CAE 331/513

Building Science
Fall 2017

September 14, 2017

Fenestration (doors and windows)

Advancing energy, environmental, and

Dr. Brent Stephens, Ph.D.
sustainability research within the built environment Civil, Architectural and Environmental Engineering
www.built-envi.com Illinois Institute of Technology
Twitter: @built_envi brent@iit.edu
 Last time and this time

• Combined mode heat transfer and energy balances

– Multiple modes of heat transfer in parallel to calculate combined R-
values and U-values
– Energy balances and solving for unknown parameters
– Sol-air temperatures (effective surface temperatures)

• HW #2 is due now

2
HEAT TRANSFER THROUGH FENESTRATION

3
 Definition: Fenestration

• “Fenestration”
– Technically: Areas of the building enclosure that let visible
light through
• Also the term used for windows, doors, and skylights
– Fenestration concerns the units themselves, as well as
placement and shading
• Two buildings with the same windows that are located in different
positions are considered to have different fenestration

• Placement is important for building physics

– By changing the locations of windows and shading devices,
the use of electric lighting and overall building energy use
can be drastically altered (for better or worse)
4
 Fenestration and energy use

• Fenestration impacts building energy use by:

– Heat transfer
• Conduction, convection, long-wave radiation, and short-wave
radiation
• Example: we should utilize solar heat gain in cold climates
and restrict it in warm climates
• We should also use appropriate materials/assemblies to
minimize heat transfer
– Air leakage
• Penetrations in walls and roofs for fenestration can be
problematic for creating pathways for air leaks (advection)
– Daylighting
• Utilize to reduce lighting requirements

5
 Fenestration components

Fenestration consists of three main components:

1. Glazing
– The main part of fenestration that lets the light through
– Usually glass
• Occasionally plastic
– A layer is called a glaze or a pane or a lite
2. Framing
– The material that holds the glazing in place
• Attaches it to the rest of the enclosure
– Usually wood, metal, plastic or fiberglass
3. Shading devices and/or screens
– A unit may or may not have shading
– Either from other building components or shading devices that may or
may not be an integral part of the overall assembly

6
 Fenestration and total heat gain

• The total heat gain of fenestration is the sum of two terms:

– The amount of heat gain from solar radiation passing through
– Combined conductive/convective/LWR thermal heat gain or loss from
the temperature difference between the interior and exterior

• In the summer, both terms are positive towards the interior

and add to heat gains

• In the winter, solar is positive inwards (gain) but conduction/

convection/LWR is negative towards the exterior (loss)
– Net heat gain may be in either direction

7
 Solar radiation and fenestration

• Solar radiation through a single window pane

100% incident
Isolar 8% reflected
hconv+rad
ε 80% transmitted
Thus 12% absorbed
8% rad/conv outward
4% rad/conv inward
ρ
84% total transmitted
𝜏
hconv+rad
outside ε inside

α
8
 Total heat gain through windows

• Calculating the heat gain/loss through a window based on

indoor/outdoor temperature differences is relatively easy:

Q = UA(Tin − Tout ) = UAΔT

• Accounting for solar heat gain is more complicated
– Need to include absorption of solar energy and re-radiation of thermal
energy
– Need to include spectral and angular characteristics of radiation and
glazing
• We can do this with a simplified metric
– The solar heat gain coefficient (SHGC):

( )
Qsolar,window = I solar A SHGC

9
 Glazing U-values

• U-values include the thermal resistance of the glass (or

glazing assembly), as well as the interior and exterior “film”
thermal resistance
k/L
1 1 Lglass 1
R= = + +
hint U hint k glass hext

kglass = ~1 W/mK
Lglass = ~5 mm (~0.2 inches)
hext Typical Rint = 0.12 m2K/W
Typical Rext = ~0.04 m2K/W

10
 Solar heat gain coefficient, SHGC
• For a single pane of glass:
U
SHGC = τ + α
hext
U=k/L

hint
( )
Qsolar,window = I solar A SHGC

𝜏 ( )
qsolar,window = I solar SHGC

hext

11
 Solar heat gain coefficient, SHGC

• For double glazing with a small still air gap:

U ! 1 1 $
SHGC = τ + α outer pane + α inner paneU ## + &
&
hext h
" ext hairspace %

1 1 Louter pane 1 Linner pane 1

R= = + + + +
U hint kouter pane hairspace kinner pane hext

outer pane and

*R Rinner pane are negligible

It gets complicated quickly!

