R-value calculator for insulation and walls

R-value, insulation thickness and U-factor in one place

This R-value calculator gives the R-value of one layer, the thickness needed for a target R, the total R-value and U-factor of a wall, roof or floor, and compares two insulation materials. It starts in inches and US R-values; metric units are one click away.

How to use it

1. Pick “R-value of a layer”, “Thickness for a target R”, “Whole wall (R and U)” or “Compare two insulations”.
2. Choose a “Material”: its typical value fills “R per inch”, which you can replace with the product's rated figure.
3. For a whole assembly, set the “Assembly type (direction of heat flow)”, then list the layers from inside to outside with “Add a layer”.
4. “Copy link” keeps the calculation for later.

Conductivity, R-value and U-factor, simply

Thermal conductivity λ (W/m·K) belongs to the material: the lower it is, the better it insulates. The R-value adds the thickness. In US units it is simply R = thickness (in) × R per inch; in metric, R = thickness (m) ÷ λ. The U-factor is 1 divided by the total R of the assembly: lower U means less heat loss.

Exact examples: 6 in of insulation at R-3.5 per inch gives 6 × 3.5 = R-21. To reach R-30 with the same material you need 30 ÷ 3.5 = 8.57 in. In metric, 200 mm at λ = 0.035 gives 0.2 ÷ 0.035 = 5.71 m²·K/W, which is R-32.4 in US units (multiply by 5.678).

These are typical figures. Hollow concrete block has no true λ because its cores hold air, so the tool uses an equivalent value. Unvented air spaces use the table values of the EN ISO 6946 standard.

Whole assemblies and surface resistances

Layer R-values add up, plus two air films: the inside surface resistance Rsi and the outside surface resistance Rse. The tool uses the usual EN ISO 6946 values: Rsi = 0.13 m²·K/W for a wall, 0.10 for a ceiling or roof, 0.17 for a floor, and Rse = 0.04 (0.74 and 0.23 in US units for a wall).

Example from the tool's default wall: 1/2 in drywall (R-0.29), 5.5 in of fiberglass at R-4.12 per inch (R-22.66) and 1/2 in wood sheathing (R-0.56). With the air films, total R = R-24.5 and U = 0.041 BTU/h·ft²·°F (0.232 W/m²·K). The calculation treats the wall as continuous insulation: in a real stud wall, the studs lower the whole-wall R-value.

How thick for each insulation?

For the same R-value, thickness goes down as R per inch goes up. For R-20, wood fiber at R-3.61 per inch needs 5.54 in, while polyiso at R-6.01 per inch needs 3.33 in: 2.21 in less, about 40%.

Limits of the calculation

The tool ignores thermal bridging (studs, joists, fasteners), moisture and installation quality. Compressed batts or gaps between boards insulate less.

Code requirements and rebates: check official sources

Minimum R-values and rebate rules depend on the country, the state or province, the climate zone and the program, and they change. In the US, check with your local building department and the energy code your state has adopted; in the UK, the Building Regulations guidance on GOV.UK; in Canada, your provincial code.

Frequently asked questions

How do I calculate the R-value of insulation?

Multiply the thickness in inches by the R per inch: 3.5 in of XPS at R-5 per inch gives R-17.5.

How do I convert an R-value to a U-factor?

Take the total R of the assembly, air films included, and divide 1 by it: R-25 gives U = 0.04 BTU/h·ft²·°F.

What is the difference between RSI and R-value?

RSI is the metric value in m²·K/W used in Europe and Canada; the US R-value is in ft²·°F·h/BTU. Multiply RSI by 5.678 to get the US figure: RSI 5.71 equals R-32.4.

Does doubling the insulation halve the heat loss?

Through the insulation itself, yes, but not through the whole wall: the other layers, air films and thermal bridges stay the same, so the U-factor drops a little less than half.