R Value Calculator

Build up a wall, roof or floor layer by layer — insulation, sheathing, drywall, siding and air films — and the calculator totals the R-value, converts it to a U-factor and metric RSI, and estimates heat loss through the area for your temperature difference.

What R-value means

R-value measures resistance to heat flow through a material or assembly. The higher the R-value, the better it insulates. In the US, R-values are in ft²·°F·h/BTU; in metric countries the equivalent RSI is in m²·K/W (multiply R by 0.1761 to get RSI). For layers stacked in series — drywall, insulation, sheathing, siding — the R-values simply add up. The U-factor is the inverse of the total R-value and tells you how many BTUs per hour pass through each square foot for each degree of temperature difference.

How to calculate the R-value of a wall

1. List every layer from inside to outside, including the still-air films on each surface.
2. For each material, multiply its R-value per inch by its thickness, or use its fixed R-value.
3. Add the layers to get the total R-value through the insulated part of the wall.
4. If you want the whole-wall value, account for studs or joists, which conduct more heat than insulation (thermal bridging).
5. Take the inverse to get the U-factor: U = 1 ÷ R.

Heat loss through the assembly is U × area × temperature difference, in BTU per hour. This is the conduction part of a heat loss calculation; air leakage and windows are separate.

R-values of common materials

These are typical values from building science references and manufacturer data; actual products vary. Use the R-value printed on the insulation product where you can. Rigid foam and spray foam values vary by product and some decline slightly as the foam ages.

Worked example: 2×4 wall

A typical 2×4 wall has ½ inch drywall (R-0.45), 3½ inches of fibreglass batt (R-11.2), 7/16 inch OSB sheathing (R-0.55), vinyl siding (R-0.61), plus the inside air film (R-0.68) and outside air film (R-0.17). Through the insulation the total is about R-13.7, a U-factor of 0.073. With about 25% of the wall area taken by studs, plates and headers (wood about R-4.4 through a 3½ inch stud), the effective whole-wall R-value drops to roughly R-10. For 1,000 square feet of wall at a 58 °F temperature difference, that is about 5,800 BTU/h of heat loss.

Thermal bridging and continuous insulation

Studs, joists and rafters interrupt cavity insulation and conduct heat more easily. The fraction of a wall that is framing — often 20–25% for 16-inch stud spacing once plates, headers and corners are counted — lowers the whole-wall R-value noticeably. Continuous insulation (rigid foam or mineral wool boards over the sheathing) covers the framing and reduces thermal bridging, which is why energy codes in colder climates often call for cavity insulation plus continuous exterior insulation.

Recommended R-values

These are broad ranges reflecting typical US guidance; exact requirements depend on your climate zone and the edition of the energy code adopted locally. Check the International Energy Conservation Code (IECC) tables for your zone, ENERGY STAR recommendations for existing homes, or your local building department.

Improving an existing wall or attic

• Attics are usually the cheapest place to add R-value: blow cellulose or fibreglass over the existing insulation after sealing air leaks.
• Walls can be dense-packed with cellulose through small holes, or upgraded with exterior rigid foam when re-siding.
• Floors over crawl spaces or garages can take batts or spray foam between joists.
• Air sealing often saves as much energy as extra insulation, because moving air bypasses insulation.

Limitations of R-value

R-value describes conductive heat flow under test conditions. It does not account for air leakage through gaps, moisture in insulation, installation defects such as compressed or gapped batts, or radiant heat transfer, which radiant barriers address differently. A well-installed, air-sealed assembly performs closer to its rated R-value than a poorly installed one with a higher label.

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