12
 Manufacturer supplied SHGC
• Glazing manufacturers will measure
and report SHGC values for normal
incidence according to the methods of
NFRC 200
– National Fenestration Rating Council
has developed methods for rating and
labeling SHGC, U factors, air leakage,
visible transmittance and
condensation resistance of
fenestration products
• In reality, SHGC is a function of
incidence angle (θ)

Simplified: ( )
Qsolar,window = I solar A SHGC
More
accurately: Qsolar,window = I direct SHGC(θ )A + (I diffuse+reflected )SHGCdiffuse+reflected A
13
 Complex SHGC
• SHGC, solar transmittance, reflectance, and absorptance properties for
glazing all vary with incidence angles of solar radiation (θ)
• The ASHRAE Handbook of Fundamentals 2013 Chapter 15 provides
data for a large variety of glazing types

14
 What about window assemblies?
• In addition to glazing material, windows also include framing, mullions,
muntin bars, dividers, and shading devices
– These all combine to make fenestration systems

15
 What about window assemblies?
• In addition to glazing material, windows also include framing, mullions,
muntin bars, dividers, and shading devices
– These all combine to make fenestration systems
• Total heat transfer through an assembly:

( )
Qwindow = UApf Tout − Tin + I solar Apf SHGC

Where:
U = overall coefficient of heat transfer (U-factor), W/m2K
Apf = total projected area of fenestration, m2
Tin = indoor air temperature, K
Tout = outdoor air temperature, K
SHGC = solar heat gain coefficient, -
Isolar = incident total irradiance, W/m2

16
 Window assembly U-values

• U-values (or U-factors) for windows include all of the

elements of the fenestration system
– Center of glass properties (cg)
– Edge of glass properties (eg)
– Frame properties (f)
• The overall U-factor is estimated using area-weighted U-
factors for each:

U cg Acg +U eg Aeg +U f Af
U=
Apf

Edge of glass is typically 2.5 inches wide

17
Assembly U-value data: ASHRAE 2013 HOF Ch. 15 (SI)

18
Assembly U-value data: ASHRAE 2013 HOF Ch. 15 (IP)

19
Assembly U-values and multiple layers of glazing

• Insulated glazing units (IGUs)

– 2 or more panes of glass
• Separated with a spacer to keep air-tight
– Double glazing: 2 sheets
– Triple glazing: 3 sheets
• Much less common (expensive)

20
 U-values and multiple layers of glazing
• We can separate glass panes with air-tight layers of air or other gases

Center of glass U-values for double pane glazing

Q: Why does argon filled have lower U value than air filled? kair = 0.025 W/mK
kargon = 0.016 W/mK
2013 ASHRAE Handbook of Fundamentals: Chapter 15 kkrypton = 0.0088 W/mK
21
 U-values and multiple layers of glazing
• We can separate glass panes with air-tight layers of air or other gases

Center of glass U-values for triple pane glazing

2013 ASHRAE Handbook of Fundamentals: Chapter 15 22

 How are window U and SHGC values determined?

• Measurements
– NFRC 102, 201, 202, etc.

• Models
– LBNL THERM and WINDOW software

23
http://www.intertek.com/building/standards/
 Window U-value measurements

24
http://www.intertek.com/building/standards/nfrc-102/
 Window U-value measurements

NFRC 102 test procedure

25
http://ftl-incinfo.com/images/thermal_chamber.jpg
 Window U-value models

NFRC 100
model
(software)

26
https://windows.lbl.gov/software/therm/6/THERM63_Features.html
Assembly U-value data: ASHRAE 2013 HOF Ch. 15 (IP)

27
Multiple layers of glazing under a vacuum
New window on the north side of Alumni Hall

28
Multiple layers of glazing under a vacuum

29
 Multiple layers of glazing under a vacuum
• In addition to replacing the fill gas with a lower conductivity
gas, you can also attempt to evacuate an insulated cavity by
placing it under a vacuum
– Eliminates any chance of convection (no air)
– Can reduce the thermal conductivity of the gas left in the cavity

30
http://www.iea-ebc.org/fileadmin/user_upload/docs/EBC_Annex_39_Report_Subtask-A.pdf
 Multiple layers of glazing under a vacuum

