Insulation Calculator
The total R-value and U-value of layers in a wall, roof or floor, the insulation thickness for a target R or U, and the rolls or packs to cover an area with the cost. SI and US units side by side.
Result…
How it's calculated
R of a layer = thickness (m) ÷ conductivity λ (W/m·K) (US: thickness (in) ÷ k) Total R = R₁ + R₂ + … (layers in series); U = 1 ÷ R R (US, ft²·°F·h/BTU) = R (SI, m²·K/W) × 5.678263 Thickness for a target = (target R − R already there) × λ Rolls = area × layers × (1 + waste %) ÷ coverage, rounded up
Adds up the thermal resistance of the layers of a wall, roof or floor: each layer's R-value is its thickness divided by its thermal conductivity (λ, or k in US units), layers in series add, and the U-value is 1 ÷ R. Results are shown in SI units (m²·K/W and W/m²·K) and US units (ft²·°F·h/BTU and BTU/h·ft²·°F), converted with the factor 5.678263 that follows from the exact definitions of the foot and the BTU. It also finds the insulation thickness that reaches a target R or U, and the rolls or packs needed to cover an area with the cost. Conductivities come from your product datasheets, and the required values come from your local building rules, which vary by country and climate.
Your numbers, step by step
About the Insulation calculator
This insulation calculator adds up the R-value of the layers in a wall, roof or floor and gives the U-value, in SI units (m²·K/W and W/m²·K) and US units (ft²·°F·h/BTU and BTU/h·ft²·°F) side by side. Enter each layer's thickness and thermal conductivity from its datasheet, plus any layers you only know by their R-value. It also works out the insulation thickness needed to reach a target R or U, allowing for the layers already there, and the rolls or packs to cover an area with the cost. Each layer's R is its thickness divided by its conductivity, layers in series add, and U = 1 ÷ R. SI and US values convert with the factor 5.678263. Required values come from building rules that vary by country and climate, so you enter your own target.
How to use it
- Choose SI or US units and what to work out: “R-value of layers”, “Thickness for a target” or “Rolls for an area”.
- For layers, list the “Layer thicknesses” and “Their conductivities” in the same order, and any “Other R-values (optional)”.
- For a thickness, pick an R or U “Target”, enter the “Insulation conductivity” and the “R-value already there”.
- For rolls, enter the “Area to insulate”, the “Coverage per roll or pack”, the “Layers of insulation” and an “Extra for waste”.
- Add a “Price per roll or pack (optional)” and read the result and the working.
Worked example
A wall with 150 mm of insulation at λ 0.035 W/m·K and 12.5 mm of board at 0.21 W/m·K, plus surface resistances of 0.13 and 0.04 m²·K/W from a local standard. R = 0.15 ÷ 0.035 + 0.0125 ÷ 0.21 + 0.13 + 0.04 = 4.286 + 0.060 + 0.17 = 4.515 m²·K/W. That is R-25.64 in US units (4.515 × 5.678263). U = 1 ÷ 4.515 = 0.221 W/m²·K, or 0.039 BTU/h·ft²·°F.
Updated
Questions people ask
How do I calculate the R-value of a wall?
Divide each layer's thickness in metres by its conductivity in W/m·K to get its R-value, then add the R-values of all the layers. In US units, divide the thickness in inches by k in BTU·in/h·ft²·°F.
How do I convert U-value to R-value?
R = 1 ÷ U in the same unit system. To move between systems, multiply an SI R-value by 5.678263 for the US value, or divide a US value by 5.678263 for SI; the R-value article gives the same factor.
How thick should insulation be?
Subtract the R-value of the other layers from your target R, then multiply by the insulation's conductivity: (5 − 0.5) × 0.035 = 0.1575 m, or about 158 mm. Round up to a thickness you can buy.
What R-value or U-value does the building code require?
It depends on the country, region, climate zone and the part of the building, and rules change over time. Check your local building regulations or energy code and enter the figure as your target.
Why can a real wall perform worse than the calculation?
Studs, joists and fixings bridge the insulation, and gaps, compression and damp lower its performance. A full method such as ISO 6946 allows for these; the heat transfer calculator shows the heat flow through a layer.
What does this insulation estimate leave out?
It adds layers in series and ignores thermal bridging, air leakage and moisture. For a building that must meet a code, or for condensation risk, ask a building professional or energy assessor.