The familiar NFRC sticker found on most new windows sold in the U.S. includes a number in the upper left-hand box labeled “U-factor.” For many homeowners and builders, an encounter with this sticker represents their first exposure to U-factor.
Once people understand that a window has a U-factor, they might learn that walls and ceilings can have a U-factor, too. At that point, confusion may begin.
U-factor is literally a factor — it’s part of a mathematical formula involving multiplication. (We all remember factors from the 7th grade, right?) It is part of the well-known heat loss formula used by builders to determine transmission losses through floors, roofs, and walls:
Q = A • ΔT • U
In other words, the rate of heat flow through a building assembly (in Btu/h) is equal to the area of the assembly (in ft²) times the ΔT (in F°) times the U-factor (in Btu/ft² • hr • F°).
Delta-T (ΔT) is the difference between the outdoor temperature and the indoor temperature. For example, if the outdoor temperature is 20°F and the indoor temperature is 70°F, then the delta-T is 50 F°.
In short, the U-factor was invented to make this formula work for the peculiar British (or Imperial) units we use in the U.S. It’s the number that you have to use to multiply a certain product (area times delta-T) in order to end up with a value we are interested in: Q, or the rate of heat flow in Btu/h.
The U-factor in this equation is not a constant. U-factor varies. U-factor will be relatively low for building assemblies with thick insulation (resulting in a low rate of heat flow through the assembly), and U-factor will be relatively high for building assemblies with thin insulation (resulting in a high rate of heat…