The National Fenestration Ratings Council (NFRC) has developed benchmarks for rating window performance under specific circumstances. Their designation AA refers to residential windows; BB refers to windows designed for commercial use.
- The most significant rating of NFRC is the U factor, which measures the flow of heat through window material. The lower the rating, the less heat flow. Ratings range from .10 to 1.20 (pay close attention to the decimal points).
- The Solar Gain Heat Coefficient (SHGC) measures the effectiveness of a window in blocking heat radiation from the sun. This rating is measured in increments between zero and one; the lower the rating, the less solar heat is radiated through the windows.
- Visible Transmittance, or VT, measures how much light comes through the glass. The rating is between zero and one. The higher the rating, the more light gets through. As an example, a VT rating of .92 indicates that 92 percent of the light hitting the window gets through its layers. If you're concerned about sun damage to window coverings and furniture, choose windows with a lower VT rating.
You'll notice that many replacement windows carry the Energy Star logo. This indicates that the windows meet the US Environmental Protection Agency (EPA) criteria for energy efficiency. If you want to conserve energy and reduce your heating and air conditioning costs, Energy Star windows are a good choice.