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Insulation Thickness Calculator

Work out insulation thickness instantly with clear inputs, formula shown and shareable results.

About Insulation Thickness Calculator

Thermal resistance is the reciprocal of U-value, so the additional resistance needed is 1/target minus 1/existing. Thickness is that resistance times the material's conductivity — with a 0.032 W/mK board, reaching 0.18 from 1.6 needs about 160 mm.

Formula

Insulation thickness

R needed = 1/U_target - 1/U_current; thickness = R x conductivity

Worked example

With the default values below, Insulation Thickness Calculator returns Insulation thickness required of 158. Change any input above and every figure here is recalculated in your browser.

Inputs used

Target U-value
0.18 W/m²K
Existing U-value
1.6 W/m²K
Insulation conductivity
0.032 W/mK
Element area
45 m²

Results

Insulation thickness required
158 mm
Additional thermal resistance
4.93 m²K/W
Heat loss saved per degree
63.90 W/K
Annual energy saved
3,374 kWh

Step by step

  1. 1Values used
  2. 2Insulation thickness
  3. 3Insulation thickness required= 158 mm
  4. 4Additional thermal resistance= 4.93 m²K/W
  5. 5Heat loss saved per degree= 63.90 W/K
  6. 6Annual energy saved= 3,374 kWh

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is there diminishing return?

U-value is the reciprocal of resistance. The first 50 mm cuts loss dramatically; going from 150 to 200 mm changes it by only a few percent.

What conductivity should I assume?

0.038-0.044 W/mK for mineral wool, 0.022-0.028 for PIR boards, and 0.032 for phenolic. Lower conductivity means less thickness for the same result.

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