Energy Efficient BuildingsWindows and Solar Heat Gain

IntroductionA major pathway of heat loss and gains into buildings is through windows. Heat is lost and gained through windows through:

conduction solar radiation air infiltration through cracks/leaks

This chapter discusses: physical mechanisms of these pathways high performance window construction Uwin (conduction) SHGC (solar heat gain coefficient)

Thermal Properties of Windows Single Pane

Single Pane & Storm Window

Rules of Thumb: Single Glaze: R = 1 (hr-ft2-F/Btu)

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RT=Rhi+Rg+Rho

=. 68+ (3 /32 ) /12.45

+. 17=. 68+. 0174+ .17

=. 87 [Btuhr⋅ft2⋅° F ]

−1

RT=Rhi+Rg+Rspace+Rg+Rho=. 68+. 0174+1.01+. 0174+. 17

=1 .89[Btuhr⋅ft2⋅° F ]

−1

Double Glaze: R = 2 (hr-ft2-F/Btu)Triple Glaze: R = 3 (hr-ft2-F/Btu)

These calculations show that heat loss/gain through windows is 5 to 15 times greater than through walls. This provides strong motivation to develop high performance windows

High Performance Windows

1 st task is to stop air movement between panes Air movement from leakage reduces Rspace = 1.01 above, and essentially makes

storm windows the same as a single pane window Air movement between panes causes a convection cell which increases heat

transfer

When the space between windows < 1-inch then Fviscous > Fbuoyant

air doesn’t move and have conduction instead of convection Increasing space decreases conduction, but increases convection

Optimum appears to be 0.5-1.0 inches (Pella windows are 14

32} {¿

~1

2} {¿

)

2 nd task is to find & stop greatest heat loss

σ=. 1714×10−8[Btuhr⋅ft2⋅R4 ] εglass ~. 94 kair=. 18[Btu⋅in

hr⋅ft2⋅° F ]Case 1 Regular glass, air

q̇rad=σ (T 1

4−T 24 )

1ε1+1ε2

−1=

.1714×10−8×(5304−4804 )1. 94

+1. 94

−1=39 . 2[Btu

ft2⋅hr ]q̇conduction=

kΔΤΔx

=.18×50.5

=18 . 0[Btuft2⋅hr ]

q̇win=q̇rad+q̇conduction=57 .2[Btuft2⋅hr ]

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Case 2 Low ε, air (ε = .10)

q̇rad=. 1714×10−8×(5304−4804 )1. 1

+1. 1

−1=2. 3[Btu

ft2⋅hr ]q̇cond=18 . 0[Btu

ft2⋅hr ]

q̇win=q̇rad+q̇conduction=20 .3 [Btuft2⋅hr ]

Case 3 Low ε, argon (ε = .10, kargon = .108)

q̇ rad=2 . 3[Btuft2⋅hr ]

q̇cond=( . 108×50 ). 5

=10 . 8[Btuft2⋅hr ]

q̇win=q̇rad+q̇conduction=13. 1[Btuft2⋅hr ]

The center-of-glass U value would be:

Q/A = U dTU = Q/A dt = 13.1 Btu/ft2-hr / 50 F = 0.26 Btu/hr-ft2-F

Source: ASHRAE Fundamentals

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Total Conductive Heat Loss/Gain through WindowsThermal resistances of:

center-of-glass (cg) edge-of-glass (eg) frame (f)

are different.

Thus, the total conductance of the window must include all three. In general, Ueg > Ucg > Uframe.

U win=( UA )cg+(UA )eg+ (UA )f

Awin

Thus, the conduction heat transfer through a window is

Q̇win=( UA )win (T ia−Toa )=Uwin Awin (T ia−Toa )

Use Toa instead of Tsa since T sa=T oa+

Iαh≈Toa

and α << 1.

Window TypesWindow type influences air leakage and frame fraction. For passive solar, look for windows that seal shut, such as casement or awning windows rather than sliding windows to reduce air leakage. Also look for windows with high ratios of Aglass / Awin to maximize Qsol and reduce edge-of-glass heat loss. Finally, choose high quality wood or vinyl frames since low-quality frames can warp and allow air and moisture (condensation) between glazings.

