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How to cut Kingspan by sheet thickness

Updated: October 2026

The sheet is foil-faced PIR — Kingspan, Celotex, or a merchant equivalent — and the cut has to stay square enough to jam into the bay without a draught line. How to cut Kingspan with a hand insulation saw is a thickness question first. Thin board scores and snaps. Around 75 mm the wave saw in this set reaches its retail cap and UK reviews start to split. The six listings are in the [insulation knife comparison](/hub/cutting-tools/insulation-knife). Stroke and sheath sit on [how to cut insulation](/hub/cutting-tools/insulation-knife/sub/insulation-knife-how-to-cut). Peter F. wrote this for DIY & tools. A qualifying purchase can earn a commission at no extra cost to you.

The sheet is foil-faced PIR — Kingspan, Celotex, or a merchant equivalent — and the cut has to stay square enough to jam into the bay without a draught line.

Our top picks

Trade 4 You Trade 4 You insulation saw

Best overall

Trade 4 You Trade 4 You insulation saw
5.8/10about £11.95
View offer
Everhard Everhard MK46000 insulation knife

Alternative

Everhard Everhard MK46000 insulation knife
7.4/10about £19.01
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Bahco Bahco PC-22-INS insulation saw

Rank 3

Bahco Bahco PC-22-INS insulation saw
8.8/10about £15.92
View offer
On this page
  1. 1.Under about 40–50 mm
  2. 2.Around 75 mm and above
  3. 3.Blades in this comparison
  4. 4.Thickness first

Under about 40–50 mm

For thin foil board, support the sheet, mark both faces, score along a straight edge, and snap over the bench. Under about 40 mm that is usually enough. Around 50 mm you can still finish with a sharp utility knife or a serrated insulation saw. UK reviews of the Bahco PC-22-INS still describe 50 mm Kingspan as a clean, low-dust cut when the edge is sharp. That is a review of that saw, not a licence to snap every 50 mm sheet. The friction fit matters more than the brand of knife. Approved Document L (2026) cares about continuous insulation, with most English work following from 24 March 2027. The knife does not set the U-value. A gap along the rafter does. Measure the bay, add a little, and cut on a board so the sheet does not sag. IKO’s FFP3 note for PIR cutting is manufacturer guidance, not the mineral-wool workplace exposure limit. Wear a named mask. If you only have a utility knife and a 25 mm sheet, score and snap and stop.

Around 75 mm and above

Wickes and PrimeTools cap EPS and XPS at 75 mm on the Bahco PC-22-INS. UK reviews are often fine around 50–70 mm and then talk about binding, flexing or hard work at 90 mm, 100 mm and 150 mm PIR. The Predator file is harsher: one review calls it useless on Kingspan, and foil facing blunts the edge quickly. Do not claim an easy 100 mm cut with either listing. BuildHub users have said a wavy Bahco was not great above about 50 mm. If the sheet is 100 mm Kingspan and the cut is a long square, keep a fine-tooth wood saw for that pass. The insulation blade can start the line. The blade wanders because it is thin and the foam grabs. Short strokes against a clamped straight edge matter more once the sheet is 75 mm. There is no grind angle in the files. Phenolic board is not PIR. If the old material in the roof might be asbestos, do not cut it at all.
Trade 4 You Trade 4 You insulation saw

Testsieger

Trade 4 You Trade 4 You insulation saw
5.8/10about £11.95
View offer

Blades in this comparison

The Bahco PC-22-INS at £15.92 is the 550 mm wave saw with 4.1 from 540 reviews, and it is the top pick for a long hand saw with a real UK file. Use it on PIR at or under the 75 mm retail line if you accept mixed reviews above that. The Trade 4 You YHSC, single £11.95 and two-pack £18.50, claims XPS, PIR and polystyrene and includes a scabbard. Its 5.0 from six shared ratings is not a second test; one text describes a three-pack. A sheathed serrated saw is a look if you want a cover. It is not a ranked winner. The Spear & Jackson Predator at £11.48 is the best value for fibre, not for thick foil. The Everhard MK46000 at £19.01 is a 3-5/8 in roofing knife. The 4.7 from 371 sits on United States texts about polyiso. It will not cross a 100 mm sheet in one pass. Named picks sit on the best insulation knife page.

Thickness first

If the foil board is under about 40 mm, score both faces and snap. If it is around 50 mm, a sharp knife or a serrated saw against a straight edge is the usual finish. If it is at the 75 mm EPS and XPS cap on the Bahco, expect mixed reviews and keep a fine-tooth wood saw for a long square cut in 100 mm Kingspan. If the pack is phenolic, or the old roof material might be asbestos, do not treat it as this PIR method.

Common questions

Is score and snap enough for 50 mm?
Sometimes, if the score is deep on both faces and the sheet is well supported. About 40 mm is the comfortable snap band. At 50 mm a serrated insulation saw or a sharp utility knife against a straight edge is the usual finish. Do not snap a 100 mm sheet and expect a clean face.
Why does the blade wander?
The plate is thin and the foam grabs, especially on a long freehand stroke. UK reviews of the Bahco and the Predator describe that on thick PIR. Clamp a straight edge, use short strokes, and switch to a fine-tooth wood saw when the insulation blade starts to flex.
Does foil blunt the edge?
Yes, in the review files. A Predator text says foil facing blunts it quickly. Bahco reviewers sharpen often. Hone as soon as the saw skids instead of slicing. There is no grind angle stated, so do not invent one.
Is a bread knife the same tool?
No. Community posts use a bread knife as a cleaner, harder-to-steer stand-in. The insulation saws here are long waved, toothless or serrated plates sold for wool and foam. A kitchen blade has no sheath story, no 550 mm option, and no place in the six-row table.
What mask is for PIR dust?
IKO’s FFP3 note is manufacturer guidance for PIR cutting, not the mineral-wool workplace exposure limit of 5 mg/m³ and 2 fibres/ml. For a dusty loft the FEF factsheet still points at FFP1 or FFP2 to EN 149. Wear a named mask. Do not call the cut dust-free because a review said very little dust versus a wood saw.

Related pages

Cut rock wool without dust

Insulation knife

Best insulation knife UK: loft wool or foil boards

Insulation knife buying guide: edge, length, sheath

How to cut insulation board: mark, guide, short cut

Sources

Public UK sources from the research digest. Listing prices, stars and review counts are the amazon.co.uk lines used for this comparison, checked on 19 October 2026.

  1. GOV.UK — Buying and carrying knives (retrieved 19 October 2026)
  2. Criminal Justice Act 1988, section 141A (retrieved 19 October 2026)
  3. ROCKWOOL UK mineral wool safety data sheet (retrieved 19 October 2026)
  4. FEF mineral wool health and safety factsheet (retrieved 19 October 2026)
  5. PrimeTools — Bahco PC-22-INS insulation saw (retrieved 19 October 2026)
  6. Spear & Jackson — Predator insulation saw B98INS (retrieved 19 October 2026)
  7. Centre for Sustainable Energy — loft insulation (retrieved 19 October 2026)
  8. GOV.UK — Approved Document L 2026 (retrieved 19 October 2026)