Model energy performance. Installing too many windows in a house can introduce too much solar gain, causing overheating, which may force the air conditioner to work harder (particularly if those windows are east- or west-facing). Too many windows can also create excessive heat loss, depending on the U-factor of the windows. To determine how the windows will affect the home’s energy performance, nothing beats energy modeling. There are excellent software tools for modeling performance, including REM/Design ($347) and EnergyPlus (free)—though these programs take some training or experience to use effectively. Using these tools during a home’s design can influence the placement and size of the windows—leading to lower energy bills and also greater thermal comfort.
Look to the NFRC labels. When shopping for windows, make sure you’re comparing apples to apples. Use NFRC labels to compare different products’ U-factors, visible transmittance, and solar heat gain. Some NFRC labels also include numbers for air leakage and condensation resistance. The Efficient Windows Collaborative (efficientwindows.org) has an excellent online tool for help with window selection.
Define energy performance targets. This depends on both the climate and the location of the windows in the house. Given the many glazing options to choose from, it increasingly makes sense to “tune” windows by orientation—in other words, specify a certain type of glazing depending on the window’s location. With this approach, the primary variables are U-factor and SHGC.
If you want passive solar heating, especially in northern climates, specifying high-SHGC windows for the south makes sense, even if it means a sacrifice in the U-factor. For windows located on east and west walls, glazing with lower SHGCs will help limit unwanted solar gain. For north-facing windows, the SHGC value doesn’t matter much; a low U-factor window is more important. For practical reasons, it usually makes sense to specify one type of glazing for south-facing windows, and another type for all of the other windows. Some designers and builders like to design the south windows to be slightly different dimensions to ensure that they aren’t mixed up with windows slated for other walls (see the “Tuning Windows for Orientation & Climate” table).
Egress considerations. Certain windows must be designed for getting out if there’s a fire. These windows have to be a specified minimum size, and they should be readily operable—usually without separate storm windows or window attachments that require opening. In general, egress is easier with casement than double-hung windows. A building official will be able to advise on egress requirements in your area.
Warranties. Warranties make an important difference. Many have 20-year warranties or 20/10 warranties (20 years for the glass and 10 years for the non-glass components). Labor to replace failed windows is often covered for a much shorter period of time. Some manufacturers offer “limited lifetime” warranties, though understanding the fine print on “lifetime” tends to be an art form.
Alex Wilson is the founder and executive editor at BuildingGreen, which publishes Environmental Building News. He also is president of the nonprofit Resilient Design Institute, which works to advance measures to improve the resilience of our buildings and communities.
Efficient Windows Collaborative • efficientwindows.org
National Fenestration Rating Council (NFRC) • nfrc.org
Alpen High Performance Products • alpenhpp.com
Cardinal Glass Industries • cardinalcorp.com
Guardian Industries • guardian.com
Hurd Windows and Doors • hurd.com
Marvin Windows and Doors • marvin.com
Pilkington Glass • pilkington.com
Quanex Building Products (Super Spacer) • quanex.com
Sage Electrochromics • sageglass.com • Dynamic glass
View • viewglass.com • Dynamic glass