Thermal performance
Thermal resistance of a plaster or board layer
- where
- thermal resistance of the layer (m²K/W)
- layer thickness (mm)
- thermal conductivity (W/mK)
Explain this equation
Works out the thermal resistance of a single layer such as a coat of plaster, a render coat or a plasterboard. Use it to see how much (or how little) a layer contributes to a wall's insulation.
d is the thickness in mm and λ the material's thermal conductivity in W/mK; the 1000 converts mm to metres. R is the resistance in m²K/W: the bigger it is, the better the layer resists heat flow.
Heat flow through a flat layer is proportional to conductivity and inversely proportional to thickness, so resistance is thickness divided by conductivity. Doubling the thickness doubles R.
Sense check: 13 mm of dense gypsum plaster (λ about 0.43) gives about 0.03 m²K/W, and 12.5 mm plasterboard about 0.066. Compare 50 mm of PIR at roughly 2.0: ordinary plaster adds almost nothing to insulation.
Watch out: λ depends on density and moisture. Use the manufacturer's declared value where you have one.
Works out how much a single layer of plaster, render or plasterboard resists heat flow.
Enter your values to see the result.
Good to know
Typical design values (BS EN ISO 10456 / CIBSE Guide A): gypsum plaster about 0.43 W/mK at 1200 kg/m³; lightweight gypsum plaster about 0.18; plasterboard 0.19–0.25; cement–sand render about 1.0. A 13 mm coat of dense gypsum plaster adds only about 0.03 m²K/W, so ordinary plaster has almost no insulating effect: compare 50 mm of PIR at about 2.0 m²K/W.
Common mistake
Expecting a thicker coat of plaster or "thermal" paint to fix a cold wall. Even lightweight plaster contributes very little resistance compared with insulation.
Limits
One homogeneous layer with steady one-dimensional heat flow. Use the product's declared λ where available; values vary with density and moisture content.
Sources
- BS EN ISO 10456 Hygrothermal properties – tabulated design values (as reproduced in the SEAI DEAP guide: gypsum plaster 1200 kg/m³ 0.43; plasterboard 900 kg/m³ 0.25; cement–sand render 1.00; brick outer leaf 0.77; PIR 0.025 W/mK)
- BS EN ISO 6946:2017 Building components and building elements – Thermal resistance and thermal transmittance – Calculation methods (Rsi = 0.13, Rse = 0.04 m²K/W for horizontal heat flow)
- British Gypsum product data sheet: Gyproc WallBoard 12.5 mm (minimum 7.7 kg/m², λ = 0.19 W/mK)
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the manufacturers’ system and product data, BS 8212, BS EN 13914 and Building Regulations. Coverage rates vary by product, background and thickness. Check prices, VAT and CIS rules with your own records and HMRC.