10.3: Factors in Window Selection (II)

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Solar Heat Gain Coefficient

Solar Heat Gain Coefficient (SHGC) measures how well a window blocks heat from the sunlight.

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.

Visible Transmittance

Visible Transmittance (VT) measures how much visible light comes through a window.

The visible transmittance is an optical property that indicates the amount of visible light transmitted.
VT is expressed as a number between 0 and 1.
The higher the VT, the more light is transmitted.

The following video explains more about VT.

The light-to-solar gain ratio(LSGR) provides a gauge of the relative efficiency of different glass types in transmitting daylight while blocking heat gains. It is determined by the ratio between VT and SHGC, i.e.,

\[ LSGR = \dfrac{VT}{SHGC} \]

The higher the ratio number, the brighter the room is without adding excessive amounts of heat.

Table 10.3.1 lists typical SHG, VT, and LSG values for different types of glass according to:

Total Window
Center of Glass (in parentheses)

Table 10.3.1. Typical SHG, VT, and LSG values

Window and glazing types	SHG (0 to 1 scale)	VT (0 to 1 scale)	LSG (VT / SHG)
Single glazed, clear	0.79 (0.86)	0.69 (0.90)	0.87 (1.04)
Double glazed, clear	0.58 (0.76)	0.57 (0.81)	0.98 (1.07)
Double glazed, bronze	0.48 (0.62)	0.43 (0.61)	0.89 (0.98)
Double glazed, spectrally selective	0.31 (0.41)	0.51 (0.72)	1.65 (1.75)
Double glazed, spectrally selective	0.26 (0.32)	0.31 (0.44)	1.19 (1.38)
Triple glazed, new low-e	0.37 (0.49)	0.48 (0.68)	1.29 (1.39)

The following two parameters are not required to be reported on NFRC label but are optional:

Air leakage
- Indicated by an air leakage rating expressed as the equivalent cubic feet of air passing through a square foot of window area (cfm/ft²).
- Heat loss and gain occur by infiltration through cracks in the window assembly by convection.
- The lower the AL, the less air will pass through cracks in the window assembly.
Condensation resistance
- 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.
- While this rating cannot predict condensation, it can provide a credible method of comparing the potential of various products for condensation formation.
- CR is expressed as a number between 0 and 100.