算BuildCalc

Insulation Thickness Calculator

Insulation thickness from a target U-value.

Quick answer

With the default inputs: Target U-value 0.45 W/(m²·K) · Base layer thickness 200 · Base conductivity 1.74 W/(m·K) etc.Insulation thickness required 110 mm

Edit the inputs below to recalculate.

Inputs

W/(m²·K)
W/(m·K)
W/(m·K)
m²·K/W
m²·K/W

Result

Insulation thickness required
110mm
Required total R2.222m²·K/W
Surface resistances0.15m²·K/W
Base layer R0.115m²·K/W
Insulation R required1.957m²·K/W
Corrected conductivity0.0540W/(m·K)
Calculated thickness106mm
Rounded thickness110mm
Actual total R2.302m²·K/W
Actual U-value0.434W/(m²·K)

R = d/λ, resistances add in series, and U = 1/(Ri + ΣR + Re) with Ri typically 0.11 and Re 0.04. The correction factor accounts for moisture, ageing and compression - commonly 1.15-1.30 for external wall insulation and 1.2-1.4 for roofs. U-value limits by climate zone and element are in GB 55015 and local standards; this tool does thermal conversion only, not the energy compliance calculation.

Formula

R = d / λd 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 × correctionmoisture 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.

How to work it out

  1. Fix the target U-value from the energy code, then total resistance R = 1/U.
  2. Deduct the surface resistances: Ri 0.11 and Re 0.04 m²·K/W, 0.15 together.
  3. Deduct the substrate resistance: R = thickness ÷ conductivity. For 200 mm concrete at 1.74 W/(m·K) that is 0.115.
  4. What remains is the resistance the insulation must provide: R insulation = R total − Ri − Re − R substrate.
  5. Multiply by conductivity and the correction factor: δ = R × λ × factor. The factor covers workmanship, ageing and moisture, normally 1.2 and 1.3 for warm roofs.

Where it goes wrong

Material and cost

EPS 25-40 CNY/m² at 50 mm, XPS 40-65, mineral wool 35-60, sprayed PU 80-150 at 30 mm. A complete external insulation system with adhesive, render, mesh and finish runs 120-220 CNY/m².

On site

Basis

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.

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Insulation Thickness Calculator. 2026-10-25. https://jianzhujisuan.com/en/insulation-thickness

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.