Rating windows based on energy efficiency.
All reputable windows manufacturers rate their window's energy efficiency in one of two ways. They either provide the r-value or the u-value for each window they produce.
R-value is a unit of measurement used to compare the insulating values of one window versus another. It's calculated using Fahrenheit degrees x hours x square feet per Btu. The higher the r-value of a window the better that window is at insulating heat.
U-value is the opposite of r-value. Instead of measuring a windows insulating value, it's a measure of a windows rate of heat loss. It's expressed in Btu's per hour per square feet. The lower the u-value the less heat that is transferred through the window.
R-value and u-value can used to easily compare one windows energy efficiency versus another's as long as you keep the following things in mind.
1. You must compare values for the same size and style of window.
2. Be sure the calculation is based on the entire window including the frame on not just of the glazing.
3. Be sure the manufacturer is abiding by guidelines setforth by U.S. Department of Energy's EnergyStar program or the National Fenestration Rating Council.
Remember a high r-value or low u-value equals a good insulating window.
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