How do I calculate the R-value of a wall?

Add the R-value of every layer, then add the interior and exterior air films. Each layer's R-value is its thickness in inches multiplied by the material's R per inch — 3½ inches of fibreglass at R-3.2 per inch gives R-11.2.

What is the difference between R-value and U-factor?

They are reciprocals. R measures resistance to heat flow and higher is better; U measures how much heat flows and lower is better. Windows are rated in U, opaque assemblies in R, which is why the two are rarely compared directly.

How do I convert RSI to R-value?

Multiply RSI by 5.678, or divide R by that figure to go the other way. RSI 3.5 is about R-19.9, which is why R-20 and RSI 3.5 appear as equivalents in code tables.

What R-value do I need in an attic?

Around R-60 in cold climates and R-38 in mild ones, against R-20 and R-13 for above-grade walls. Attics get the highest target because heat rises and the space is cheap to fill.

Why does framing lower a wall's R-value?

Wood conducts heat far better than insulation, so studs act as thermal bridges around the cavity. With roughly a quarter of the wall area framed, a nominal R-13 assembly typically performs like R-11 overall.

Insulation R-Value Calculator

Add up an assembly's R-value layer by layer, then see the U-factor, whole-wall figure and heat loss.

The Insulation R-Value Calculator runs entirely in your browser. Assembly details and fuel prices you enter are computed on your device and never uploaded.

Open the BTU Calculator

About Insulation R-Value Calculator

Layers of a building assembly sit in series, so their thermal resistances simply add: drywall, cavity insulation, sheathing, siding and the thin films of still air clinging to each face. The U-factor is the reciprocal of that total, and heat loss is U multiplied by area and temperature difference. What catalogue figures leave out is framing. Studs, plates and headers bypass the cavity insulation entirely, and averaging the two parallel paths is why a nominal R-13 wall performs closer to R-11 in practice. This calculator builds the stack from a material list, shows what each layer contributes, and puts a seasonal fuel cost on the result.

Features

Build an assembly layer by layer from a material list with R per inch
Insulation, sheathing, structure and finish materials included
Interior and exterior air films added, or left out for a component figure
Metric RSI and W/(m²·K) alongside imperial R and U
Whole-wall R-value averaged across cavity and framing paths
Hourly heat loss in BTU from area and temperature difference
Seasonal energy and fuel cost from heating degree days
Comparison against typical targets for attics, walls, floors and basements

How to use the Insulation R-Value Calculator

Add each layer of the assembly, inside to outside, with its thickness
Read the total R-value, RSI and U-factor
Pick the assembly type and climate to compare against a target
Enter area, temperature difference and fuel price to cost the heat loss

Example

Input

½″ drywall + 3½″ fibreglass batt + ½″ OSB + 1″ EPS

Output

R-17.10 · RSI 3.01 · U-0.0585
Whole-wall with 23% framing: R-13.9

The continuous inch of foam board is worth more than it looks, because it is the only layer the studs do not bypass.

Common errors & troubleshooting

The total is higher than the wall actually performs. — You are looking at the centre-of-cavity figure. Set a framing factor — 23 per cent is typical of 16-inch stud spacing once plates and headers are counted — and use the whole-wall number instead.
R-values from a European datasheet look tiny. — They are RSI, the metric form. One RSI is 5.678 imperial R, so RSI 3.5 is R-19.9. Both are shown here side by side.
Adding more cavity insulation barely moves the number. — Diminishing returns are real. Going from R-13 to R-20 cuts heat loss by about a third; going from R-30 to R-40 cuts it by a quarter of that. Continuous exterior insulation usually buys more than a thicker cavity.

Frequently asked questions

How do I calculate the R-value of a wall?: Add the R-value of every layer, then add the interior and exterior air films. Each layer's R-value is its thickness in inches multiplied by the material's R per inch — 3½ inches of fibreglass at R-3.2 per inch gives R-11.2.
What is the difference between R-value and U-factor?: They are reciprocals. R measures resistance to heat flow and higher is better; U measures how much heat flows and lower is better. Windows are rated in U, opaque assemblies in R, which is why the two are rarely compared directly.
How do I convert RSI to R-value?: Multiply RSI by 5.678, or divide R by that figure to go the other way. RSI 3.5 is about R-19.9, which is why R-20 and RSI 3.5 appear as equivalents in code tables.
What R-value do I need in an attic?: Around R-60 in cold climates and R-38 in mild ones, against R-20 and R-13 for above-grade walls. Attics get the highest target because heat rises and the space is cheap to fill.
Why does framing lower a wall's R-value?: Wood conducts heat far better than insulation, so studs act as thermal bridges around the cavity. With roughly a quarter of the wall area framed, a nominal R-13 assembly typically performs like R-11 overall.

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