How do you cut celotex straight without it crumbling everywhere?
Cutting rigid foam insulation freehand is a fight nobody wins. The knife wanders, the saw shreds the foil, and you're left with a wavy edge and a floor full of yellow crumbs. Shane Green got sick enough of it to build a jig. When you know, you know.
Applies to
- Process
- Cutting rigid foam insulation to width
- Materials
- Celotex / Kingspan / PIR board, foil-faced foam
- Setup
- Two timber rails as fence + guide, a knife or fine-tooth saw
- Best for
- Repeated rip cuts to the same width between joists or studs
Why it matters
Foam board has no resistance, so a freehand blade follows the path of least effort, not your line. A fence gives the board a hard edge to sit against and the rail gives the blade a wall to run down — the jig does the straightness so your hand doesn't have to.
Cutting to a repeated width between joists is the whole job on an insulation run. Set the fence once and every piece comes off the same, instead of measuring and wandering fifty times.
What good looks like
- The cut edge is straight and square through the full thickness
- Every piece off the jig is the same width without re-measuring
- The foil face scored clean, not torn
How to apply it
- Build a flat base with a fence set at your cut width and a straightedge rail to guide the blade.
- Drop the board against the fence so it can't shift.
- Run a sharp knife or a fine-tooth saw down the rail in one pass — score the foil first if you're knifing it.
- For a different width, move the fence, not your aim.
Exceptions & tradeoffs
Thick, high-density PIR is better sawn than knifed — a knife bogs and wanders halfway through. Fine-tooth handsaw, or a tracksaw with dust extraction, because the dust is horrible.
Foam dust and offcuts get everywhere and the foil edges are sharp. Cut over a drop sheet and wear a mask.
Common mistakes
- Freehanding it because "it's only insulation"Gappy insulation is wasted insulation — the jig makes the cut that closes the gap
- One deep knife pass through thick boardScore, then deepen; one heavy pass wanders and tears the foil
A shop-made timber jig with a fence and guide rail for cutting rigid PIR insulation board straight and repeatably — We all hate cutting celotex so I've made a jig ↗ at 0:08–0:28.
Summarized and independently edited by Start Here from Shane Green Build's Facebook clip. I read the jig off the frames — timber rails, board captured, cut along the guide — with no audio, so the exact fence geometry is my reading and the dust and density cautions are mine. Watch it before you build to it.