How should I evaluate the energy performance of a window? Look for products that are tested and approved by the National Fenestration Rating Council (NFRC). This includes testing the U-value of a window. Look for the sticker on the window. Another place to look for information on energy efficiency in windows is: U.S. EPA Energy Star Homes Program web site. NFRC and Energy Star give you a measure of a windows "Thermal Performance Only".
How Should I evaluate structural, air infiltration and other performance issues? AMMA and NWWDA ratings have set test specifications standards for windows and doors. AAMA stands for American Architectural Manufacturers Association, while NWWDA stands for National Wood Window and Door Association. AMMA and NWWDA standards work by defining four mandatory performance requirements for a completely fabricated window:
Structural integrity to withstand wind loads
Resistance to Water Leakage
Resistance to air infiltration
Forced Entry resistance
Actual windows are tested by independent labs and graded by their "Design Pressure" performance. The following table shows the minimum Performance Grade required for each class.
Structural Test Pressure
Water Resistance Pressure
Light Commercial (LC)
Heavy Commercial (HC)
A window rated "R" 15 just passed for residential use only. An "R" 30 or 45 rated window will perform much better.
What is air infiltration? Air infiltration refers to the movement of air at a different temperature through the different interfacing surfaces of the sash and frame. Look for windows that are designed for absolute minimum air infiltration.
What is a "rough opening"? The rough opening is the opening in the wall that the window or patio door fits into. Normally, the rough opening must be sized 1/2 inch larger than the frame in both width and height.
What does it mean to have a "clad" wood window? "Clad" refers to the covering on the exterior window or patio door unit. From the inside, you see the beauty of wood while the aluminum clad "shell" on the outside ensures the life of your wood windows for many years.
What does the term "failed" mean and how can you tell when a window has "failed"? You know a window has failed if there's moisture in between the panes of glass. There are two types of window failures:
1. a seal failure: moisture (condensation) is leaking into the space between the panes.
2. a window failure: the window is no longer keeping out the elements.
Can old-style windows be custom made? Yes. One affordable option that also maintains the historic integrity of a home is to replace an old wood window with a new wood window of the same style.
What window material do you recommend for a house in the D.C. Metro area? Window material choices vary by region. In the D.C. area, houses use mostly vinyl, fiberglass and wood windows, whereas in the Southwest, more aluminum windows are used. This has a lot to do with the building codes, and the fact that aluminum conducts heat and cold. Fiberglass and Vinyl are very popular in this region.
Please explain the (U) and (R) values and how they rate a window's insulation capabilities. One way that insulation is created is with dead-air space between panes of glass. Therefore a double-paned window gives more insulation than a single-paned one. Other factors contribute to the insulation qualities of a window, such as the gasket, the material type, and the gas used in a gas-filled window. The U/R value tells how the product insulates. It is a formula derived from the efficiency of the product, and that formula takes into account many of the factors just mentioned. A good R measurement is a high one, while a good U measurement is low since U is the reciprocal of the R. Homeowners are more interested in the R value, while builders are interested in the U value.
What is low-E? Low-E, or low emissivity, refers to how a window reflects certain length waves. A window with a good low-E lowers your energy bill because in the summer it reflects sunlight and in the winter it absorbs it to help in heating your home.
What are some reasons why I might want tinted windows in my home? Tinting on glass is used for more than just keeping out the sun. The tinting, which comes in either gray or bronze, cuts light into the home, and keeps heat out.
What is the reason for injecting gas into double paned windows? Two types of gas are used to fill the space between panes of glass: argon and krypton. These gases are denser than air, which contributes better insulation. Krypton is even denser than argon, hence slightly more expensive.
3853-A Pickett Road
Fairfax, Virginia 22031
3000 Jefferson Davis Hwy.
Alexandria, Virginia 22305