SettingsSettings for this calculationUS
The standard allowance most suppliers and estimating guides assume for ordinary work.
The elevation being wrapped, with window and door openings taken out.
Deduct openings here rather than leaving them in as a safety margin. Continuous insulation runs past the sheathing over a whole elevation, so an un-deducted patio door is a real over-order, and the waste allowance below is meant for cutting rather than for windows you decided not to subtract.
Tools needed: Tape measure, Elevation drawing or a sketch with opening sizes
The long dimension of one board, taken off the pack.
Read it from the product rather than assuming a standard: unlike rigid foam, semi-rigid mineral wool board has no single market face size, and the ranges differ between density grades from the same manufacturer.
The short dimension of one board.
The two dimensions are multiplied, so it does not matter which you call length and which width. What matters is that both come from the same product you are ordering.
How many thicknesses of board go on, one behind the other.
Two thinner layers with staggered joints outperform one thick layer of the same total depth, because the joints in one layer are covered by the body of the other. Each layer is a full-area order — the offcuts from one cannot make up the next, since staggering is exactly what stops their joints lining up.
Extra boards for cutting, corners and damage in handling.
10% suits a plain elevation cut with a serrated blade. Raise it for a wall broken up by openings, for many external corners where boards are cut to interlock, and for the lower density grades, which are easier to crush at the edge while fixing than rigid foam is to snap.
Mineral wool boards needed
60 boards
- Face area of one board
- 8 ft²
- Area over all layers
- 430 ft²
- Area including waste
- 473 ft²
They open the calculator with your figures already in it
Exterior Mineral Wool Board Insulation Calculator: 60 boards — 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)
- Board count = area x layers x (1 + waste) / the face area of one board, rounded up — the face area being measured from the product rather than assumed, because semi-rigid mineral wool board is sold in several face sizes that differ by manufacturer and density grade
- A multi-layer build-up is a full-area order per layer: staggering the joints between layers means neither layer can be made up from the other's offcuts
Inputs used
- Area to Insulate
- 430 sq ft
- Board Length
- 48 in
- Board Width
- 24 in
- Number of Layers
- 1
- Waste Factor (%)
- 10
Intermediate steps
- Face area of one board
- 8 ft²
- Area over all layers
- 430 ft²
- Area including waste
- 473 ft²
What this calculation does not cover
- This counts boards to cover an area. It does not choose a thickness, and thickness is the decision that matters most on an exterior wall: the board has to be thick enough to hold the sheathing behind it above the dew point of air reaching it from inside, and a layer that is too thin makes the wall wetter than no exterior insulation at all rather than merely less efficient.
- Fixings are not counted or sized here, and they are not an afterthought on this assembly. Fastener length has to cover the board thickness plus the furring and still reach its embedment in the structure, and mineral wool is compressible, so over-driving a fixing dishes the board and loses the very continuity it was installed for. The Continuous Insulation Fastener Count Calculator handles that.
- The waste allowance is a flat uplift, not a cutting layout. Boards are cut to interlock at external corners and trimmed around every opening, and the offcut from one window head is rarely the size the next one needs — a broken elevation generates far more waste than a plain one of the same area.
- Nothing but the boards is included: no furring or Z-girt to carry the cladding, no fixings or washers, no membrane, and no flashing or reveal detailing at heads, sills and jambs, all of which get harder as the insulation gets thicker because the cladding has moved further off the wall.
- The board is treated as flat area only. Reveals into window and door openings, soffit returns and the wrap around a projecting element are additional pieces this does not add, and cutting them from the deducted opening area is not always possible once the reveal depth exceeds the offcut.
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.
Part of a bigger job
This trade is one line of a job takeoff. Run the whole job and every other trade comes back with it, off the same measurements.
also measures10 other trades
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 2026-09-05 · in the site-wide review of 2026-09-06 · v1.0.0
Regulatory standards & verification citations2
- Board count = area x layers x (1 + waste) / the face area of one board, rounded up — the face area being measured from the product rather than assumed, because semi-rigid mineral wool board is sold in several face sizes that differ by manufacturer and density grade
- A multi-layer build-up is a full-area order per layer: staggering the joints between layers means neither layer can be made up from the other's offcuts
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 needsHide tools
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.