rate (ACH)Unwanted infiltration represents an increasing proportionof heat losses from buildings as the insulation levelsincrease. Test for infiltration with door blower test.



23/25

Steady State Heat Loss

The sizing of heating equipment is based on the calculation of total heatloss due to conduction and infiltration. This calculation assumes a steady

. .temperature that represents a worst case (usually coldest winter day,during the occupied hours).

The calculated heat loss (Btu/h) for this condition can be used to determine

the maximum amount of heating that needs to be supplied to a space inorder to achieve thermal balance.


This calculation summarizes all conductive heat losses through the

enclosing construction elements (based on their size, temperature

difference and thermal resistance) and infiltration losses (based on theinfiltration rate and the room volume).

Conductive Heat Loss

Having identified the U-Value ofeach of facade elements, the

TUAQ

rate of Heat Flow through thegiven surface area of the

element is calculated by theequation:

Q

U

.


T = Tin-Tout

Inside Outside


24/25


25/25

Calculation Alternative Windows

Improving the windows in this class room to double pane windows reduces the steady state heatloss by 22%.

3 o ume

8,750

Surface Area U-Value Temp Difference U*A*T(ft

2) (Btu/h-ft

2-0F) (

oF)

Wall adjacent to exterior 780 0.079 56 3,451

Wall adjacent to interior 315 0.24 10 756

Window adjacent to exterior 210 0.42 56 4,939

Floor 700 0.084 20 1,176

Roof 721 0.044 56 1,777

Total Fabric Loss 12,098


Infiltration Rate 1.0 AC/hr 58 9,135

Total Heat Loss (btu/h) 21,233

The end

Next week: building fabric gains


Week 04 Building Fabric Losses 100215

Documents

Transcript of Week 04 Building Fabric Losses 100215