Thermal resistance and U-values
Thermal resistance of a layer (R = d / λ)
- where
- thermal resistance of the layer (m²K/W)
- layer thickness (mm)
- thermal conductivity (W/mK)
Explain this equation
Works out how much a single layer of material resists heat flow. It is the building block of every U-value calculation.
λ (lambda) is how readily the material conducts heat: low for insulation, high for brick, concrete and metal. A thicker layer resists more, so resistance is thickness divided by λ. The thickness is entered in millimetres, so the 1000 converts it to metres. The calculator also shows 1/R, the U-value the layer would have on its own.
Heat conduction through a slab is proportional to λ and inversely proportional to thickness, so resistance is d/λ.
Sense check: 100 mm of PIR at 0.022 gives about 4.5 m²K/W; the same thickness of brick at 0.77 gives only 0.13.
Watch out: use the declared or design λ for the actual product, not a generic figure. Thin, high-λ layers such as plaster add very little.
Works out the thermal resistance of one uniform layer of insulation, board or masonry from its thickness and thermal conductivity.
Enter your values to see the result.
Good to know
Use the manufacturer's declared λ (the λD on the data sheet or the DoP) for insulation, and BS EN ISO 10456 design values for masonry, timber and plaster. Typical λ: foil-faced PIR about 0.022, phenolic 0.018–0.020, mineral wool loft roll 0.044 (Knauf Loft Roll 44), EPS 0.030–0.038, softwood 0.13, brick 0.77, plasterboard 0.19–0.25 W/mK. Many board data sheets also quote R directly, rounded down to 0.05 m²K/W.
Common mistake
Entering thickness in metres, or using an old or generic λ when the product on site has a different declared value. Halving λ doubles R just as surely as doubling thickness.
Limits
One homogeneous layer, steady one-dimensional heat flow. Not valid for air layers (use the BS EN ISO 6946 air-layer tables) or for layers bridged by timber or steel (use the bridged-layer calculator).
Sources
- BS EN ISO 6946:2017 Building components and building elements – Thermal resistance and thermal transmittance – Calculation methods (surface resistances; combined method; Annex F corrections for air voids and mechanical fasteners)
- BS EN ISO 10456 Hygrothermal properties – tabulated design values (as reproduced in the SEAI DEAP guide: gypsum plaster 1200 kg/m³ 0.43; plasterboard 0.25; cement–sand render 1.00; brick outer leaf 0.77 W/mK; softwood 0.13 W/mK as used by BR 443)
- Knauf Insulation Loft Roll 44 (Combi-Cut): λ 0.044 W/mK, 1140 mm wide; 100 mm 13.89 m², 150 mm 9.18 m², 170 mm 8.01 m², 200 mm 6.84 m² per roll (standard lengths)
- Kingspan and Celotex PIR board data sheets (declared λ typically 0.022 W/mK for foil-faced PIR; phenolic about 0.018–0.020); use the declared value on the product you are fitting
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS EN ISO 6946, BS 5250, PAS 2035, Building Regulations Part L and manufacturers’ data. Whole-house retrofit needs a qualified retrofit assessor and coordinator.