EngineerHub.orgDrawing No. EH–TH–018 // Thermal Engineering & HVAC
Simplified Heat Transfer Estimator
Reviewed August 2026
Estimate steady-state heat transfer through a wall or barrier, combining convection on both faces with conduction through the material.
Drawing No. EH–TH–018 // Thermal Engineering & HVAC
Reviewed August 2026
Estimate steady-state heat transfer through a wall or barrier, combining convection on both faces with conduction through the material.
A 10 m² wall with 100 mm of insulation, still air inside and wind outside.
Rtotal = 1/hi + L/k + 1/ho U = 1/Rtotal Q̇ = U·A·ΔT
With hi = 7.7, ho = 25 W/m²K, k = 0.035 W/mK and L = 0.10 m: R = 0.130 + 2.857 + 0.040 = 3.03 m²K/W, so U = 0.330 W/m²K. Across a 25 K difference (20 °C inside, −5 °C outside) the wall loses Q̇ = 0.330 × 10 × 25 = 82.6 W.
Note where the resistance sits: the insulation contributes 2.86 of the 3.03, and the two surface films only 0.17 between them. That is why adding insulation works and why fiddling with surface coefficients does not.
Points that come up most often with this calculation.
Wind strips heat from the outer surface far faster than still indoor air does, so h is larger and 1/h smaller. On a sheltered façade the outside coefficient falls and the wall performs slightly better than this estimate.
No. This is one-dimensional conduction through a uniform build-up. Studs, joists, lintels and wall ties short-circuit the insulation and can raise the real U-value substantially — a fabric assessment is needed for a compliance figure.
The defaults are conventional values for a vertical surface with horizontal heat flow. They are approximations: real values vary with wind speed, surface emissivity and temperature difference.
No. Treat it as a screening estimate. Building-regulation U-values must be calculated to the method your national standard specifies, including bridging and correction terms.
Include the inputs and assumptions used.