31
http://www.nsg-spacia.co.jp/Laminated/performance.html
 Multiple layers of glazing under a vacuum

https://www.pilkington.com/en/us/products/product-categories/thermal-insulation/pilkington-spacia#brochures

32
 Doors

• Doors are often overlooked in terms of thermal integrity of

the envelope in many buildings
– Represent a small area fraction of the shell
• But U value is usually quite large
• Net impact is usually larger than the area fraction
• Doors are much bigger issues for some industrial buildings
– Overhead loading bay doors

• Issue for air leakage too

33
 Doors
• U-values for typical doors

34
 What about shading?
• Shading devices, including
drapes and blinds, can mitigate
some solar heat gain

• We can attempt to describe this

with an indoor solar
attenuation coefficient (IAC)

• Heat gain through a window can

be modified as follows:
( )
Qwindow = UApf Tout − Tin + I direct Apf SHGC(θ )IAC(θ ,Ω) + (I diffuse+reflected )Apf SHGCdiffuse+reflected IACdiffuse+reflected

IAC is a function of incidence angle, θ, and the angle created by a shading device

Or more simply: ( )
Qwindow = UApf Tout − Tin + I solar Apf SHGC ⋅ IAC
35
IAC for blinds and drapes: ASHRAE HOF 2013

36
 Example fenestration problem

Example 3 from 2013 ASHRAE HOF Ch 15:

• Estimate the overall average U-factor for a multi-floor curtain
wall assembly that is part vision glass and part opaque
spandrel.
– The typical floor-to-floor height is 12 ft, and the building module is 4 ft
as reflected in the spacing of the mullions both horizontally and
vertically. For a representative section 4 ft wide and 12 ft tall, one of
the modules is glazed and the other two are opaque. The mullions
are aluminum frame with a thermal break 3 in. wide and centered on
the module. The glazing unit is double glazing with a low-e coating (e
= 0.40) and has a 1/2 in. gap filled with air and a metal spacer. The
spandrel panel has a metal pan backed by R-20 insulation and no
intermediate reinforcing members.

37
 Example fenestration problem

• Solution steps:
1. Calculate the U-factor for the glazed module and for the opaque
spandrel modules
2. Calculate an area-weighted average to determine the average U-
factor for the overall curtain wall assembly

38
 Typical convective surface resistances

• We often use the values given below for most conditions

Surface Horizontal Upwards Downwards

Conditions Heat Flow Heat Flow Heat Flow
Indoors: Rin 0.12 m2K/W (SI) 0.11 m2K/W (SI) 0.16 m2K/W (SI)
0.68 h⋅ft2⋅°F/Btu (IP) 0.62 h⋅ft2⋅°F/Btu (IP) 0.91 h⋅ft2⋅°F/Btu (IP)

Rout: 6.7 m/s wind 0.030 m2K/W (SI)

(Winter) 0.17 h⋅ft2⋅°F/Btu (IP)
Rout: 3.4 m/s wind 0.044 m2K/W (SI)
(Summer) 0.25 h⋅ft2⋅°F/Btu (IP)

We can still sum resistances in series,

even if it involves different modes of heat transfer

39
Summary: Modes of heat transfer in a building

40
 Summary to date: Modes of heat transfer in a building

Conduction Convection Radiation

k Long-wave
q=
L
(Tsurf ,1 −Tsurf ,2 ) (
qconv = hconv T fluid − Tsurf ) ( 4
σ Tsurf ,1
4
− Tsurf ,2 )
k 1 q1→2 =
=U = 1 1− ε1 A1 1− ε 2 1
L R Rconv = ε1
+
A2 ε 2
+
F12
hconv
1
Rtotal =
U total (
qrad ,1→2 = hrad Tsurf ,1 − Tsurf ,2 )
Rtotal = R1 + R2 + R3 + …
Advection 3
4σ Tavg 1
• hrad = Rrad =
Qbulk = m C p ΔT 1 1 hrad
For thermal bridges and + −1
ε1 ε 2
combined elements:
A A
U total = 1 U1 + 2 U 2 + ...
4
(
q1→2 = ε surf σ F12 Tsurf ,1
− T 4
surf ,2 )
Atotal Atotal
Solar radiation: qsolar = α I solar
Window (combined modes) (opaque surface)
Transmitted solar radiation: qsolar = τ I solar
Qwindow = UApf (Tout − Tin ) + I solar Apf SHGC ⋅ IAC (transparent surface) 41