The building code provides requirements for energy performance of window and other fenestration assemblies, including the maximum allowable fenestration U-factors. It can be challenging for designers to meet the U-factor requirements, when working on new and existing buildings. Understanding the energy code requirements and how to properly calculate the fenestration U-factors is an important skill to develop to inform the building-enclosure design. This article provides a review of fenestration U-factors, related code requirements, an example calculation, and other practical considerations for understanding U-factors.
The International Energy Conservation Code (IECC, 2018 referenced herein) defines U-factor (thermal transmittance) as “the coefficient of heat transmission (air to air) through a building component or assembly, equal to the time rate of heat flow per unit area and unit temperature difference between the warm side and cold side air films [(Btu/h • ft2 • °F) [W/(m2 • K)].” In other words, a U-factor defines the rate of heat loss through a component or assembly. U-factor is the reciprocal R-value, so a lower U-factor means greater resistance to heat flow and improved thermal performance.