Stack a wall, read its R and U, check the energy code

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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Preliminary: verify against your adopted energy code
This calculator sums nominal, center-of-cavity R-values and does not apply a framing (thermal-bridging) factor, account for fasteners or thermal bypass, or run the IECC U-factor or REScheck compliance paths. Published R-values vary by product, temperature and age. Use it to compare schemes and size insulation; confirm final compliance against the adopted code and the manufacturer's rated values.
Assembly layers (exterior → interior)
Total assembly R-value
R-27.0
U-factor 0.037 · insulation R-24.3
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Cómo 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.
MaterialR-valueBasis

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.
ZoneCeilingWood-frame wallFloor
Marine zone 4 follows the zone-5 rows. Confirm the edition and amendments adopted in your jurisdiction.

Frequently Asked Questions

Cómo do you calculate a wall's R-value?

Add the R-values of every layer in series: interior air film, gypsum board, insulation, sheathing, cladding and the exterior air film. For materials rated per inch, multiply the R-per-inch by the thickness. The sum is the assembly's total R-value; its reciprocal (1 ÷ R) is the U-factor. This calculator builds the stack for you and adds the air films automatically.

R-value vs. U-value?

R-value measures resistance to heat flow. Higher is better and R-values add up through an assembly. U-value (U-factor) measures how much heat passes through (lower is better) and is simply 1 divided by the total R-value. Energy codes state wall and ceiling requirements both ways: a prescriptive insulation R-value, or a maximum assembly U-factor as an alternative path.

Qué hacemos R-value does my climate zone need?

Under the 2021 IECC, wood-frame walls need about R-13 in zones 1-2, R-20 (or R-13+5 continuous) in zone 3 and R-30 (or R-20+5ci) in zones 4-8. Ceilings run from R-30 in zone 1 to R-49-60 in colder zones, and floors from R-13 to R-38. Pick your zone in the calculator and it compares your assembly's insulation against the minimum.

Por qué does continuous insulation matter?

Yes. Cavity insulation between studs is interrupted by framing, which conducts heat around it, so a wall's real performance is lower than the batt's nominal R-value. Continuous insulation (rigid board on the outside of the framing) is uninterrupted and is why the code offers alternatives like R-13+5ci (cavity plus continuous) that outperform a higher cavity-only number. This tool sums nominal R-values; a full check accounts for the framing factor.

Qué hacemos's the R-value of a 2x6 wall?

A 2x4 cavity (3.5 in) holds about R-13 to R-15 of batt; a 2x6 cavity (5.5 in) holds about R-19 to R-21. Adding drywall, sheathing, cladding and air films brings a filled 2x6 wall to roughly R-22 to R-25 total, and continuous exterior insulation adds directly on top. Build your exact assembly above for the precise number.

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