When shopping for new residential windows, there are a number of features to consider. These include three key features regarding energy: the Solar Heat Gain Coefficient for resisting the sun’s heat, the U-factor for insulating against cold winter temperatures or very hot summer temperatures, and the visible transmittance which indicates how much light actually gets through. Additionally, if maximizing your home for natural ventilation you may be concerned about the operation type of window with casement windows providing larger openings and awning windows providing better protection from rain versus the typical single hung, double hung and slider windows that only provide about 50% opening. If you are in a hurricane zone you may also want to consider impact resistance glass and this may limit the efficiency levels you can achieve.
Solar Heat Gain Coefficient (SHGC)
Windows are labeled with an SHGC and it represents the amount of solar heat gain that the window allows in. SHGC includes the thermal impact of the sun transmitting through the window as well as the amount of sun-induced heat absorbed by the glass and frame that conducts through to the living space. The SHGC is always between 0 and 1.0 with 0 being no solar heat gain. Some of the best residential windows for reducing solar heat gain have SHGC values less than 0.20.
The U-Value represents the rate of which heat conducts through the window assembly with a value of 0 being no heat conduction. In the U.S. this value is given in units of Btus/(hr)(ft2)(°F), in most other countries it is W/(m2)(K), so make sure you are comparing the same values. Some manufacturers might report center-of-glass U-factor which is often a better (lower) value than the whole window assembly. To reduce U-factor some manufactures use thermally isolated frames and argon gas between layers of glass. In Florida the extra benefit of very low U-factors is not that important — better to concern yourself with the SHGC. However, if you are buying windows for a home in Minnesota very low U-factor is essential. High performance double pane, low-e windows can have U-factors of less than 0.30.
This value can sometimes be more difficult to find for a given product. Visible transmittance, sometimes called daylight transmittance, represents the ability of the glass portion of the windows to let in the visible part of the sun’s spectrum. Many low-e coatings on windows are spectrally selective so as to allow a greater portion of the sun’s visible spectrum while reflecting the non-visible portion. However, there is variability in these values. To maximize daylight and clear views look for a high visible transmittance for a given desired SHGC.