Building Science
R-Value
A measure of resistance to conducted heat, per unit area. Product R-value describes a material in isolation; the assembly containing it almost always performs worse, because framing and gaps conduct around it.
In more detail
R-value measures resistance to conducted heat per unit area — ft²·°F·h/Btu in the US, RSI in m²·K/W elsewhere. It is a laboratory property of a material at a stated mean temperature, and in the US the FTC's R-value Rule governs how it may be advertised.
Two figures are routinely confused. A product's R-value describes the material in isolation. An assembly's R-value describes the wall or roof containing it, including the framing that conducts around the cavity, the sheathing, the cladding, and any gaps or compression from installation. Because framing typically occupies a fifth or more of a wall's area, whole-assembly performance is regularly well below the nominal cavity figure.
R-value also covers conduction only. It says nothing about air leakage, which can move more heat than conduction in a leaky assembly, nothing about radiant exchange at surfaces, and nothing about thermal mass. Some materials' R-value per inch varies with temperature, so a rated figure and cold-weather performance are not always the same number.
Why it matters
R-value is the number in every insulation conversation and the one most likely to be compared between things that are not comparable. A specification naming a cavity R-value without addressing framing factor, air sealing and continuity has described one input to comfort rather than the outcome. Whole-assembly U-factor is the figure that predicts what the house will actually do.
The common misunderstanding
That doubling R-value halves heat loss through the assembly. Returns diminish: the same increment matters less as total resistance rises, and once conduction is well controlled, air leakage and thermal bridging usually dominate what remains. Money spent past a certain R-value would often do more elsewhere in the enclosure.
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.