R-values, whole-product U-values and center-of-glass U-values can be confusing for both professionals and homeowners. It’s important to recognize that, when selecting windows and doors, the only performance criterion that should be considered is whole-product U-value.

Whole-product U-values are an indication of the performance of the product’s glass and its other components and characteristics. The thermal properties of the frame, spacer system, weather stripping and even grids, if the product has them, all have a bearing on the product’s whole-unit U-value. Whole-product U-values are the only form of measurement recognized and accepted by the ENERGY STAR program for determining whether products qualify for the ENERGY STAR label.

To arrive at a whole-product U-value, a particular model—in a specified, pre-determined size—is tested by an independent testing laboratory, and measurements are acquired from as many as 20 different locations on the product. In addition to the center of the glass, other locations include various points on the glass and around the sash and master frame.

A related—and often confusing—form of measurement is center-of-glass U-value. This measurement always appears better (i.e., a lower number) than whole-product U-values, because it’s based solely on the most thermally efficient section of the product—ONLY one spot on the glass. The U-value will be the same for that glass, regardless of the type of product the glass is in or what size that product is. It is NOT a measurement of the rest of the product, and does not reflect the thermal efficiency of the frame, spacer system and other components—only a small portion of the glass.

Another, older form of measuring a product’s thermal efficiency is R-value. R-values became well known when insulation manufacturers used them in advertising their products. Basically, R-value is the measure of the resistance of heat flow. R-values are an appropriate form of measurement for products like roof and wall insulation—but not fenestration products. R-values are typically shown as center-of-glass measurements, so they can be misleading as well.