Understanding consumer ratings for replacement windows

Improved energy efficiency is one of the top reasons homeowners and businesses replace their windows. The National Fenestration Rating Council (NFRC) provides consumer ratings for replacement windows to help you determine which windows are most effective.

The NFRC rating consists of five factors:

1. U-factor – The U-factor (or U-value) indicates how well a window prevents air from escaping through gaps in the sash and frame. This rating is measured between 0.20 and 1.20, and you want a U-factor of 0.35 or lower for best energy efficiency. U-factor also includes the R-value, which shows how well windows insulate heat. R-value is inversely proportional to U-factor – you want a window with an R-value of R-3 or higher.

2. Solar heat gain coefficient (SHGC) – The SHGC displays how well a window can block heat from sunlight. SHGC measures how much of the sun’s heat is released into the home. Rated between 0 and 1, the lower the SHGC, the less solar heat will be transmitted.

3. Visible transmittance (VT) – VT, expressed as a rating between 0 and 1, measures light visibility through windows. A higher VT value means more light is transmitted. This rating is most important for areas of the home where you want a lot of natural light, such as a skylight or garden window.

4. Air leakage (AL) – This refers to the amount of air that the window allows to pass through. The AL is measured in cubic feet per square foot of window area, and it indicates how much heat is lost or gained through openings in the window. AL is measured between 0 and 1, with lower AL values translating to better heat conservation.

5. Condensation resistance (CR) – The CR measures how well a window resists interior condensation. It is calculated by comparing multiple windows and seeing how well they resist condensation. CR is measured between 0 and 100, and higher CR means better resistance against condensation.

NFRC ratings are printed on labels and affixed to the window. In addition to the NFRC ratings, the label may also note whether the window is meant for residential use (marked “AA” or “residential”) or commercial purposes (marked “BB” or “nonresidential”). See the image to the right for an example of a typical NFRC label.

While the NFRC label is an effective way to gauge energy efficiency, manufacturers are not required to use it. However, we recommend that you shop for windows that include this label to guarantee the level of efficiency you need.

The one thing the label will not detail is how durable the window is. This is a key reason to look for an experienced installer when you’re ready to purchase windows. An installer can provide a clearer estimate for how long a window will last.

Choosing the right U-factor
Of the five NFRC ratings, the U-factor is the most important. In order to decide how low the U-factor needs to be for your energy-efficient windows, you should consider your particular home situation.

For instance, if you plan to stay in your home for only a few years and you live in an area where the winters don’t get exceptionally cold, you don’t need to spend extra to get the lowest U-factor. Instead, you can purchase double-pane windows with a U-factor of 0.35 or higher and still get lower energy costs.

If you intend to live in your home for at least 10 years and you face expensive heating bills, however, it’s worth spending more on windows with a U-factor of 0.35 or lower. Triple-pane windows with a very low U-factor are priced at a premium, but you’ll see the difference in your heating costs almost immediately.

Glazed custom windows for improved efficiency
Glazing is one way window manufacturers can greatly improve their U-factor of their windows. A glazed window is coated with a transparent metallic oxide that prevents heat loss. Windows can be single-glazed, with the coating on one side of the glass, or double-glazed, with coating on both the inside and the outside. It’s an inexpensive way to improve the insulating qualities of your custom windows while providing protection from harmful UV rays.

A step up from standard glazed windows is low-emissivity (or “Low-E”) windows. These windows fill the space between inner and outer panes with inert gasses such as argon, krypton, and carbon dioxide. Low-E windows provide great insulation and clear visibility year-round, but are also the most expensive.