1. Chapter 5A: HEAT GAINS THRU WINDOWS
Agami Reddy (rev Dec 2017)
Basic considerations and window characteristics
Window performance metrics
Inter-pane convective and radiative interactions
Overall U values of window
Surface temperature of glazing
Solar heat gains and SHGC
Radiative properties of glass
Thermal loads due to solar radiation
HCB-3 Chap 5A: Window Heat Gains

2. HCB-3 Chap 5A: Window Heat Gains
Basic Considerations
Window and glazing choices should be considered holistically.
Issues to consider include:
Heat gains and losses
Visual requirements (privacy, glare, view)
Shading and sun control
Thermal comfort
Condensation control
Ultraviolet control
Acoustic control
Color effects
Daylighting
Spacer material for windows with two or more glass panes are made of metal or insulators placed all along the perimeter to keep the panes apart. They act as undesirable thermal bridges and increase U value of center of pane
Figure 5.1. Sketch of a double pane window
HCB-3 Chap 5A: Window Heat Gains

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4. HCB-3 Chap 5A: Window Heat Gains
Fig. 5.1(b)
Right photo: Low-e double pane glass sash with stainless steel spacers
and rollers at bottom (not visible). The I-section provides rigidity
and also insulates the glass frame.
Left photo: The frame for a double sliding window on which the sash slides.
The several discrete compartments are meant to provide added rigidity and also
increase thermal resistance.
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Fig. 5.2 Different Types of Residential Windows
HCB-3 Chap 5A: Window Heat Gains

6. Window Performance Metrics
Important characteristics needed to specify window system:
Window U-value
- typical aluminum frame single glazed window:
1.3 Btu/(h·ft²·°F) or 7.4 W/m2.K
- multi-paned, high-performance window with low-emissivity coatings and insulated frames: 0.2 Btu/ (h·ft²·°F) or 1.1 W/m2.K
Window Solar Heat Gain Coefficient (SHGC) of a particular window- ratio of solar heat gain to incident solar radiation
Glass Visible Transmittance - same as above but limited to visible solar spectrum, this is proportional to the luminous efficacy of transmitted daylight
Air leakage
About 1/3rd of heat losses are thru windows
HCB-3 Chap 5A: Window Heat Gains

7. Fig. 5.3 Example of National Fenestration Rating Council Label
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8. HCB-3 Chap 5A: Window Heat Gains
Solar Heat Gains
Figure 5.6
Distribution of beam radiation falling on glazing product #1a of Table 5.1 at normal solar incidence.
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9. HCB-3 Chap 5A: Window Heat Gains
Notice that the sum adds up to Explain why?
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Heat Gain Equations
Total instantaneous heat flow thru glass = radiation transmitted through glass + inward flow of absorbed solar radiation + conduction heat gain
Solar heat gains
Could be inwards
or outwards depending
on season
Q= UA (Tout – Tin) +A. (SHGC) IT
Incident solar radiation
Overall heat loss coefficient Solar Heat Gain Coefficient (non-dimensional)
HCB-3 Chap 5A: Window Heat Gains

11. Conduction: Single Pane Window
Figure 5.8
Energy balance of a single-pane window (neglecting heat capacity of glass).
Physical configuration.
Thermal network without solar radiation.
Thermal network with solar radiation (and neglecting resistance in glass).
Glass accounts for about
2% of total resistance
- negligible
BTU/h-ft-F
HCB-3 Chap 5A: Window Heat Gains

12. Inter-Pane Heat Transfer from Chap 2:
For infra-red radiation, glass behaves like an opaque material
(2.69)
Using the concept of radiative coefficient:
Good approximation when surface temperatures are close
or A.htotal =A.hcon+A.hrad
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13. HCB-3 Chap 5A: Window Heat Gains
Double Pane Window
For double pane glass: convective heat loss from one pane to another
roughly equal to radiation
HT between panes can be reduced:
- by suppressing convection: inert gases, small inter-pane gap
- by reducing radiation: selective films
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Figure 5.9 Variation of center-of-glass U values with gap width for different window emissivities and gas fill materials. (a) Double-pane window. (b) Triple- pane window.
Argon and krypton are less conductive gases than air
One could try creating a vacuum between both panes, but that is not practical
Experimental studies indicate that convection is suppressed when spacing
< 13mm, and heat transfer is by conduction thru air. For larger spacing, increase in heat transfer is offset by thicker air layer.
No point in increasing separation between panes > 10 mm (0.4 inches)
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Example 5.1:
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Table 5.3
HCB-3 Chap 5A: Window Heat Gains

