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How to fit insulation boards

Updated: 23 October 2026

You have a knife, a board, and a floor or a shed wall that still looks unfinished. How to fit insulation boards depends on which job you are actually doing. A timber shed lining is a cut-and-fix afternoon. A solid ground floor that has to meet an energy limit, or a shower with an electric mat, is where the work stops being a knife job. The five listings in the [insulation slab comparison](/hub/diy-supplies/insulation-slab) are small EPS sheets, a 10 mm XPS board, and two foam backers. These boards were not fitted in a lab. A purchase through our links can earn a commission at no extra cost to you.

You have a knife, a board, and a floor or a shed wall that still looks unfinished. How to fit insulation boards depends on which job you are actually doing. A timber shed lining is a cut-and-fix afternoon.

Our top picks

Triplast Triplast EPS 70 50 mm six-pack

Best overall

Triplast Triplast EPS 70 50 mm six-pack
7.4/10about £47.59
View offer
Thermopanel Thermopanel 15 mm tile backer

Best value

Thermopanel Thermopanel 15 mm tile backer
7.2/10about £15.99
View offer
EkoBoard EkoBoard 10 mm XPS underlay

Alternative

EkoBoard EkoBoard 10 mm XPS underlay
6.2/10about £7.99
View offer
On this page
  1. 1.A shed lining is not a house floor
  2. 2.Cut square, and deal with the beads
  3. 3.Joints, fixings, and keeping the board dry
  4. 4.Stop and get advice

A shed lining is not a house floor

On a shed or workshop wall, United Kingdom reviews of the Triplast EPS sheets describe a light board you cut and place, sometimes with wallpaper paste, sometimes between timbers. There is no fixing system in the box. You are lining an outbuilding, not producing a U-value. On a solid house floor the order changes. The PIR floor note of November 2024 describes a damp-proof membrane, often 1200 gauge, lapped and turned up, then boards in a staggered bond. That method is not for a Thermopanel backer or a 10 mm XPS sheet. GOV.UK puts the membrane down first, and it flags lime or earth floors, where you should ask before you close them in.

Cut square, and deal with the beads

Cut the board so the edge meets the next piece without a finger gap. Thermopanel and EkoBoard reviewers use a Stanley knife. On Triplast EPS the cut throws beads, and the sheet can snap. Sweep as you go. A crumbled edge left in place is a gap. Do not treat a hot wire as the method. HSE notes treat hot-wire cutting of EPS as a fire risk, and HSG92 says cutting dust should not build up. Bag the offcuts, and do not burn them. One Thermopanel review says wear gloves, because the material field only says Foam.
Triplast Triplast EPS 70 50 mm six-pack

Testsieger

Triplast Triplast EPS 70 50 mm six-pack
7.4/10about £47.59
View offer

Joints, fixings, and keeping the board dry

On a PIR floor, lay boards in a staggered bond and leave no gaps for the screed. Tape the membrane laps, and tape a tile backer when reviewers sealed it before tiling. Do not call a taped 10 mm XPS edge that method. The membrane and the U-value stay on the floor guide. Thermopanel’s box has no adhesive, washers, screws, tape, or heating mat. One shared review says four of ten boards arrived damaged, so check the edges before you tile. The wet-area line names no tanking standard. Store boards dry. A foil PIR datasheet that says do not use wet boards is not a test of the EkoBoard sheet.

Stop and get advice

Call a professional when a floor must hit a U-value, when an external wall might sit over 11 m, when electric underfloor heating is a system, or when the element is a flat roof. None of these five listings is that design. GOV.UK lists insulation under timber floors among work that does not need an application. LABC says replacing more than half of a ground-floor thermal element can need one. Do not read the timber-floor line as permission to strip a solid floor. Ask your building control body before you lift it. Choose the material on the buying guide before you cut.

Common questions

Which way up does the board go, and what goes under it on a solid floor?
On a normal solid floor, the usual pattern in the GOV.UK best-practice note and the PIR floor guidance is a damp-proof membrane under the insulation, with the membrane lapped, taped, and turned up. Foam boards in this set are not a reason to skip that membrane. XPS that a manufacturer says may go below the membrane is a claim about that specified board, not about the 10 mm EkoBoard, which prints no such line. Ask before you insulate a lime or earth floor.
How do I cut EPS without filling the room with beads?
You will not get a bead-free cut from these Triplast sheets. Reviewers say the beads come off and the board is fragile. Use a sharp knife, cut on a board you can sweep, and keep the edge square so the next sheet butts up. A crumbly edge left in place is a gap. Bag the debris rather than letting it drift across the floor.
Can I use a hot wire on these sheets?
Do not take a craft hot-wire table as a fitting method. HSE notes treat hot-wire cutting of EPS as a fire risk, and cutting dust from cellular plastics should not be left to build up. A knife is what the tile-backer and underlay reviews describe. Do not burn the offcuts when you are finished.
What is not in the Thermopanel box?
The board is what you get. Adhesive, washers, screws, tape, and the heating mat are not listed. Reviews describe buying the fixings separately and cutting with a Stanley knife. The wet-area claim does not name a tanking standard. If the picture looked like several sheets, the listing that matches these boards is one board, 0.75 m².
Do I tape the joints?
On a PIR floor build-up, the membrane laps are taped, and the boards are laid staggered with no gaps. On a tile backer, reviewers tape and seal before they tile. That is not a universal instruction to tape every 10 mm XPS edge as if it were a foil PIR system. Tight joints matter more than a decorative strip of tape.
When should I stop and call building control?
Before you replace a large part of a ground floor. GOV.UK lists insulation under timber floors among work that does not need an application. LABC says replacing more than half of a ground-floor thermal element can need approval. Ask your building control body which side your job is on. A shed lining is not that question, and a U-value floor, a tall external wall, an electric heating system, or a flat roof is a professional job.

Related pages

Best insulation boards

Insulation slab

Insulation board buying guide: nation, then lambda

Insulation boards for floors: why 10 mm is not 0.18

PIR vs EPS insulation boards: thinner, then fire class

Sources

Public UK sources and the amazon.co.uk listings compared on this page. Prices are the amazon.co.uk figures stored on 21 October 2026. Updated 22.10.2026.

  1. Approved Document L 2026, GOV.UK (retrieved 22.10.2026)
  2. Scottish building standards handbook 2022, section 6.2 (retrieved 22.10.2026)
  3. Ban on combustible materials, GOV.UK (retrieved 22.10.2026)
  4. Circular letter 02/2022, combustible materials (retrieved 22.10.2026)
  5. Recticel Eurothane GP datasheet (retrieved 22.10.2026)
  6. Polyfoam XPS: ground floor insulation (retrieved 22.10.2026)
  7. IMA: PIR floor insulation guidelines, November 2024 (retrieved 22.10.2026)
  8. HSE cellular plastics, HSG92 (retrieved 22.10.2026)