Insulation Thickness Calculator
Insulation thickness from a target U-value.
Inputs
Result
Formula
| R = d / λ | d in m, λ in W/(m·K) |
| U = 1 / (Ri + ΣR + Re) | Ri = 0.11, Re = 0.04 |
| d = (1/U − Ri − Re − Rbase) × λcorrected | |
| λcorrected = λ × correction | moisture and ageing |
Target U = 0.45, 200 mm concrete base (λ = 1.74), insulation λ = 0.045 with a 1.2 correction: required R = 2.222, surfaces 0.15, base 0.115, so insulation must give 1.957; corrected λ = 0.054 gives 105.7 mm, rounded to 110 mm and U = 0.435.
FAQ
Why apply a correction to the conductivity?
Laboratory values are dry, at room temperature, uncompressed. In service the board absorbs moisture, gets compressed and ages, so real conductivity runs 15-30% higher. Without the correction the design U-value looks compliant while the building does not perform - one of the most common failures in energy inspections.
Is thicker insulation always cheaper to run?
No - there is an economic optimum. Material cost grows linearly while savings fall logarithmically; past a point the extra never pays back. The optimum depends on energy prices, service life and climate. In practice, meet the code minimum (a floor, not an optimum) and then optimise by payback - often 10-30 mm above the limit.
Does internal or external insulation change the calculation?
The thermal thickness is the same; the detailing and risk are not. External insulation wraps the structure, removes thermal bridges and protects the fabric - the mainstream choice. Internal insulation is easier for retrofits but costs floor area and creates bridges at slab junctions, causing condensation and mould. Cold regions should prefer external.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.