About this calculator
Calculates the thermal transmittance (U-value) of a layered building element (wall, roof, or floor) from each layer's thickness and thermal conductivity, plus the interior and exterior surface resistances.
Formula
R (layer) = thickness (m) / lambda (W/(m·K))
R (total) = R_si + Σ R (layer) + R_se
U = 1 / R (total)
This is the ISO 6946 method for a plane building element with layers perpendicular to the heat flow direction.
Worked example
Defaults: 200 mm concrete (λ = 1.7 W/(m·K)) + 100 mm insulation (λ = 0.035 W/(m·K)), R_si = 0.13, R_se = 0.04.
R concrete = 0.2 / 1.7 = 0.1176 m²K/W
R insulation = 0.1 / 0.035 = 2.8571 m²K/W
R total = 0.13 + 0.1176 + 2.8571 + 0.04 = 3.1448 m²K/W
U = 1 / 3.1448 = 0.318 W/(m²K)
FAQ
Why are the surface resistances (R_si, R_se) plain inputs instead of fixed values? ISO 6946 Table 1 gives different standard surface resistance values depending on the element's orientation and heat flow direction (horizontal for a wall, upward or downward for a roof or floor) — this calculator doesn't ask which one applies, so it defaults to the horizontal-flow wall values (R_si = 0.13, R_se = 0.04 m²K/W) and lets you change them for a roof or floor.
Where does the thermal conductivity (λ) for each layer come from? It's a material property specific to the exact product, not a fixed constant per material name — use the value from your product's technical data sheet or declaration of performance.
Assumptions and limits
Assumes a plane element with layers perpendicular to the heat flow direction — no thermal bridging, air gaps, or cavity resistance is modelled. Surface resistances default to the horizontal-flow (wall) values from ISO 6946 Table 1; change them for a roof or floor.