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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
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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
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=
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
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Storage
Thermal Storage vs. Daylighting
Natural Ventilation vs. Convection
Heat Gain/ Natural Lighting vs.Conduction
Floor Losses
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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
Week 4: Building Heat Loss
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
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CONVECTION
inside outside
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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
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,
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.
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Note: 1in. Polystyrene hasthe same conductance as
5in. glass
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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
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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
Week 4: Building Heat Loss
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
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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
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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)
External Surface resistance 0.34
Brick outer skin 6 5 1.20
Unventilated Cavity 1.02
Insulation 6 0.2 30.00
Gyp. Board 0.375 1.2 0.31
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Internal Surface resistance 0.69
Total Resistance 33.56
U-Value (btu/h-ft-F) 0.03
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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)
External Surface resistance 0.34
Brick outer skin 6 5 1.20
Unventilated Cavity 1.02
Insulation 2 0.2 10.00
Brick inner skin 0.375 1.2 0.31
Brick outer skin 6 5 1.20
Unventilated Cavity 1.02
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Insulation 2 0.2 10.00
Brick inner skin 0.375 1.2 0.31
Internal Surface resistance 0.69
Total Resistance 26.10
U-Value (btu/h-ft-F) 0.04
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
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X
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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
Week 4: Building Heat Loss
14F, 75% RH
Tt
ToTi Rt
Temperature Gradient
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
,,,,
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To=14oF, 75%RH
T12
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Temperature Gradient
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
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T5 (gyp. board) 0.375 1.2 0.31 68.6
Ti 0.34 70.0
Total Resistance 13.56
U-Value (btu/h-ft-F) 0.07
Air at 70oF and 50% RH has a dew point temperature of 50oF
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Moisture Movement in Structures
INSIDE OUTSIDE
Warm, moist
air
@
High Vapor
Pressure
Cool air,
possibly
saturated but
@
Low Vapor
Pressure
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n ers a
Condensation in
Structure
Dewpoint
Location of Vapor Barriers
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Cold Bridges
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Cold Bridges*>12.4C
10.0
12.0
*>14.7C
10.0
12.0
14.0
*
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Cold Bridges
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Cold Bridges
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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:
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The vapor barrier is always
located on the warm side of theinsulation.
Ground Heat Losses
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Insulation against Ground Heat Losses
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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
Week 4: Building Heat Loss
Glazing U-Value =0.9 1.0 Btu/h-ft2-F
Inside Outside
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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%
Week 4: Building Heat Loss
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.
Week 4: Building Heat Loss
Glazing U-Value = 0.29-0.35 btu/h-ft2-F
Inside Outside
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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
Week 4: Building Heat Loss
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.
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Thermal Mass
outside
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Thermal Mass
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Construction Comparison
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Construction Comparison
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Thermal Mass
Week 4: Building Heat Loss
Overview
INFILTRATION
Infiltration = uncontrolled by cracks or openings
Ventilation = controlled for fresh air demand
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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
Week 4: Building Heat Loss
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
Week 4: Building Heat Loss
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:
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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.
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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.
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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.
Week 4: Building Heat Loss
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
.
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T = Tin-Tout
Inside Outside
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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
Week 4: Building Heat Loss
Infiltration Rate 1.0 AC/hr 58 9,135
Total Heat Loss (btu/h) 21,233
The end
Next week: building fabric gains
Week 4: Building Heat Loss
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