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The total R-value required by your frost-protected shallow foundation design.
This target comes from your local frost depth and climate zone requirements, not a fixed universal number.
The rated thermal resistance per inch of thickness for the specific rigid insulation board.
Check the manufacturer's data sheet — common rigid foam boards range roughly from R-3.6 to R-6.5 per inch depending on the foam type.
Required insulation thickness
2 in
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.
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Foundation Frost-Protection Insulation Depth Calculator: 2 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- 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
Inputs used
- Target R-Value (Per Frost-Protection Design)
- 10
- Board R-Value per Inch
- 5
Confidence note: 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.
What this calculation does not cover
- The rated R per inch is a dry laboratory figure at a mean temperature around 75°F, and buried foam is neither dry nor at 75°F. Boards in wet ground take up water and lose resistance as they do, extruded polystyrene sheds blowing agent over decades toward a lower aged value, and polyiso — which is what the top of this input's range describes — absorbs water badly and is generally not accepted below grade at all. Choose a product the FPSF detail permits underground first, then read its R per inch off that product's sheet.
- The answer is a raw division, not a stock thickness. Foam comes in fixed thicknesses, so 3.33 in (85 mm) means two layers, and two layers only deliver the calculated figure if their joints are offset — aligned seams leave a straight thermal path through the whole assembly at every one. The portion above grade also needs a protective covering against sunlight and impact, which adds thickness the trench width and the finish detail have to accommodate.
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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.
Computed in your browser — nothing you enter is uploaded. 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.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- 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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