Do you know how your windows perform?

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:

Rating Values
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.
Condensation Resistance
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.



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