Building Science
U-Factor
The rate of heat transfer through an assembly per unit area per degree of temperature difference — the reciprocal of total R-value. A window's rated U-factor includes frame and edge effects, not just the glass.
In more detail
U-factor is the rate of heat transfer through an assembly, per unit area, per degree of temperature difference — Btu/(h·ft²·°F) in the US, W/(m²·K) elsewhere. It is the reciprocal of total thermal resistance, so lower is better where higher R-value would be.
It is the more useful of the two figures for whole assemblies. Because resistances in series add while areas in parallel do not, an assembly's U-factor is calculated as an area-weighted average including framing, glazing and every other path. That makes it the number that reflects thermal bridging rather than ignoring it, and the number energy codes use to set envelope requirements.
For windows the distinction is consequential. NFRC rates whole-window U-factor including the frame and the edge-of-glass region, both of which perform worse than the center of the glass. A center-of-glass figure from a glass datasheet will always look better than the window installed in the wall.
Why it matters
U-factor is how enclosure performance is actually compared, because it can be computed for a wall containing several materials or a window containing frame and glass. When a builder quotes R-value and a code official asks for U-factor, they are not answering the same question: one describes a product, the other describes what gets built. Modeling, load calculations and code compliance all run on the latter.
The common misunderstanding
That U-factor and R-value are interchangeable with a flipped sign. They are reciprocals for a single path, but assembly U-factors are area-weighted across parallel paths, so you cannot average R-values and invert the result. That shortcut consistently overstates performance, and by more as the framing factor rises.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.