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Foundation Frost-Protection Insulation Depth Calculator

Calculate the rigid insulation thickness needed to achieve a target R-value for frost-protected shallow foundations.

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 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Required insulation thickness

2 in

Check your inputs

Target R-value for frost-protected shallow foundations (FPSF) depends on your local frost depth/climate zone per IRC Appendix AF or local code — confirm the required R-value for your specific location before sizing insulation thickness.

For the dimensions entered, expect a required insulation thickness of 2. Moderate confidence — sound arithmetic, but allow for the variation any real site introduces. Set for United States against IRC 2024. The market selector changes both the units and the code cited.

Add the equipment this sizes

This result is a specification — 2 in — 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

  • Required thickness = target R-value ÷ the insulation board's rated R-value per inch, the same relationship used for any rigid foam insulation thickness sizing

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Frequently asked questions

Where does the target R-value come from?
It depends on your local frost depth and climate zone, as set by IRC Appendix AF (Frost-Protected Shallow Foundations) or your local building code — it is not a single fixed value, so confirm the required target for your specific site before sizing insulation.
Why does board R-value per inch vary between products?
Rigid foam insulation types (EPS, XPS, polyiso, etc.) have different rated thermal resistance per inch — always use the specific rated R-value per inch from your chosen board's manufacturer data sheet, not a generic assumption.
Does this account for insulation placed vertically versus horizontally (wings)?
No — this only converts a target R-value into a required thickness for a single insulation layer. FPSF designs typically also require horizontal wing insulation extending outward from the foundation; size that separately per your code table.