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Window Thermal Performance DataDESIGNER SERIES : CLAD CASEMENT WINDOWS

A B A BSMARTSASH II SYSTEM 3/32" Clear with 3/32" low-E DGP 0.38 0.40 0.40 2.48* 2.51 0.52 75 0.03SMARTSASH III SYSTEM 5/8" InsulShield IG with 3/32" glass 0.24 0.35 0.33 2.9 3.06 0.33 73 0.035/8" InsulShield IG with 3/32" low-E DGP 0.15 0.26 0.24 3.85** 4.17 0.26 61 0.03INSULSHIELD IX GLAZING5/8" InsulShield HM IG with 3/32" low-E DGP 0.11 0.23 0.21 4.35 4.76 0.18 55 0.03

A = 36" x 60" frame size * costs approximately $275Unit size and type are based on ASTM E1423-91 B = 48" x 72" frame size ** costs approximately $340

Types of Glazing Total-Unit "U" Value Total-Unit "R" ValueWinter Center-

Glass "U" Value LBL Damage

Function

% Visible Light Center-Glass Transmission

Air Infiltration

Data Source: Pella

Solar Heat Gain Coefficient All solar radiation incident on a window is absorbed, transmitted or reflected. From an energy balance, the sum of the absorbtivity, transmittance and reflectivity equals 1.0

Solar transmittance of glass is a function of wavelength and the angle of incidence. In general, glass transmits visible light, but is opaque to infrared light. Since most of the radiation from the sun lies in the visible part of the spectrum, windows transmit most of the energy in solar radiation into the building. After the solar radiation is absorbed by building surfaces, the surfaces reradiate the heat as low temperature infrared radiation. Since glass is opaque to infrared radiation, this radiation remains inside the building

The transmittance of glass is also highly dependent of the angle of incidence; at perpendicular angles most of the light is transmitted and at low angles of incidence most of the light is reflected.

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Solar heat gain coefficient, SHGC, is the fraction of solar radiation incident on fenestration that is transmitted into the building. SHGC includes fraction of solar transmitted plus fraction of solar that is absorbed by glass and radiated into the building:

SHGC=τ+ fα

Solar heat gain coefficients are shown in the tables below.

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In the Tables above, note that the transmittance of visible light, labeled as “Center Glazing Tv” is different than the “Center of Glazing” SHGCs. Most daylighting applications seek to maximize the transmittance of visible light.

In the Tables above, note that “Total Window” SHGC is proportionally less than the “Center of Glazing” SHGC by the proportion of glass area to total window area (Aglass / Awin). Thus, to calculate solar heat gain using the total window area Awin use the “Total Window” SHGC. The SHGC reported on window labels in the “Total Window SHGC at Normal Incidence”.

The SHGC is different for diffuse solar radiation, which is spread evenly over the sky, and beam solar radiation which is directional. Thus, the total solar energy into a building, Qsol, is:

Q̇sol=Awin×[ (SHGCbeam×I beam )+(SHGCdiffuse×I diffuse) ]

In the northern hemisphere, most solar radiation is from the south. Thus, to simplify

calculations, the average solar heat gain coefficient SHGC can be approximated as:

SHGC≈. 9 SHGCnormal

Thus, using the average solar heat gain coefficient, the solar heat gain through a window can be calculated as:

Q̇sol=Awin×SHGC× Itotal

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Selective and Non-Selective Low-e Coatings While low-e coatings and multiple glazings significantly reduce U, they also significantly reduce SHGC. This is good for commercial buildings, warm climates, and E,W windows that allow more Qsol in summer than winter. But it is bad for passive-solar heating applications. Original low-e coatings had higher SHGCs. But new low-e coatings are “selective” and reduce transmittance in the IR range, thus reducing SHGC. Fortunately, old-style, non-selective, low-e coatings are available if specified in the US, or as the standard, in Canada.

GLAZING TYPE U-Value SHGCNON-COATED GLASSSingle Pane 1.11 0.85Dual Pane IGU 1/8"g, 3/4" IG Air 0.49 0.75Dual Pane IGU 1/8"g, 3/4" IG Argon 0.46 0.75Tripane IGU 0.32 0.68

SELECTIVE LOW-E COATINGSMarvin Low E2 (Dual Pane) 0.32 0.41Cardinal Low E2 - 145 (Anderson)(Dual Pane) 0.27 0.32Cardinal Low E2 - 171 (Anderson)(Dual Pane) 0.24 0.41Loewen Heat Smart Plus 1 (Tripane) 0.24 0.41Loewen Heat Smart Plus 2 (Tripane) 0.18 0.37Marvin High R Tripane 0.14 0.34Loewen Heat Smart Plus 3 (Tripane) 0.12 0.34Super Quad SC (4-Pane - 2 suspended PET films) 0.08 0.24

