R-Value & U-Value Assembly Calculator

Build a wall, roof or floor one layer at a time from a library of real materials, and watch the total R-value, U-factor and IECC climate-zone check update as you go. The interior and exterior air films are added for you, continuous and cavity insulation are tallied separately, and a section drawing shows where the resistance actually is. Below the tool, a full insulation R-value chart to size any layer.
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How the Assembly R-Value Adds Up

Heat crossing a wall passes through each layer in turn, so the resistances add in series — the same way resistors add end to end. The total R-value is simply the sum of every layer plus the two air films that cling to the inside and outside surfaces.

Per-inch vs. rated. Continuous materials — rigid foam, lumber, concrete — are rated in R per inch, so their resistance is R-per-inch × thickness. Batts and finished products carry a fixed rating for the whole layer.

Air films. A thin film of still air on each face adds real resistance — about R-0.68 inside and R-0.17 outside for a wall. The tool adds them automatically and adjusts the interior film for roofs (heat flowing up) and floors (heat flowing down).

U-factor. The reciprocal of the total, U = 1 ÷ R. It is what the code's U-factor alternative and most heat-loss math actually use — lower is better.

Cavity vs. continuous. Insulation between studs is short-circuited by the framing around it, so its real, whole-wall performance is lower than the batt's label. Continuous exterior insulation has no framing to bypass it, which is why the code rewards assemblies like R-13+5ci. This tool reports nominal R-values; a framing-factor or U-factor analysis refines them.

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Insulation & Material R-Value Chart

Nominal R-values for common building materials. Per-inch values are multiplied by thickness; fixed values apply to the whole layer as noted. Continuous-insulation (ci) products can be installed unbroken on the outside of the framing.

IECC Climate-Zone Minimums (2021 IECC, Residential)

Prescriptive insulation R-values by climate zone. Walls list a cavity value or a cavity-plus-continuous alternative (ci). A U-factor path is also permitted.
Marine zone 4 follows the zone-5 rows. Confirm the edition and amendments adopted in your jurisdiction.

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Frequently Asked Questions

‍How do you calculate a wall's R-value?
Add every layer's R-value in series, including the two air films; for per-inch materials multiply R-per-inch by thickness. The total is the assembly R; U-factor is 1 ÷ R.

‍R-value vs. U-value?
R resists heat and adds up (higher is better); U = 1 ÷ R is how much heat passes (lower is better). Codes use both.

‍What R-value does my climate zone need?
2021 IECC walls: ~R-13 (zones 1–2), R-20 or 13+5ci (zone 3), R-30 or 20+5ci (zones 4–8); ceilings R-30 to R-60; floors R-13 to R-38. Pick your zone above.

‍Why does continuous insulation matter?
Framing conducts heat around cavity batts, lowering real performance; continuous exterior board isn't bypassed, so R-13+5ci beats a higher cavity-only wall.

‍What's the R-value of a 2x6 wall?
Roughly R-22 to R-25 total with a filled cavity, drywall, sheathing, cladding and films — more with continuous insulation. Build it above for the exact figure.