Rating Window Efficiency Systems

When looking at different models of windows you want to be cognizant of the rating that the manufacturer applies. That rating will either be a “U Value” which gives an indication of that windows tendency to transfer heat either in or out, or by an “R Value” which describes the windows resistance to transferring heat in or out.

U-Values are a measure of heat conductance

R-Values are a measure of heat flow resistance

If a window's R-value is high, it will lose less heat than one with a lower R-value. Conversely, if a window's U-value is low, it will lose less heat than one with a higher U-value. In other words, U-values are the reciprocals of R-values (U-values = 1/R-value). NOTE: Most window manufacturers use R-values in rating their windows.

Usually, window R-values range from 0.9 to 3.0 (U-values range from 1.1 to 0.3), but some highly energy-efficient exceptions also exist.

When comparing different windows, you should ensure that all U- or R-values listed by manufacturers:

• Are based on current standards set by the American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE)


• Are calculated for the entire window, including the frame, and not just for the center of the glass


• Represent the same size and style of window

1. The type of glazing material (e.g., glass, plastic, treated glass)


2. The number of layers of glass


3. The size of the air space between the layers of glass


4. The thermal resistance of conductance of the frame and spacer materials


5. The "tightness" of the installation (i.e., air leaks-- see previous discussion).

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