Why does only the edge of the slab get this, and not the whole area?

Because the edge is where the heat actually leaves. The middle of a ground-bearing slab sits over soil that has been warmed by the building for years and is a poor path to anywhere cold; the perimeter is centimetres from outside air and from the ground surface that freezes. Heat takes the short route, so the slab edge is a thermal bridge running the whole way round the building, and insulating it is worth far more per square metre than insulating the middle. That is also why this quantity follows the PERIMETER and the depth of the upstand rather than the floor area: doubling the size of the building barely changes the edge condition per metre, while a re-entrant corner or a bay adds edge without adding floor. Under-slab area insulation is a separate decision, and on a heated slab it is often the right one, but it answers a different question.

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.

Materials & Quantities

Under-Slab Perimeter Rigid Foam Board Calculator

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

Answers as you typeNo submit button
Every formula citedNamed published standards
Calculated in your browserNo account, nothing uploaded

SettingsSettings for this calculationUS

Market

Imperial · sales tax

Foundation Perimeter Length

The total length of the foundation perimeter to be insulated.

Measure the full run of slab edge or foundation perimeter where the rigid foam strip will be installed.

Standard board length

The length of a single rigid foam board as sold.

Common rigid foam boards are sold in 8 ft (2.4 m) lengths; check your specific product's dimensions.

Foam boards needed

17 boards

High confidence

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

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

Inputs used

Foundation Perimeter Length: 130 ft
Standard board length: 8 ft

Final result17 boards

What this calculation does not cover

The count divides perimeter by board length and nothing else. There is no upstand depth and no board width input, so it assumes one board covers one board length of run at the full depth required: a 2400 x 1200 mm (47 in) sheet ripped into four 300 mm (12 in) slab-edge strips actually covers 9.6 m (31 ft) of run, while a skirt deeper than the board width needs a second course and doubles the figure.
There is no waste allowance and no corner allowance in this number. It assumes every offcut is carried onto the next run, which a plan with re-entrant corners, steps and door thresholds does not allow; the related area-based foam board calculator adds 10 per cent for exactly this reason.
Where boards sit under a thickened slab edge they are a load-bearing layer, and this is a piece count that says nothing about compressive strength grade or long-term creep under sustained load. A board specified for a vertical face is not necessarily graded to bear beneath an edge beam.
Nothing here covers protecting the foam once it is in. Exterior perimeter foam is a concealed route for termites, and jurisdictions differ on whether an inspection gap or termite shield is required and whether exterior below-grade foam is permitted at all; the above-grade portion also needs render or a protection board against UV and impact, which is a separate material line.

Estimated cost — your price

Price per board ($)

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.

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

Part of a bigger job

Or plan the space itself: measure it once, doors and windows included, and this figure comes back worked out on that space, with what goes with it.

Slab planner

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 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations1

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

Cite this page

Your workspace

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.

Tools and safety for this job

To cut and fit rigid insulation board. Generic types, no brands, no prices.

Protection this work requires

Mineral and glass wool irritate skin, eyes and airways: cover your arms, wear a P2/N95 respirator while cutting, and rinse in cold water — hot water drives fibres in.
Knives cause more site injuries than any power tool: cut away from your body, change blades often, and wear cut-resistant gloves to EN 388 level C for repeated cutting.

Show the 4 tools this job needs

Essential

Tape measure
Utility knife

Recommended

Hand saw

Optional

Caulking gun

Also needed as materials: caulk and sealant cartridges, replacement blades.

what each insulation tool is for, and the spec that decides which to buy where one does.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Building the Floor of a Cold Store So It Does Not Heaveuses this calculator
The frost front under a freezer never retreats, so the insulation, the vapour barrier and the sub-floor heating grid all have to be right in one pour.
Converting an Integral Garage Into a Roomuses this calculator
Building up from a slab poured to drain: the floor stack, the infill under the old lintel, and holding the damp line unbroken across the threshold.
Installing Radiant Floor Heatinguses this calculator
A radiant floor gives up only what its surface temperature limit allows, and spacing, water temperature and finishes all follow from that cap.

Called something else where you work? Damp-proof membrane under a floor · Ground floor slab — the term in each market, how close the equivalence really is, and the standard that governs it.

How to calculate under-slab perimeter rigid foam board in 3 steps

Foundation Perimeter LengthThe total length of the foundation perimeter to be insulated.
Standard board lengthThe length of a single rigid foam board as sold.
Foam boards neededThe tool computes the foam boards needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Foam boards needed by foundation perimeter length

Page defaults, not your figures above.

Foundation Perimeter Length	Foam boards needed (boards)
100 ft	13
150 ft	20
200 ft	26
250 ft	32

Frequently asked questions

Why does only the edge of the slab get this, and not the whole area?: Because the edge is where the heat actually leaves. The middle of a ground-bearing slab sits over soil that has been warmed by the building for years and is a poor path to anywhere cold; the perimeter is centimetres from outside air and from the ground surface that freezes. Heat takes the short route, so the slab edge is a thermal bridge running the whole way round the building, and insulating it is worth far more per square metre than insulating the middle. That is also why this quantity follows the PERIMETER and the depth of the upstand rather than the floor area: doubling the size of the building barely changes the edge condition per metre, while a re-entrant corner or a bay adds edge without adding floor. Under-slab area insulation is a separate decision, and on a heated slab it is often the right one, but it answers a different question.
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.

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.