Solar Heat Gain Through Windows

Understanding Solar Heat Gain Coefficient

• The SHGC is the quantity of solar radiation that enters through a window. The term applies to both the amount that is directly transmitted and absorbed, and the amount that is released inward. SHGC is calculated using variables such as latitude and longitude, ground surface and reflectance, structure orientation and type and size of the window. The coefficient is shown as a number between 0 and 1, and the lower the number, the less solar heat transmitted by the window.

Measuring SHGC

• The National Fenestration Rating Council (NFRC) promotes a nationally recognized, independent rating method to measure the SHGC and the energy performance of windows and other solar inlets. This measurement takes account of the whole window, including the effect of the frame, as opposed to other systems that describe the effect of the glazing alone. This enables architects, builders and energy programs to compare products available and make informed choices.

Other Ways to Measure Heat Gain

• Energy transmission through windows is also calculated according to the U-factor, visible transmittance and air leakage. The window's insulating value, known as the U-factor, is the heat gained or lost through the window and its frame. Visible transmittance (VT) measures the amount of visible light that is transmitted through the glazing, and air leakage measures the quantity of air that passes through the window when pressurized. All these factors have an effect on the solar heat gain of the window.

Windows That Reduce Solar Heat Gain

• In the 1990s, a new technology resulted in a low solar gain e-coating for glazing, which works by reflecting approximately half of the solar radiation back to the exterior while still allowing the full amount of daylight to enter. This makes it possible for the window to have the full brightness of untinted glass while reducing the quantity of energy that enters, subsequently reducing the heat gain through the window. When you buy window glazing, look for a VT number of 0.6 to 0.8, or 60 to 80 percent, as well as an SHGC that is as low as possible, around 0.4 --- 40 percent --- or lower. The VT and SGHC numbers can be read on the NFRC's energy efficiency label found on all U.S. window glazing.