R = 0.13 + Σ(d/λ) + 0.04; U = 1/R. One-dimensional steady-state estimate for a vertical wall. Replace example conductivities with design material data. Thermal bridges, moisture and non-uniform layers are excluded.
ZeroHouse
Wall thermal resistance
Transparent assumptions and a preliminary result.
Enter layer properties in the stated units
For each homogeneous layer, enter thickness d in metres and conductivity λ in W/(m·K). Layer resistance is d/λ, expressed in m²·K/W. Do not enter millimetres as metres: 200 mm is 0.20 m. Use a conductivity appropriate to the material and conditions; a trade name without a stated property is not sufficient input.
How the tool derives R and U
The calculator adds layer resistances and a fixed surface term of 0.17 m²·K/W, then calculates U = 1/R in W/(m²·K). This is a simplified flat-wall model, not a universal method for every assembly. The fixed term should not be transferred to a roof or floor without checking the method. An empty or inactive layer does not represent an additional physical resistance.
Interpret the exclusions
The result excludes thermal bridges, fixings, frame inhomogeneity, moisture and heat capacity. Walls with the same R can have different thermal inertia and drying behaviour. The number does not establish compliance or junction suitability. Use it to compare homogeneous layers and retain the inputs; a design decision also needs actual junctions and applicable calculation conditions. PDF download follows email entry and consent. Treat the saved calculation as a record of assumptions, not a certificate that the proposed wall is suitable for construction. Do not assign an arbitrary solid-material conductivity to an air gap; its resistance needs a method accounting for geometry and heat-transfer conditions.