Wall insulation & dew point

Enter layers from inside to outside. Templates supply illustrative conductivities λ; replace them with design data for the actual product and conditions. R of a solid layer is thickness in metres ÷ λ. Known layer R accepts a supplied thermal resistance directly, for example a separately determined air-space resistance. It does not assume every air gap has the same R. Arrows change the order; up to 32 layers are supported.

Total R is the sum of layer and surface resistances; U = 1/R. The example surface values Rsi = 0.13 and Rse = 0.04 m²K/W describe a simplified wall assumption and can be edited. Heat flux q = U × (indoor − outdoor temperature); positive means outward, negative inward. Temperatures drop in proportion to accumulated R. Reordering changes interface temperatures, not total U.

The indoor-air dew point uses a Magnus/Bolton approximation over liquid water: γ = ln(RH/100) + 17.67T/(243.5+T), Td = 243.5γ/(17.67−γ), temperatures in °C. At 0% humidity there is no finite dew point. Surface margin = inside-surface temperature − Td; zero or negative means the indoor surface reaches the dew threshold in this model. It is not a mould-risk assessment; frost-point behaviour is not modeled.

Do not compare temperatures deep in the wall with the indoor-air dew point to infer interstitial condensation. Vapour pressure varies across materials. This tool does not calculate vapour diffusion, moisture accumulation or drying and gives no vapour-barrier recommendation.

One-dimensional stationary, homogeneous series model: no thermal bridges, parallel studs, ventilated cavities, radiation, air leakage, transient heat or moisture effects. The displayed profile distinguishes both air temperatures from the actual wall surfaces. This is not a complete standard-compliant building assessment.