Floor-layer R, nominal U and heat flow
Enter slab, insulation and screed thicknesses and conductivities, chosen surface resistances, area and temperature difference. This homogeneous series model calculates R, its reciprocal U and steady heat flow; it does not model ground heat transfer.
Calculator: illustrative inputs — Floor-layer R, nominal U and heat flow
Layer method and example
Rtotal = Rsi + Σ(d [m] / λ) + Rse; U = 1/Rtotal; Q = U × A × ΔT
The displayed inputs are examples: 120 mm at λ = 1.70, 150 mm at 0.035, 60 mm at 1.20, plus Rsi = 0.17 and Rse = 0.04 give R ≈ 4.6163 and U ≈ 0.2166. At 50 m² and 20 K, heat flow is about 216.62 W. Replace all conductivities and surface terms with values justified for your case. Millimetres are converted to metres before division.
Ground and thermal bridges
A slab on ground also depends on perimeter, geometry, soil and boundary conditions. ISO 13370 addresses ground heat transfer; this simple reciprocal is not that calculation. Floors over outside or unheated air have different boundary conditions. Framing, edges, fixings, gaps and moisture are outside this model. It does not prove compliance or determine load-bearing capacity.