Insulation Calculator
Calculate insulation thickness and quantity to reach a target R-value.
About Insulation Calculator
Thickness follows from the R-value shortfall multiplied by the material conductivity, so a better insulator needs less depth for the same performance. Spray foam and PIR reach a given R-value in roughly 60 per cent of the thickness mineral wool needs. Where depth is constrained, such as between existing rafters, conductivity rather than price per square metre determines whether the target is achievable at all.
Formula
Insulation
Thickness = required R-value × thermal conductivity; U = 1 ÷ total R
- R-value
- Thermal resistance, higher is better
- U-value
- Heat transfer per square metre per degree, the reciprocal of total R
Worked example
With the default values below, Insulation Calculator returns Thickness Required of 157.5. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Area to insulateminimum 0 m2
- 90 m2
- Target R-valueminimum 0.1 m2K/W
- 6 m2K/W
- Existing R-valueminimum 0 m2K/W
- 1.5 m2K/W
- Materialone of Mineral wool batts, Blown cellulose, Closed cell spray foam, Rigid PIR board
- Mineral wool batts
- Coverage per pack or bagminimum 0.01 m2
- 5.5 m2
Results
- Thickness Required
- 157.5 mm
- Additional R-Value Needed
- 4.500 m2K/W
- Packs or Bags Required
- 17
- Thermal Conductivity
- 0.0350 W/mK
- U-Value Achieved
- 0.1667 W/m2K
Step by step
- 1Values used
- 2Insulation
- 3Thickness Required= 157.5 mm
- 4Additional R-Value Needed= 4.500 m2K/W
- 5Packs or Bags Required= 17
- 6Thermal Conductivity= 0.0350 W/mK
- 7U-Value Achieved= 0.1667 W/m2K
Assumptions and limits
- The result is derived only from the 5 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 4 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
How is Insulation calculated?
Thickness = required R-value × thermal conductivity; U = 1 ÷ total R Thickness follows from the R-value shortfall multiplied by the material conductivity, so a better insulator needs less depth for the same performance. Spray foam and PIR reach a given R-value in roughly 60 per cent of the thickness mineral wool needs.
Why does Insulation matter?
Where depth is constrained, such as between existing rafters, conductivity rather than price per square metre determines whether the target is achievable at all.
What values do I need to enter?
This calculator takes 5 inputs: Area to insulate, Target R-value, Existing R-value, Material, Coverage per pack or bag. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.