What is R Value?
R value is a measurement of window energy efficiency. Along with U factor, this measurement can tell buyers what kind of energy performance they can expect to get out of their windows. The energy efficiency of windows can vary significantly between brands and models so it is important to get a handle on these numbers and to keep them in mind as you shop. Since there are actually two different numbers it can get confusing. To keep it simple remember that windows with a high R value and a low U factor are the most efficient.
R Value and Thermal Resistance
R value measures thermal resistance, which is the ability of a material to resist heat flow. Higher R value is better because it means your window is doing a good job of preventing heat flow into or out of a home. Many newer homes are well insulated, but in older homes the insulation is lacking. Thus especially in these houses, replacement windows with a high R value are extremely important. Often these homes also have to be retrofitted with insulation blown into the walls or applied in some other way to help the home perform in a more energy efficient fashion. Even the best windows can't do it all on their own if the rest of the house is not well insulated. A single glazed window only offers an R value of around R 1, while a double glazed window delivers twice the R value. High performance glazing or triple glazing can add even more to the insulation ability of the window.
U Factor and Heat Loss
Unlike R value, U factor applies the variable of time to the measurement of heat loss, noting the amount of heat that escapes through a window per hour.