Insulating Values

R-Value usually applies to building materials like the pink fluffy stuff in the walls of your house. Windows use another rating unit that is called, "U-Factor". You may see both these rating styles as some manufacturers use R-value because it is more widely known.

Let's explain each of them and then we will compare how the two are pretty much the same.

R-Value; Expressing numerically, the resistance to the flow of heat through a certain thickness of material. The higher the number, the better the value for use as insulation.

U-Factor; The measure of heat loss through a window. The lower the number the more energy efficient the window.

"R-Value" measures resistance to heat loss and "U-Factor" measures the amount of heat lost. Sounds the same but one is the attempt and the other is the result. The two measurements are directly correlated to each other, if you know one then you can apply an equation to find the other. The important thing to remember is that "R-value" is good to be a high number and "U-factor" is better when it is a low number. Example; an "R-value" of "3", would equal a "U-factor" of ".33".

Reducing heat transfer is critical to controlling heating costs. In many windows the center-of-glass region can have the best performance values; to be accurate and comparable the overall U-factor of the window should be used as this includes the effects of the frame material and the edge-of-glass.