EGEE 102
Energy Conservation and Environmental Protection

Factors in Window Selection, page 2


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.
Sarma is standing in a house that is 70 degrees F. Outside is 0 degrees F and the sun is shining in the window.
Solar Heat Gain

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.
Visible Transmittance

Click here to open a text description of the Visible Transmittance activity

Visible Transmittance

The scenario illustrating Visible Transmittance is of Dr. Pisupati lying in bed on a sunny day with his dog Buddy on the floor next to him. The blinds on the nearby window are shut, making the room too dark to see. He doesn't know where Buddy is because the room is so dark, so he opens the blinds to let light into the room. When the sunlight enters the room, he is able to see that Buddy is on the floor next to him.

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.


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

The table below lists typical SHG, VT, and LSG values for different types of glass according to:

  • Total Window
  • Center of Glass (in parentheses)
Typical SHG, VT, and LSG Values
Window and Glazing Types SHG
(0-1 scale)
(0-1 scale)
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

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). 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

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. 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.

Instructions: For each of the following, calculate the R-value and LSG. (Round your answers up to two decimal places.) After you enter your answers in the boxes below, check your work by clicking one of the “check answer” buttons below.

Calculating R-Value and LSG

Click here to open a text description of the Calculating R-Value and LSG activity

Calculating R-Value and LSG

Based on the following energy performance ratings, calculate the R-Value and LSG for each.

  1. Window #1
    • U-Factor: 0.30
    • Solar Heat Gain Coefficient: 0.36
    • Visible Transmittance: 0.59
    • Air Leakage: 0.2
  2. Window #2
    • U-Factor: 0.35
    • Solar Heat Gain Coefficient: 0.30
    • Visible Transmittance: 0.46
    • Air Leakage: 0.2
  3. Window #3
    • U-Factor: 0.32
    • Solar Heat Gain Coefficient: 0.45
    • Visible Transmittance: 0.58
    • Air Leakage: 0.3


  1. Window #1
    • R-Value = 3.33
    • LSG = 1.64
  2. Window #2
    • R-Value = 2.86
    • LSG = 1.53
  3. Window #3
    • R-Value = 3.13
    • LSG = 1.29