Thermal resistance and U-values
Solid ground floor U-value (BS EN ISO 13370)
- where
- characteristic dimension of the floor (m)
- floor area (m²)
- exposed perimeter (m)
- equivalent thickness (m)
- full thickness of the external walls (mm)
- ground thermal conductivity (W/mK)
- internal surface resistance (m²K/W)
- thermal resistance of the floor construction (m²K/W)
- external surface resistance (m²K/W)
- floor U-value (W/m²K)
Explain this equation
Works out the U-value of a solid ground-bearing floor using BS EN ISO 13370, allowing for the ground under it.
Heat from a floor escapes mainly near the edges, through a short path in the ground to the outside air. B′ compares the floor area with its exposed perimeter: big floors with little perimeter lose less per square metre. d_t turns the wall thickness and floor resistances into an equivalent thickness of ground. Lightly insulated floors use the first formula; well-insulated floors (d_t at least B′) use the simpler second one.
The formulas come from the heat flow pattern in a semi-infinite ground under a slab, simplified for design use.
Sense check: a 104 m² floor with 42 m exposed perimeter on clay with about 5.7 m²K/W of floor construction comes out around 0.13 W/m²K.
Watch out: only count perimeter next to outside air or unheated space, not party walls. Suspended floors need a different method.
Works out the U-value of a slab-on-ground floor, allowing for the ground under it. Bigger floors lose less heat per square metre because most of the floor is far from the cold edges.
Enter your values to see the result.
Good to know
B′ is the floor area divided by half the exposed perimeter; count only walls to outside or unheated spaces, not party walls. dt is the "equivalent thickness": the wall thickness plus the ground conductivity times all the floor's resistances. Thin, poorly insulated floors (dt < B′) use the logarithmic formula; well-insulated floors use the simpler one. BR 443 gives ground λ of 1.5 W/mK for clay or silt (the commonest UK ground), 2.0 for sand or gravel and 3.5 for rock; Rsi = 0.17 and Rse = 0.04 m²K/W. Leave out hardcore below the slab. The default inputs reproduce a published 13370 calculation (U = 0.132 W/m²K).
Common mistake
Using the whole room perimeter in an extension or terrace, including party walls and the wall to the existing house. Only exposed perimeter counts, so a small extension often has a high P/A and a worse U-value.
Limits
Slab-on-ground without edge insulation, using the simplified BS EN ISO 13370 method. Suspended floors, basements and edge insulation have their own procedures in the standard. Thermal bridging in the floor layers should be included in Rf via the combined method.
Sources
- BS EN ISO 13370 Thermal performance of buildings – Heat transfer via the ground – Calculation methods (slab-on-ground: B′ = A/(0.5P), dt = w + λ(Rsi + Rf + Rse))
- BRE BR 443 (3rd edition, 2019) Conventions for U-value calculations (Rsi 0.13 / 0.10 / 0.17 and Rse 0.04 m²K/W; air-gap levels 0 / 0.01 / 0.04; timber fractions; ground λ clay 1.5, sand/gravel 2.0, rock 3.5 W/mK; ignore corrections under 3%)
- Back to Earth: solid ground floor U-value calculation to EN ISO 13370 (P = 42 m, A = 104 m², w = 0.30 m, λ = 1.5 W/mK, ΣR = 5.869 m²K/W, U = 0.132 W/m²K)
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.