Wall U-Value and Condensation Calculator

The homogeneous-layer series-resistance approach follows EN ISO 6946:2017; the diffusion and condensation assessment follows the steady-month principles of EN ISO 13788:2012, as explained in Atkinson (2020), pp.7–17, 28 and 41–49. This implements the heat and diffusion balance, not every compliance procedure in either standard.

For each inside-to-outside layer, $R_j=d_j/\lambda_j$ and $s_{d,j}=\mu_j d_j$. Then $R_T=R_{si}+\sum R_j+R_{se}$, $U=1/R_T$ and $q=U(T_i-T_e)$. A face temperature is $T=T_i-q(R_{si}+\sum R_j)$. The Open University wall example supplies the illustrative surface resistances.

Saturation pressure in Pa is $610.5\exp[17.269T/(237.3+T)]$ for $T\geq0\,^{\circ}\mathrm C$, and $610.5\exp[21.875T/(265.5+T)]$ below zero (over ice), with numerical temperatures in Celsius. Inverting the applicable branch gives dew/frost point. Boundary pressures are RH fractions times saturation at the air temperatures.

The dry-wall pressure interpolates linearly in cumulative $s_d$. Each material is split so sublayer resistance is at most $0.25/\text{refinement}$ m²K/W. A lower convex envelope of saturation constraints and the fixed boundary pressures gives the Glaser profile. Its slope changes represent vapour sinks; $g=\delta_a(p_a-p_b)/(s_{d,b}-s_{d,a})$, and local deposition is incoming minus outgoing flux. Deposition mass is the rate multiplied by the entered duration. Positive values mean water gained, unlike the source's negative vapour-loss sign. Conservation and saturation bounds are checked.

For sizing, $d_{req}=\lambda_{selected}\max(0,1/U_{target}-R_{other})$. This is the total replacement thickness. The displayed moisture profiles remain those of the entered wall.

One-dimensional, steady heat and vapour diffusion through 1–5 homogeneous solid layers. Layers are dry initially; properties stay constant. Surface films have thermal resistance but no vapour resistance. Air permeability is fixed at $\delta_a=2\times10^{-10}$ kg/(m·s·Pa). Atkinson's example vapour resistivities are converted to dimensionless μ using this value: μ = 0.2ζ when ζ is expressed in MN·s/(g·m).

The material selector contains selected worked-example inputs from Atkinson pp.23 and 41, with each row's source shown. They are illustrative, including unusually insulating blockwork; replace them with product-specific design data for a real wall. No standards table is reproduced.

RH is relative to liquid water at nonnegative temperatures and ice below zero. There is no annual carry-over, evaporation from stored water, moisture-dependent conductivity, latent-heat coupling, rain, air leakage or liquid transport. Surface condensation invalidates the fixed-vapour-pressure boundary assumption and holds back the internal-deposition results. The 80% inside-surface RH message is a screening flag, not a mould-growth prediction.

Use for planar, unbridged solid wall paths and a constant or monthly mean climate, between −40 and 60 °C (indoor range −30 to 60 °C), with positive layer thickness and μ ≥ 1. Internal calculations use SI units; Celsius/Fahrenheit entry is explicit because the catalog unit switch does not convert offset temperatures. °R in the US thermal results denotes a temperature interval equal to one °F.

Sublayer calculations approximate a continuous saturation curve. Increase mesh refinement and compare deposition near a threshold; there is no single universal “dew-point depth”. The interior/exterior face temperatures are distinct from air temperatures; surface-film drops appear in the calculation steps, not as fictitious material thicknesses on the charts. Layers in the section have scaled physical widths and a schematic height; very thin membranes may be hard to see.

For a thin membrane with known sd, custom mode may use its finite thickness with μ = sd/d and an appropriate conductivity; the thermal treatment is approximate. Unventilated air gaps, ventilated cavities, repeated studs, ties, mortar paths, metal bridges, ground contact and walls with leakage are outside this tool's homogeneous-solid scope. A full annual Glaser assessment needs monthly climates, stored-condensate tracking and drying checks; use a transient hygrothermal model where storage, rain or capillarity matters. The U target is a design input, not a code limit. Check against the standard in force and local requirements. Installation work belongs to a qualified person.