Solve backwards
How much wall will my foam insulation boards cover?
- 20 boardsThe example worked below
- Foam Board Insulation CalculatorEvery figure is its own
You have
How much foam insulation board?
Worked by the Foam Board Insulation Calculator at the settings below.
The settings it uses
- Cutting Waste (%)
- 10
What it will do
582 sq ft of wall
54.1 m²
One wall. Enough for a wall 72.7 ft long and 8 ft high.
Or several. About 1 wall of the size the Foam Board Insulation Calculator opens on: a wall 53.7 ft long and 8 ft high.
Check it forwards
- Area to Insulate
- 582 sq ft
- It asks for
- 20 sheets (4x8 ft)
- You have
- 20 sheets (4x8 ft)
The Foam Board Insulation Calculator asks for 20 sheets (4x8 ft) at that size, and 21 sheets (4x8 ft) one step larger.
If you had more, or less
Around your amount
Each worked by the Foam Board Insulation Calculator at the same settings.
Half
10 boards
291 sq ft
a wall 36.4 ft long and 8 ft high
Yours
20 boards
582 sq ft
a wall 72.7 ft long and 8 ft high
Half as much again
30 boards
873 sq ft
a wall 109 ft long and 8 ft high
Double
40 boards
1,164 sq ft
a wall 145 ft long and 8 ft high
Common amounts
What each amount of foam insulation board covers
The sizes it is usually sold in, each worked by the Foam Board Insulation Calculator at the settings above.
| You have | It covers | As one wall | Average walls |
|---|---|---|---|
| 5 boards | 145 sq ft(13.5 m²) | a wall 18.2 ft long and 8 ft high | under 1 |
| 10 boards | 291 sq ft(27 m²) | a wall 36.4 ft long and 8 ft high | under 1 |
| 20 boards | 582 sq ft(54.1 m²) | a wall 72.7 ft long and 8 ft high | 1 |
| 50 boards | 1,455 sq ft(135 m²) | a wall 182 ft long and 8 ft high | 3 |
Full boards, the market's size
The count is for the market's standard full board: 4 by 8 feet in North America, 32 square feet, and 2400 by 1200 mm in metric markets, 2.88 m². It does not cover the 2-foot-wide XPS or 1200 by 600 mm half boards some ranges are sold in, and shiplap or tongue-and-groove edges cover a little less than their face.
Bays waste more than a flat wall
A continuous layer over a flat wall with few openings cuts cleanly, and the offcut often starts the next row — the calculator's ten per cent. Boards fitted between rafters or studs, around many penetrations or on a hipped roof leave strips too narrow for the next bay, and there fifteen or more is honest, so the wall shrinks.
One layer, and a barrier over it
The answer is one layer. A staggered two- or three-layer build-up is a full-area order for each layer, so the same boards cover half or a third as much wall. Rigid foam is combustible, and building codes generally require a thermal or ignition barrier between it and an occupied space — a separate material from these boards.
How the answer is worked
Nothing here is a second formula. The Foam Board Insulation Calculator runs forwards — a size in, a quantity out — and this page grows its own starting wall in proportion until the quantity it asks for is what you have. Where it rounds up to whole containers, the answer is the largest size before the next one would be needed.
Where the calculator adds an allowance for cuts or breakage when you order, the same allowance comes off here, because it is the same calculator: a stack you already own finishes less than its face value. Where it adds none, neither does this page — the settings beside the answer say which.
Foam Board Insulation Calculator
runs it forwardsEstimate how many 4x8 ft (2400x1200 mm) rigid foam insulation sheets (XPS, EPS, or polyiso) you need.
Work out what to buy insteadSolve backwards
Other things you might already have
How big a room will my drywall sheets board?
Drywall, worked by the Drywall Calculator.
How much subfloor will my plywood sheets cover?
Plywood or OSB, worked by the Plywood and OSB Sheet Calculator (Subfloor, Wall and Roof).
How much shower wall will my backer boards cover?
Backer board, worked by the Tile Backer Board Calculator.
How big a ceiling will my plasterboard sheets cover?
Ceiling plasterboard, worked by the Ceiling Plasterboard (Gypsum Board) Sheet Calculator.
How much wall will my mineral wool boards insulate?
Mineral wool board, worked by the Exterior Mineral Wool Board Insulation Calculator.
How much roof will my cover board sheets cover?
Cover board, worked by the Roof Insulation Cover Board Calculator.
Solve backwards
Start from what is already in the yard
Paint, sheets, rolls, bags and pieces, each read the other way by the calculator that orders it.
The calculator's own figures
Nothing is estimated twice: the page runs the same calculator you would order with.
The largest size that fits
Where a calculator rounds up to whole containers, the answer stops before the next one.
Both unit systems
Gallons and litres, square feet and square metres, side by side.
Frequently asked questions
- How much wall does 20 boards of foam insulation board cover?
- At the Foam Board Insulation Calculator's own settings, 582 sq ft (54.1 m²) — a wall 72.7 ft long and 8 ft high. That is about 1 wall of the size the calculator opens on: a wall 53.7 ft long and 8 ft high. Every figure follows the settings, which you can change in the calculator.
- How many square feet does a sheet of foam board insulation cover?
- A standard 4 by 8 foot board covers 32 square feet, 2.97 m², and a 2400 by 1200 mm metric board 2.88 m², 31 square feet. With a ten per cent offcut allowance a board insulates about 29 square feet (2.6 to 2.7 m²) of a plain wall in one layer.
- Does foam board thickness change how much wall it covers?
- No — the area a board covers is its face, whatever its thickness. Thickness decides the insulation value, and two thinner layers staggered over each other cover half the wall a single layer of the same boards would. The calculator counts one layer and leaves the R-value to your climate zone.
- Can foam board insulation be left exposed indoors?
- Generally not. Rigid foam is combustible, and building codes generally require a thermal or ignition barrier — commonly half-inch (12.5 mm) drywall — between it and an occupied space. Some products are rated for exposure in specific locations; the code and the product listing decide.