Insulation R-Values
Thermal Resistance
Learn R-ValuesR-value measures thermal resistance: how well a material resists heat flow. Higher R-values mean better insulation. Unlike thermal conductivity, which measures how easily heat flows, R-value measures how well heat is blocked.
What R-Value Measures
R-value is the thermal resistance of a layer of material. It tells you how strongly that layer opposes conductive heat flow for a given thickness. The relationship is straightforward: R equals thickness divided by thermal conductivity (R = L / k). A thick layer of a low-conductivity material earns a high R-value and lets very little heat pass. The reciprocal of R is the U-factor, or thermal transmittance, which states how much heat an assembly conducts.
Units and How R-Value Is Measured
Two unit systems are common, and mixing them causes most confusion around R-value. In SI, thermal resistance (written RSI) uses square-meter-kelvin per watt, m²·K/W. In US customary units, R-value uses square-foot-degree-Fahrenheit-hour per BTU, ft²·°F·h/BTU. One fixed factor links the two.
| Conversion | Multiply by |
|---|---|
| RSI to US R-value | 5.678 |
| US R-value to RSI | 0.1761 |
In the lab, R-value comes from a guarded hot plate or a heat-flow meter that holds a known temperature difference across a sample and records the heat passing through.
Worked Examples
Begin from conductivity. A typical fibrous insulation has a thermal conductivity near 0.04 W/(m·K). A 100 mm (0.1 m) layer therefore has an R-value of 0.1 ÷ 0.04 = 2.5 m²·K/W. Converting to US units, 2.5 × 5.678 gives about R-14.
The reverse works the same way. An attic rated R-30 in US units equals 30 × 0.1761, or roughly 5.3 RSI. Because R and the U-factor are reciprocals, an R-20 wall has a U-factor of 1 ÷ 20 = 0.05 BTU/(h·ft²·°F). Layers in series add up, so putting another 50 mm of the same insulation over the first 100 mm raises the total from about R-14 to about R-21.
Insulation R-Values per Inch
| Material | R-value/inch |
|---|---|
| Fiberglass batts | R-2.9 to R-3.8 |
| Cellulose (blown) | R-3.2 to R-3.8 |
| Spray foam (closed) | R-6.0 to R-7.0 |
| Spray foam (open) | R-3.5 to R-4.0 |
| Rigid foam (XPS) | R-5.0 |
| Mineral wool | R-3.0 to R-3.3 |
Where R-Values Are Applied
Builders use R-value to size insulation for each part of a building. Attics carry the highest targets, because heat rises and the roof covers a large area. Walls, floors over unheated spaces, basement walls, and crawl spaces each have their own recommended ranges, set by building codes and keyed to climate zone.
One practical caveat: the R-value printed on a batt describes the insulation by itself. Wood studs, fasteners, and gaps conduct heat around it, an effect called thermal bridging, so the whole-wall R-value usually falls below the label figure. A continuous layer of exterior insulation narrows that gap.
Conclusion
R-values guide insulation choices. Attics need R-38 to R-60 in cold climates, while walls typically call for R-13 to R-21. Total R-value is additive, so multiple layers add up. Higher-performing spray foams reach the same R-value with less thickness.