18. Overall U-values for Windows
Spacer material for windows with two or more glass panes are made of metal or insulators placed all along the perimeter to keep the panes apart. They act as undesirable thermal bridges and increase U value of center of pane.
Area-weighted equation:
The edge effect is not applicable for single pane windows
Figure 5.10
Center-of-glass and edge-of-glass U values for a variety of spacer materials (“ideal” corresponds to a spacer with the same U value as the ­glazing).
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Example
5.2
Given: Center-of-glass U value (double glazed, airspace width 9 mm, ε = 0.10) and edge-of-glass U of for aluminum spacer
Find: Uav
Sketch: Figure 5.11
Lookup values: Ucg = 1.9 W/(m2 · K) from Figure 5.9
Ueg = 2.8 W/(m2 · K) from Figure 5.10
Uf = 10.8 W/(m2 · K) from Table 5.2
Figure 5.11
Dimensions of the frame, edge of glass (spacer), and center of glass for Example 5.2, drawn to scale.
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Dividing the sum of the UA values by the sum of the A values according to Equation 5.5
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Interior Surface Temperature
of glazing: Impacts comfort
Surface temperature of glazing (Eq. 5.8):
Assuming Ti = 20° C and hi = 8.29 W/(m2 .K).
Figure 5.12
Interior glass surface temperature versus outdoor temperature,
“Downdraft” actually caused by radiation heat loss from body to window surface
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Solar Heat Gains
Figure 5.13
Components of solar heat gain with a double-pane window.
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How should an ideal glazing behave spectrally when coupled to a conditioned space:
In cold locations, it should have high solar radiation transmittance while acting as a good reflector in the long-wave infra-red spectrum emitted by the warm interior surfaces of the building. This is the principle of low e-coating (also called high-solar gain low-e) on window glass in cold climates (i.e., low emission over the long-wave spectrum).
Figure 5.7
Conceptual illustration of two spectrally ideal selective glazing surfaces: one for hot climates and one for cold climates
In hot locations, it should only let solar radiation in the visible portion of the solar spectrum, and block both solar radiation from the other spectral regions as well as infra-red radiation from outdoors to enter the conditioned space. This type of glass is referred to as selective low-e or low-solar-gain low-e glazing system.
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Commercially available glazing
Fig. 5.4 Spectral transmittance for different types of glass
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25. Change of Radiative properties with Incidence Angle
Figure 5.5
Variation of solar optical properties as a function of incidence angle θi, for three types of glasses: A = DSA (double-strength sheet), B = 6 mm (0.25 in.) clear glass, and C = 6 mm (0.25 in.) gray, bronze, or green tinted heat-absorbing glass. (a) Transmittance. (b) Reflectance. (c) Absorptance.
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Thermal Loads due to Solar Radiation
Total instantaneous heat flow thru glass = radiation transmitted through glass
+ inward flow of absorbed solar radiation
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Center-of-Glass
Total Window
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Eq. 5.11
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Expression for SHGC
Consider the simple one pane glazing configuration (Figure 5.8). So as to simplify the derivation, let us overlook the difference between beam and diffuse radiation effects. In the presence of solar radiation, the total heat gain becomes
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SHGC approach combines the transmitted and absorbed solar radiation effects- used for simplified calculations. However, accurate cooling load calculations need to treat these separately since:
transmitted solar radiation is absorbed by room walls and furnishings and released later in time
absorbed solar radiation is released by convection almost instantaneously
ASHRAE Fundamentals Handbook allows such a separation to be made using a software called WINDOWS 5.2 (we shall not discuss this in this class)
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34. HCB-3 Chap 5A: Window Heat Gains
Outcomes
Knowledge of various issues to be considered while selecting glazing
Familiarity with different window designs in residential and commercial buildings
Understanding of window metrics: U-value, SHGC, glass visible transmittance, air leakage
Familiarity with ways to model instantaneous heat gains combining incident solar radiation and conduction heat gains
Be able to calculate overall U-values for different window designs and be familiar with the use of lookup tables
Familiarity with spectral properties of an ideal glazing either in a cold or hot location.
Understanding of how radiative optical properties (transmissivity, reflectivity, absorptivity) vary with solar incidence angle
Understanding of the concept of SHGC, basic modeling equations and use of lookup tables
HCB-3 Chap 5A: Window Heat Gains