If shopping for windows for your home, be sure to examine the energy performance numbers. Years ago window manufacturers used R-value, a measurement of a materials resistance to heat flow transfer, to rate windows. But this provided to be an ineffective way for homeowners to evaluate overall windows performance.
The National Fenestration Rating Council (NFRC), a nonprofit collaboration of window manufacturers, government agencies, and building trade associations, however, has developed a set of standardized rating procedures to help consumers compare and select windows. The NFRC labels use whole-window U-value (or U-factor), which is a measure of the amount of heat transfer through a window. U-value is the inverse of R-value, thus the lower the U-value the better the energy performance of the window. Always choose a window with a low U-value. Other criteria, such as certified air leakage rate, solar heat gain coefficient, visible light transmittance, and warranty on the seal should also be taken into consideration when purchasing windows.
In addition to using double-pane, insulated glass units for low U-values, modern window technology also incorporates one or more of the following features in the more efficient, low U-value windows available:
Low-emissivity glass (low-e glass)
Heat-absorbing (tinted) glass
Reflective glass coatings
Gas-filled windows
Low-iron glass
Any one of these features can improve window efficiency; however, it is very common to now find combinations of these features in any particular new windows. Radiant heat gain is a very big factor in the summertime, especially in houses with large window surfaces facing south or west. Low-emissivity glass or any tinted or coated windows help reduce the radiant heating factor to not only keep occupants more comfortable but also to limit solar damage to furnishings.
If you are unable to or just don’t want to upgrade older windows, simply closing the blinds or curtains on the windows is an economical way to keep out the sunlight (and heat) during the day.
Remember, these tips are only general guidelines. Since each situation is different, contact a professional if you have questions about a specific issue.