Most people are familiar with the term “R Value” when it comes to insulation. Some people are also familiar with the term “U Value” when it comes to windows. However, most people have no idea how the two terms relate to each other. You can convert U Values to R Values pretty easily.
R values rate how much heat loss the material resists from passing through it, U-values rate how much heat the component allows to pass through it. R values rate one single material while U values measure entire components. For example R values measure how much heat loss passes through fiberglass insulation while U values rate how much heat can pass through a window component (glass, air, vinyl sash). Typical insulation might have an R value of 19 while a window might have a U value of 0.35.
Converting a U value of 0.35 to an R value is as simple as R = 1/U. So a U value of 0.35 would equal an R value of 2.86. As you can see from this example windows have a MUCH lower R value than a properly insulated wall. Most homes in the northeast are insulated with a minimum of R19 in the walls. Good windows these days boast a U value of 0.32 (or an R value of 3.13) thats approximately only 16% of the R value that’s in the wall. The lesson here is buy good windows but make sure you understand that the more area of glass you install the less efficient the total wall space will be when it comes to insulation.