Back to basics: everything you need to know about fenestration u-factor

With new entrants to the glazing industry, it is beneficial to review some basic principles, such as the fenestration U-factor – what it is, how it is calculated and the implications for fenestration performance. 

U-factor is the term given to the thermal transmittance (heat flow) through a building envelope assembly due to a 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 in the other direction – from outside to inside. 

This thermal transmittance – U-factor – is the amount of energy transferred per unit area of assembly, per degree of temperature difference, per unit of time. The larger the temperature difference between 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/^oF.hr.ft², allows 1 BTU of energy to pass through 1 square foot of itself every hour for each degree of temperature difference. 

Consider a 3-by-5-foot window (15 ft² area) with a U-factor of 0.50 BTU/^oF.hr.ft² in a building in Minnesota where it is -20^oF outside and 70^oF inside. The fenestration area is 15 ft² and the temperature difference is 90^oF. Therefore, in each one-hour period, the window would allow 0.50 x 15 x 90 = 675 BTUs through it. This deficit in heat would have to be made up by the building’s heating system. 

Likewise, if the same window were in a building in Phoenix where it is 100^oF outside and 70^oF inside, in each one-hour period, 0.50 x 15 x 30 = 225 BTUs would pass through it. This heat would need to be removed by the air-conditioning system. 

But that’s not the whole story. 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 (reaching 150^oF or more). This increases the temperature difference experienced considerably (to 80^oF in this example). This increased temperature difference is closer to that seen in colder climates and results in increased heat flow. Using lower conductance framing with thermal barriers and warm-edge spacer can reduce this heat flow in hot climates. 

Fenestration (assembly) U-factor, which is 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 lower U-factor than smaller units with proportionately more frame. It is critical not to confuse the COG U-factor with assembly U-factor. Since the COG U-factor is typically lower than the assembly’s, it can cause significant overestimation of envelope thermal performance if used in place of the assembly U-factor in building energy simulation. 

USGlass Magazine November 2022