How to calculate R-value

1. What R-value actually measures

R-value is a measure of thermal resistance — how well a layer of material slows the flow of heat through it. The higher the R-value, the harder it is for heat to pass, which means less warmth escaping in winter and less heat pushing in during summer. It is the number printed on every batt, board and bag of insulation, and it is the language building energy codes use to describe how well a wall or roof has to perform.

The imperial R-value is measured in ft²·°F·h/BTU, and in metric countries the same idea is expressed as the RSI value in m²·K/W. The two are exact multiples of each other: one RSI unit equals 5.678 imperial R-value units, so an RSI-2.2 wall is an R-12.5 wall. Whenever you see a bare “R-13” batt in North America, its metric equivalent is RSI-2.3. The calculator shows both so you never have to convert by hand.

2. The formula for one layer

For a single, uniform layer the calculation is simply R = R-per-inch × thickness (in inches). Every insulation material has a published R-per-inch — a physical property of the material, not a code figure — and the thicker you make the layer, the more resistance you get. A 2-inch sheet of XPS foam board at R-5.0 per inch is R-10; a 6-inch layer of the same board is R-30. In metric, convert the thickness to inches first (divide millimetres by 25.4), or work directly in RSI using the material's thermal conductivity.

This is why two products of the same thickness can have very different R-values: closed-cell spray foam packs about R-6.5 into every inch, while a fiberglass batt manages about R-3.2 and loose blown fiberglass nearer R-2.5. When space is tight — a thin wall or a shallow rafter bay — a higher R-per-inch material earns its extra cost; where there is plenty of room, such as an open attic, a cheaper low-R-per-inch material piled deeper is often the better value.

3. R-per-inch of common materials (chart)

Typical published R-values per inch for common insulation and building materials, and the R-value each reaches at a common thickness. These are representative properties, not code minimums — real products vary by brand and density, so use your product's data sheet for a precise figure.

The pattern is clear: the foams sit at the top, the fibrous batts and loose fills in the middle, and structural materials like wood, drywall, brick and concrete are near the bottom — they carry load, not heat resistance. That is why an uninsulated masonry or concrete wall feels cold no matter how thick it is, and why the insulation layer does nearly all the work in the total.

4. Adding up a whole assembly

A real wall is a sandwich of layers, and thermal resistances in series simply add together. Work out each layer's R-value, then add the still-air films that cling to the inside and outside surfaces — about R-0.68 on the inside and R-0.17 on a wind-exposed outside, so roughly R-0.85 together. Total everything up and you have the assembly R-value; the U-value is 1 divided by that total.

Take a standard 2×4 stud wall: half-inch drywall (R-0.45), a 3.5-inch fiberglass batt (R-11.2), half-inch OSB sheathing (R-0.6), and the air films (R-0.85). Add them and the centre-of-cavity R-value is about R-13, with a U-value near 0.077. Swap the batt for a 3.5-inch layer of closed-cell spray foam (R-22.8) and the same wall jumps past R-24. One caution the calculator does not model: the wood studs themselves are only about R-4.4 and bridge straight through the insulation, so the whole-wall R-value is a little lower than the centre-of-cavity figure. For a first pass, though, the layer sum is the number everyone uses.

5. R-value versus U-value

R-value and U-value describe the same thing from opposite ends. R-value is resistance — higher is better. U-value (or U-factor) is conductance, the rate heat flows through — lower is better. They are exact reciprocals: U = 1 ÷ R. An R-20 wall has a U-value of 0.05; an R-4 window has a U-value of 0.25. Insulation is almost always sold and specified in R-value, while windows, doors and many whole-assembly energy-code targets are given in U-value, so being able to flip between them is genuinely useful.

One trap: you cannot average U-values or R-values across different areas by eye. If half a wall is R-20 insulation and half is a big R-3 window, the combined performance is much closer to the window than to the insulation, because heat rushes through the weakest path. Whole-building energy tools handle that area-weighting; for a single uniform assembly, the straight layer sum in the R-value calculator is exactly right.

6. How much R-value do you actually need?

The right R-value depends on your climate and, crucially, on the energy code where you build — which is a legal requirement, not a guideline. As a broad orientation, the US Department of Energy suggests roughly R-30 to R-60 for attics and R-13 to R-21 for wood-frame walls, with the higher end for colder zones. Treat those as a starting point only: your local energy code, climate zone map and any rebate program set the actual targets, and they change over time. This guide and the calculator tell you the R-value you have, not the one you are required to hit — always confirm the required figure with the authority having jurisdiction.

Once you know the R-value you are aiming for, the practical next step is quantity. Use the insulation calculator to turn the area and target depth into the number of batts, rolls or bags to buy, and the BTU calculator to see how better insulation shrinks the heating and cooling load a room needs. The rest of the estimating tools sit on the HVAC hub.

Common questions

How do you calculate R-value?

Multiply the material's R-per-inch by its thickness in inches to get that layer's R-value, then add the layers together and include the inside and outside air films. Total R = layer 1 + layer 2 + … + films. A 3.5-inch fiberglass batt at about R-3.2 per inch is roughly R-11; add half an inch of drywall and the R-0.85 air films and a simple stud wall reaches about R-12.5.

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

They are reciprocals. R-value measures resistance to heat flow (higher is better); U-value measures how much heat flows through (lower is better). U = 1 ÷ R, so an R-12.5 assembly has a U-value of about 0.08 BTU/h·ft²·°F. Insulation is sold by R-value; windows, doors and whole-wall energy codes are often specified by U-value.

Is a higher R-value always better?

For resisting heat flow, yes, but there are diminishing returns. R-value rises with thickness, yet doubling the insulation does not halve the heating bill because windows, doors and air leaks lose heat too. Match the R-value to your climate zone and energy code rather than simply maximising it.

How do you add up the R-values of a wall's layers?

Thermal resistances in series add together. Work out each layer's R (R-per-inch × thickness), add the still-air surface films (about R-0.68 inside and R-0.17 outside, so R-0.85 together), and total them. The calculator does this automatically once you enter each layer.

What R-value do I need for a wall or attic?

It depends on your climate zone and local energy code, so treat any figure as guidance rather than a rule. As a general guide the US Department of Energy suggests roughly R-30 to R-60 for attics and R-13 to R-21 for wood-frame walls depending on climate. Check the recommended R-values for your climate zone and the energy code in force where you build.