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Under-Slab Perimeter Rigid Foam Board Calculator

Calculate the rigid foam board pieces needed for a slab-edge/under-slab perimeter insulation strip.

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
131 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Foam boards needed

17 boards

High confidence

Running these inputs gives 17 as the foam boards needed. Currently reading for United States under IRC 2024 — pick a different market above and the figures re-cast accordingly.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

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

  • Board count = ceil(perimeter length ÷ standard board length), since slab-edge insulation is installed as a continuous vertical/under-slab strip around the foundation perimeter rather than covering a full area

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Frequently asked questions

Why is this calculated by perimeter length instead of area?
Slab-edge and under-slab perimeter insulation is installed as a continuous strip running around the foundation perimeter, not as a sheet covering the whole slab area, so the material need scales with perimeter length rather than square footage.
Why round up to a whole number of boards?
Foam boards are sold as discrete units, and a partial board still requires purchasing the whole piece. Rounding up ensures you order enough continuous strip length to complete the perimeter.
Does this account for corner cuts or waste?
No — this is the base board count from perimeter length divided by board length. Corners typically require additional cuts and some offcut waste, so ordering a board or two extra for a run with many corners is a reasonable practice.