Back to Basics: Everything You Need to Know about Fenestration U-factor

By Helen Sanders

As new people enter the glazing industry, reviewing some basic principles is beneficial. Let’s talk about fenestration U-factor: what it is, how it’s calculated and the implications for fenestration performance.

Understanding U-Factor

U-factor is the term for the thermal transmittance (heat flow) through a building envelope assembly which results from temperature difference between the interior-facing and exterior-facing surfaces. This heat flow is due to conduction and moves from hot to cold. In winter, heat flows from the inside to the outside of a building. In hot weather, heat flows from outside to inside.

U-factor is the amount of energy transferred per unit area of assembly, per degree of temperature difference, and per unit of time. The larger the temperature difference between the outside and inside, and the longer that the temperature difference is present, the more heat will flow. An assembly with a U-factor of 1 BTU/°², allows 1 BTU of energy to pass through 1 square foot of itself every hour for each degree of temperature difference.

The temperature difference driving the heat flow is the temperature of the window surfaces, not the ambient temperature. Due to solar heat gain, opaque frames, especially dark ones, can become much hotter than the ambient air. This increases the temperature difference experienced considerably (to 80° F in this example). This increased temperature difference is closer to that in colder climates and results in increased heat flow. Lower conductance framing with thermal barriers and warm-edge spacer can reduce this heat flow in hot climates.

Assembly vs. COG

Fenestration (assembly) U-factor, reported on NFRC performance labels and referenced by codes, is an area-weighted average of the U-factor of the center of glass (COG), the edge of glass and the frame. Because the assembly U-factor changes with frame-to-glass area ratio, declared U-factors typically are based on standard sizes (defined by NFRC 100) to allow apples-to-apples comparisons. Larger units will tend to have a lower U-factor than smaller units with proportionately more frame. It is critical not to confuse the COG U-factor with the assembly U-factor. Since the COG U-factor typically is lower than the assembly’s, it can cause a significant overestimation of envelope thermal performance if used in place of the assembly U-factor in building energy simulation.

Helen Sanders is the general manager at Technoform North America Inc. based in Twinsburg, Ohio. Read her blog each month at

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