How do you add R-values of layers?

R-values in series simply add together. A 3.5 inch fiberglass batt at R-3.1 per inch (about R-13) plus 1 inch of XPS rigid foam at R-5 gives R-18 for the combined layers, before adding air films and sheathing.

What are air films and why include them?

Still air clinging to interior and exterior surfaces resists heat flow. A code calculation adds about R-0.68 for an interior surface and R-0.17 for an exterior surface. They are small but real and are included in published assembly R-values.

What is the U-factor?

The U-factor is the reciprocal of the total R-value: U = 1 divided by R. It measures how much heat passes through the assembly, so lower is better. Codes often state requirements as a maximum U-factor as well as a minimum R-value.

Does this account for thermal bridging through studs?

No. This sums a clear-wall section through the insulation only. Wood or steel framing conducts heat around the insulation and lowers the whole-wall R-value. Continuous exterior rigid foam is the usual fix and is fully counted here.

What R-value do I actually need?

It depends on the assembly and your IECC climate zone. The tool flags the relevant minimum, for example R-13 to R-21 for walls and R-38 to R-60 for ceilings across the colder zones. Always confirm the value adopted by your local code.

Insulation R-Value Calculator

A wall’s insulation performance is the sum of every layer it is built from, not just the batt in the cavity. This calculator adds the R-value of each insulation and structural layer, includes the interior and exterior air films, and tells you the total assembly R-value, its U-factor, and whether it meets code.

How it works

Each layer contributes its thickness times its R-per-inch, and all the layers and air films add in series:

layer R    = thickness inches × R per inch
total R    = Σ layer R + interior film + exterior film
U-factor   = 1 / total R

R-values add directly because the layers resist heat flow one after another. The U-factor is simply the reciprocal of the total, and codes are often written as a minimum R or a maximum U.

Example and tips

A 2x6 wall with R-21 cavity batt, half-inch OSB (R-0.5), half-inch drywall (R-0.45), and 1 inch of exterior polyiso (R-6) adds up to about R-28.6 with the air films, for a U-factor near 0.035. That easily clears the R-20 wall minimum in the milder zones. Remember this is a clear-wall figure; framing reduces the real whole-wall R-value, so continuous exterior foam, which this tool counts in full, is the most effective place to add R.