Thermal Qualities for Glass Windows

Setting Standards for Glass Windows

• The National Fenestration Rating Council (NFRC) provides information that allows consumers to compare the thermal qualities of glass windows. It invites window manufacturers to submit their windows for evaluation. A label is then developed that rates the window according to several qualities, including the ”U-factor,” the solar heat gain coefficient, condensation resistance and air leakage. The information contained on this energy-performance label helps consumers figure out how well a window should perform when they are heating and cooling a home or other structure. Programs and standards such as Energy Star, building codes and tax credits are based on ratings from the NFRC.

Insulating Qualities

• The U-factor is the rate of heat loss of a window. The more the glass window is resistant to heat flow, the better it will be at insulating and the lower the U-factor rating will be. The “center-of-glass U-factor” rating describes the insulating qualities of the glazing alone, without taking the window frame into consideration. A high-performance double-pane window will have a U-factor of around .30 or lower. Triple-pane windows may have a U-factor rating of as low as .15.

Solar Heat

• The “solar heat gain coefficient” rating (SHGC) indicates how much of a “greenhouse effect” the window will allow. Windows that allow solar heat gain will help to defray energy usage and costs when heating a home or other building. The SHGC rating is between 0 and 1, with the higher number indicating a higher solar heat transmission quality.

Inside Glass Surface Temperature

• New high-performance windows are produced with new glazing technology to make a home more comfortable and to reduce energy costs. Older windows generally have lower inside glass surface temperatures. The colder the glass feels, the more heat is radiated from a person’s body toward the window. The cold glass may also make for drafts by allowing cooled air near the window to move down toward the floor, accelerating heat loss. Newer windows like double or triple windows with argon or krypton gas provide inside glass surface temperatures of up to 60 degrees or more.