Extruded polystyrene (XPS) gives about R-5.0 per inch, or RSI 0.86 per 25 mm; a fibreglass batt gives about R-3.4 per inch.
| Material | Conductivity λ (W/m·K) | R per inch | RSI per 25 mm |
|---|---|---|---|
| Closed-cell spray foam | 0.023 | 6.27 | 1.09 |
| Polyisocyanurate board (foil-faced) | 0.024 | 6.01 | 1.04 |
| Extruded polystyrene (XPS) | 0.029 | 4.97 | 0.86 |
| Expanded polystyrene (EPS type 2) | 0.036 | 4.01 | 0.69 |
| Mineral wool batt | 0.036 | 4.01 | 0.69 |
| Open-cell spray foam | 0.039 | 3.70 | 0.64 |
| Dense-pack cellulose | 0.040 | 3.61 | 0.63 |
| Fibreglass batt | 0.043 | 3.35 | 0.58 |
| Loose-fill fibreglass | 0.050 | 2.88 | 0.50 |
| Softwood lumber | 0.120 | 1.20 | 0.21 |
| Plywood | 0.120 | 1.20 | 0.21 |
| Gypsum board | 0.160 | 0.90 | 0.16 |
| Common brick | 0.720 | 0.20 | 0.03 |
| Concrete | 1.700 | 0.08 | 0.01 |
How it is computed
RSI = thickness (m) ÷ thermal conductivity λ, and imperial R = RSI × 5.678. λ values are typical figures for each material; a product's own label or data sheet gives its tested value. Total R of a layer = R per inch × thickness.
FAQ
What is the difference between R and RSI?+
Both measure resistance to heat flow. R is in ft²·°F·h/BTU (US and Canada), RSI in m²·K/W (metric); R = RSI × 5.678.
Does doubling the thickness double the R-value?+
Yes for a uniform layer, but heat also flows through studs and joists, so the whole-wall value is lower than the insulation alone.