NON SELECTIVE LOW-E COATINGSClear HM TC88 1/8" g 3/4" IG (HURD) (Dual Pane) 0.28 0.5Dual 3mm, Super Spacer, 1 Ar 1 PPG SG 500 (Accurate Dorwin) 0.35 0.65LOF Energy Advantage (3)" 3/32" g 5/8" IG Air (Dual Pane) 0.37 0.73Cardinal Low E2 - 178 (Andersen) (Dual Pane) 0.26 0.58Clear HM TC88 1/8" g 1" IG (Dual Pane) 0.21 0.5Clear HM TC88 1/4" g 1 1/2' IG (Dual Pane) 0.18 0.47PPG Sungale E-500 (3)" 3/32" g 3/4" IG (Dual Pane) 0.3 0.74Loewen Heat Smart ER 1 (Tripane) 0.27 0.69LOF Energy Advantage (3)" 1/8" g 3/4" IG Argon (Dual Pane) 0.28 0.71Super Quad TC (4-Pane 2 suspended PET films) 0.1 0.41Loewen Heat Smart ER 2 (Tripane) 0.2 0.62Tripane, 3mm, SuperSpacer, 2 Ar, 1 PPG SG 500 (Accurate Dorwin) 0.21 0.65Loewen Heat Smart ER 3 (Tripane) 0.14 0.55Tripane, 3mm, SuperSpacer, 2 Ar, 2 PPG SG 500 (Accurate Dorwin) 0.17 0.61

Center of Glass Figure of Merit for various types of glazing as a function of k-value commercially available (Data sorted for k=2)

Selective data from: Ellison, Tim, American Solar Energy Society Annual Conference, Madison WI, 2000.

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The Efficient Windows Collaborative (www.efficientwindows.org) characterizes window performance into three groups according to solar gain: low, mid and high. In many temperature U.S. climates, engineers often specify high SHGC windows for south-facing exposures and low SHCG windows for all other exposures. This enhances solar gain during winter when the sun Is primarily in the south, and reduces solar gain during summer when the sun hits north, east and west exposures.

Center-of-glass properties are:

Solar Gain Type Low Medium HighU-Factor 0.25 0.27 0.29SHGC 0.39 0.58 0.71VT 0.71 0.78 0.75

Whole window properties are:

Frame Aluminum Wood Vinyl Insulated VinylU-Factor 0.59 0.34 0.34 0.26

SHGC 0.37 0.30 0.30 0.31VT 0.59 0.51 0.51 0.55

Whole Window Properties Double-Glazed with Low-Solar-Gain Low-E Glass, Argon/Krypton Gas

Note: The thermal performance properties of specific glazings and frames can vary depending on product design and materials. The results presented here are averages. Consult specific manufacturers for NFRC rated U-factors and SHGCs for products of interest.

Frame Aluminum Wood Vinyl Insulated VinylU-Factor 0.60 0.35 0.35 0.27

SHGC 0.53 0.44 0.44 0.46VT 0.65 0.56 0.56 0.60

Note: The thermal performance properties of specific glazings and frames can vary depending on product design and materials. The results presented here are averages. Consult specific manufacturers for NFRC rated U-factors and SHGCs for products of interest.

Double-Glazed with Moderate-Solar-Gain Low-E Glass, Argon/Krypton GasWhole Window Properties

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Frame Aluminum Wood Vinyl Insulated VinylU-Factor 0.61 0.37 0.37 0.29

SHGC 0.64 0.53 0.53 0.56VT 0.62 0.54 0.54 0.58

Whole Window Properties Double-Glazed with High-Solar-Gain Low-E Glass, Argon/Krypton Gas

Note: The thermal performance properties of specific glazings and frames can vary depending on product design and materials. The results presented here are averages. Consult specific manufacturers for NFRC rated U-factors and SHGCs for products of interest.

National Fenestration Rating Council (http://www.nfrc.org)The National Fenestration Rating Council (NFRC) energy performance label can help you determine how well a product will perform the functions of helping to cool your building in the summer, warm your building in the winter, keep out wind, and resist condensation. All energy performance values on the label represent the rating of windows/doors as whole systems (glazing and frame) for 48-in by 59-in gross window size.

