R-value
Thermal resistance of a material layer or assembly, its ability to resist heat flow; the reciprocal of U-value. Higher is better.
The R-value expresses how strongly a layer resists conductive heat flow. For a homogeneous layer it equals thickness divided by thermal conductivity. Resistances of layers in series add, which makes R-values convenient for designing insulation build-ups; the overall assembly is then converted to a U-value.
R_total = R_si + R_1 + R_2 + ... + R_se; U = 1 / R_total
US units: 1 ft²·°F·h/BTU = 0.1761 m²K/W
R (US) = R (SI) x 5.678
Example: 150 mm mineral wool at λ = 0.035: R = 4.29 m²K/W ≈ R-24 (US)
In the US, insulation products are labelled with imperial R-values per installed thickness, for example R-13 to R-21 for 2x4 and 2x6 stud walls, and R-38 to R-60 for attics in cold climates (DOE recommendations). In Europe and most of the world, SI values are used, and building codes usually specify U-values rather than R-values.
Caveats: R-values assume steady-state conduction and dry material; they ignore air leakage, convection loops in poorly fitted insulation, radiation and thermal bridging through studs or fixings. A stud wall with R-19 batts may have an effective whole-wall R-value around R-14 because of framing.
Related terms
All terms →- U-value
The thermal transmittance of a building element: heat flow in watts per square metre per kelvin of temperature difference (W/m²K). Lower is better.
- Thermal bridge
A localised area of a building envelope with significantly higher heat flow than its surroundings, caused by geometry or more conductive materials.
- Passive house (Passivhaus)
A rigorous building performance standard, developed in Germany in the 1990s, achieving very low heating demand through insulation, airtightness and heat-recovery ventilation.
- Heating degree days (HDD)
A measure of how much and for how long outdoor temperature falls below a base temperature, used to estimate and normalise building heating demand.
- Energy Performance Certificate (EPC)
A standardised rating of a building's calculated energy performance, typically on an A–G scale, required in the EU and UK when buildings are sold or rented.