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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 in United States units and terminology, and each formula’s basis 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. Currently reading for United States — 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 — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.

Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

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.