When you’re choosing a window, door, or skylight, there are many things you should consider (you know, besides how nice or expensive it is). Often you’ll find a label that looks something like this:
This label is given by the NFRC, the National Fenestration Rating Council, a non-profit that rates the performance of these fenestration products. The DOE and EPA use this rating system for their ENERGY STAR designations.
Here’s a quick summary of what these numbers mean:
Tells you how well the product insulates. Essentially, it measures to what degree the heat from inside a room that can escape through the product.
|Typical range is 0.25 to 1.25 in Btu/h·ft²·°F. Lower numbers mean better insulation and, consequently, lower heating expenses.|
|Solar Heat Gain Coefficient (SHGC)|
Tells you the measure of the fraction of solar energy transmitted – or how well the product blocks heat caused by sunlight.
|It’s a number between 0 and 1. Typically ranges from 0.25 to 0.80. The lower the number, the less solar heat can pass through the product and the better it is at keeping a room cool.|
|Visible Transmittance (VT)|
Tells you how much light can pass through.
|It is a number between 0 and 1. Typically ranges from 0.20 to 0.80. Higher means more light can pass through.|
|Air Leakage (AL)|
This number tells how good the product is keeping air out.
|Typically a number between 0.1 to 0.3 cf·m/ft², or cubic feet of air passing through one square foot of window area per minute. A lower number means less air can enter the room through this product.|
Measures how well the product resists water build-up.
|Ranges from 0 to 100. Higher means less build-up.|
These tables from Energy Star map major United States climates with recommended U-Factor, SHGC, VT, and AL values.