Thermal performance
U-value of a wall lined with insulated plasterboard (BS EN ISO 6946)
- where
- total thermal resistance (m²K/W)
- internal surface resistance, 0.13 (m²K/W)
- masonry thickness (mm) and conductivity (W/mK)
- external render thickness (mm) and conductivity (W/mK)
- air gap resistance behind the lining (m²K/W)
- insulation thickness (mm) and conductivity (W/mK)
- plasterboard thickness (mm) and conductivity (W/mK)
- external surface resistance, 0.04 (m²K/W)
- thermal transmittance (W/m²K)
Explain this equation
Estimates how much heat passes through a wall, in watts per m² per degree of temperature difference, before and after lining it with insulated plasterboard. Use it to compare boards and thicknesses when quoting.
Each layer's resistance is its thickness d divided by its conductivity λ; the 1000 converts mm to metres. Rsi and Rse are the resistances of the thin air films on the inside and outside faces, 0.13 and 0.04 for walls. Rg is any still-air gap behind dot-and-dab board. R is the total and U is its reciprocal.
Layers in series add their resistances just as resistors in series do, so the total resistance is the sum and the heat flow per degree is one over that sum. Insulation dominates because its λ is around 30 times lower than masonry.
Sense check: 215 mm solid brick with render and plasterboard is about 1.9 W/m²K. Adding 50 mm of PIR at 0.025 adds 2.0 m²K/W and brings U down to about 0.39. Doubling the insulation roughly halves the U-value of a well-insulated wall.
Watch out: fixings, battens and dabs bridge the insulation, so a full BS EN ISO 6946 calculation gives a slightly higher U-value.
Estimates the U-value of a solid wall before and after lining it with thermal laminate (insulated plasterboard), by adding the resistance of each layer.
Enter your values to see the result.
Good to know
Each layer adds R = d/λ (thickness in metres divided by conductivity). Surface resistances for walls (horizontal heat flow) are Rsi = 0.13 and Rse = 0.04 m²K/W from BS EN ISO 6946. Defaults: brick outer leaf 0.77, cement–sand render 1.00 (BS EN ISO 10456 / CIBSE values), PIR 0.025 (generic; use the board's declared λ), Gyproc WallBoard 0.19 W/mK. For a laminate with a declared R-value (e.g. ThermaLine PIR 38 mm, R = 1.15), you can enter the board as one layer whose d/λ equals that R. A sealed unventilated gap behind dot-and-dab adds roughly 0.11–0.18 m²K/W depending on width (ISO 6946 air layer table); leave it at 0 to be conservative. Check the target U-value for the work in Approved Document L.
Common mistake
Typing thickness in metres into the mm box, or using a manufacturer's "aged" λ for one layer and a generic value for the same material elsewhere. Also forgetting that mechanical fixings through the insulation raise the real U-value.
Limits
Simple layer sum (upper-limit method for uniform layers). Ignores repeating thermal bridges such as timber battens or plaster dabs, mechanical fixing corrections and air-gap corrections in BS EN ISO 6946; a full calculation to BR 443 is needed for Building Regulations submissions. Condensation risk (interstitial) is not assessed.
Sources
- 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)
- BRE BR 443 (2019) Conventions for U-value calculations
- 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)
- British Gypsum product data sheet: Gyproc WallBoard 12.5 mm (minimum 7.7 kg/m², λ = 0.19 W/mK)
- British Gypsum product data sheet: Gyproc ThermaLine PIR 38 mm (R = 1.15 m²K/W, minimum 9.2 kg/m²)
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.