If you've ever been shopping for a window, you've no doubt encountered a term that you're probably not all that familiar with – U-value. U-values are a rating of energy efficiency, and they are used to rate and compare windows, exterior doors, skylights and certain other exterior building components, including exterior walls. U-values are also the standard used in the building codes for specifying the minimum energy efficiency values for all of these components. Most people are much more familiar with R-values, which are used to rate how well a material such as insulation resists the flow of heat through it. The higher an R-value is, the more heat loss it resists, and therefore the more energy efficient it is. U-values differ from R-values in two important ways, and when you're shop...
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