U-Factor The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. In U.S. units U-Factor ratings generally fall between 0.20 and 1.20 Btu/hr-ft2-F. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value.

Solar Heat Gain Coefficient The SHGC is the fraction of incident solar radiation admitted through a window (both directly transmitted and absorbed) and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits in the house.

Visible Transmittance Visible Transmittance (VT) measures how much light is transmitted through a product. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted.

Air Leakage* Air Leakage (AL) is indicated by an air leakage rating expressed as the equivalent cubic feet of air passing through a square foot of window area (cfm/sq ft). The lower the AL, the less air will pass through cracks in the window assembly.

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Condensation Resistance* Condensation Resistance (CR) measures the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the CR rating, the better that product is at resisting condensation formation. CR is a number between 0 and 100.

Net Heat Transfer Through WindowsThus, the net heat transfer through a window, not including air leakage due to infiltration, can be calculated as the sum of the solar and conduction heat transfer:

Q̇net,in=Q̇sol−Q̇win,out=[ Awin×SHGC× Itotal ] - [Uwin Awin (T ia−Toa ) ] Example: Calculate the net heat gain into a building through a south-facing window. The total area of the window is 6 ft2, the total U value of the window is 0.30 Btu/hr-ft2-F, the total normal SHCG of the window is 0.70. The outdoor air temperature is 30 F. The indoor air temperature is 70 F. The total solar radiation on each of a south exposure is 150 Btu/hr-ft2.

InputAwin (ft2) 6U (Btu/hr-ft2-F) 0.3SHGC 0.7Toa (F) 30Tia (F) 70Isouth (Btu/hr-ft2) 150

CalculationsSHGCavg = SHGC x .9 0.63Qsol,in (Btu/hr) = Awin x SHGCavg x Isouth 567Qwin,out (Btu/hr) = Awin x U x (Tia-Toa) 72Qnet,in (Btu/hr) = Qsol,in - Qwin,out 495

Windows and OverhangsIn the Northern hemisphere, the sun makes a path across the southern sky; thus, south-facing exposures on buildings get much sunlight than other exposures. In addition, in the summer the sun takes a higher path across the sky than in the winter. The seasonal variation in the path of the sun across the sky can be utilized to maximize solar gain during winter and minimize it during summer. In the Northern Hemisphere, overhangs on south facing windows create shade in the summer, but allow direct sunlight to hit the windows in the winter. Thus, overhangs on south facing windows are highly effective at reducing solar heat gain during summer and facilitating solar heat gain during winter.

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Source: http://www.byexample.com/library/illustrations/seasonal_sun/illustration_sun.jpg)

The path of the sun across the sky varies with latitude as well as with the season (see Figure below). At locations near the equator, the sun path is high in the sky and the sunset and sunrise are about 12 hours apart all year long. At locations far from the equator, the sun path is low in the sky and the number of daylight hours are large in the summer and small in the winter.

Altitude Angle at Noon

Azimuth Angle at Sunset

Sunset (hours after noon)

Vishakhapatnam India Latitude = 18 N21-Jun 85 115 6.5421-Dec 49 65 5.46

Dayton Ohio Latitude = 43 N21-Jun 70 123 7.5921-Dec 24 57 4.41

Fairbanks Alaska Latitude = 65 N21-Jun 48 160 10.5621-Dec 2 20 1.44

(CollectorSpacing_SunPosition.xlsx)

To estimate shading, it is necessary to know the position of the sun in the sky. The position of the sun in the sky can be calculated for any latitude at any given date and time (see for example: Duffie and Beckman, Solar Engineering of Thermal Processes, John Wiley and Sons, 2013). This position can be plotted in sun path charts. For example, at 40 north latitude, the sun has an altitude angle of about 57 at 60 west of south on July 21 at 2 pm (see chart below).

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The position of the sun in the sky can be calculated for any latitude at any given date and time. WeaTran calculates this position, and uses it to determine solar gain on any exposure. WeaTran can also calculate net solar gain minus shading from overhangs and wings when the geometry of the overhang or wing is specified by enabling the “Advanced Solar Options button” If the windows are protected by horizontal overhangs or vertical fins along their sides, enter the height of the window “hc”, the protrusion of the overhang or wing “po”, and the gap between overhang/fin and window “gapo” as shown in the figure below.

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