Insulation boards for floors
Updated: 23 October 2026
You want a board you can carry under a ground floor, a solid slab, or an electric heating mat. Insulation boards for floors are judged on the whole floor. In England and in Scotland the figure you will see quoted is 0.18 W/(m²·K). A 10 mm underlay can change how a mat feels in the morning. It does not, by itself, meet that limit. The five rows in the [insulation slab comparison](/hub/diy-supplies/insulation-slab) are the boards this page sorts. This is not a calculation of your floor. Prices are amazon.co.uk figures from 21 October 2026. A purchase through our links can earn a commission at no extra cost to you.
You want a board you can carry under a ground floor, a solid slab, or an electric heating mat. Insulation boards for floors are judged on the whole floor.
You want a board you can carry under a ground floor, a solid slab, or an electric heating mat. Insulation boards for floors are judged on the whole floor. In England and in Scotland the figure you will see quoted is 0.18 W/(m²·K). A 10 mm underlay can change how a mat feels in the morning. It does not, by itself, meet that limit. The five rows in the insulation slab comparison are the boards this page sorts. This is not a calculation of your floor. Prices are amazon.co.uk figures from 21 October 2026. A purchase through our links can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Thickness | Lambda | Job | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Triplast Triplast EPS 70 50 mm six-pack | 50 mm | 0.030–0.038 W/mK | Wall and floor EPS | 7.4 | about £47.59 | View offer |
| Best value |  Thermopanel Thermopanel 15 mm tile backer | 15 mm | 0.027 W/mK | Wet-area tile backer | 7.2 | about £15.99 | View offer |
| Alternative |  EkoBoard EkoBoard 10 mm XPS underlay | 10 mm | n/a | Floor underlay | 6.2 | about £7.99 | View offer |
On this page
The floor limit is 0.18, and 10 mm is not that calculation
England’s Approved Document L 2026 prints 0.18 W/(m²·K) for a new-dwelling floor and for a new floor in an existing dwelling. Those changes take effect on 24 March 2027. Scotland’s Table 6.3 also prints 0.18 for a floor. Wales and Northern Ireland have their own documents.
Layer resistance is thickness in metres divided by lambda. Fifty millimetres at 0.022 W/mK is about 2.27 m²K/W, and 50 mm at 0.038 W/mK is about 1.32. Neither is your floor’s U-value. EkoBoard, 1200 × 600 × 10 mm at about £7.99, has no lambda, so the division cannot be run.
Where the board sits relative to the DPM
Foam floor boards in these sources go above the damp-proof membrane. The PIR floor note from November 2024 describes a membrane, often 1200 gauge or 300 micron, lapped, taped, and turned up, with the boards on the dry side. Ask before you close in a lime or earth floor.
Polyfoam says its own XPS can sit below the membrane, in a lambda band of about 0.033–0.036 W/mK. EkoBoard prints no lambda and no BS EN 13164 line. Do not lay it under the membrane because another brand says its XPS may go there.
Perimeter, area, and underfloor heating
A long, thin room loses more heat at the edge than a square room of the same area. Perimeter divided by area, P/A, is how that shape enters the calculation. A Lignacite example puts many domestic floors around 0.4–0.6. A single 10 mm sheet does not cancel the edge of a narrow extension.
The Underfloor Heating Store’s table puts 10 mm of insulation under concrete at about one hour at 150 W/m² and about 45 minutes at 200 W/m². Those are the shop’s estimates, not a test of EkoBoard. Its bullet claims a shorter warm-up and a lower bill, with no figure. A warmer room under laminate is a comfort comment, not a U-value.
Which of these five belongs in a floor conversation
EkoBoard is the thin underlay at about £7.99, 4.2 from 123 reviews, many of them model-making. The heating mat is not in the box, and you cannot check it against 0.18 W/(m²·K).
Thermopanel prints 0.027 W/mK: 15 mm at about £15.99, 40 mm at about £20.89, each 0.75 m². Reviewers use them under shower tiles. That is a backer, not an extension-floor design. The Triplast 50 mm six-pack, about £47.59, prints 0.030–0.038 W/mK. It is a range, not a signed-off ground floor. Stop when the floor must hit the U-value. Cutting sits on the fitting page. Whether 0.022 W/mK earns its price is PIR versus EPS.
Common questions
Will 10 mm XPS meet a floor U-value of 0.18?
No. England’s 2026 tables and Scotland’s Table 6.3 both print 0.18 W/(m²·K) for a floor, with the English changes tied to a later commencement date. Ten millimetres of EkoBoard has no lambda in the listing, so the resistance cannot be calculated, and a layer resistance would still not be the floor’s U-value. A warmer-feeling room in one review is not that calculation.
Does the board go above or below the damp-proof membrane?
Above it, for the foam boards these floor guides describe. The membrane is lapped, taped, and turned up, and the insulation sits on the dry side. Polyfoam says its own XPS can go below the membrane because moisture does not change that product’s performance. EkoBoard does not make that claim and prints no lambda, so do not put it below the membrane on the strength of another brand’s sheet.
What is a P/A ratio, in one paragraph?
P/A is the floor perimeter divided by the floor area. A long, thin extension has a higher ratio than a square room of the same floor area, so more of the slab is near the cold edge. Published examples often put domestic floors around 0.4–0.6, and the U-value moves with that ratio and with insulation thickness. You cannot read the ratio off an amazon.co.uk title.
Can I use the Thermopanel backer as the floor insulation for a new extension?
Not as the board that shows the extension meets 0.18 W/(m²·K). The 15 mm and 40 mm boards are foam tile backers at 0.027 W/mK, 0.75 m² each, with no heating mat in the box. They belong in a tiled wet-area build-up if you follow a proper fixing method. An extension floor is a whole-element calculation, and Table 3.2 allows that calculation to use the enlarged dwelling’s perimeter and area. That is still a calculation, not a 15 mm sheet.
Why does a heating-mat advert talk about warm-up time rather than U-value?
Because warm-up is what the mat is being sold on. One shop’s table puts 10 mm of insulation under concrete at about an hour at 150 W/m² and about 45 minutes at 200 W/m². Those times are the shop’s estimates, not a test of the EkoBoard listing. A U-value is the heat loss of the whole floor. A faster warm-up does not mean the floor meets 0.18 W/(m²·K).
When do I stop and get the floor calculated?
When the floor has to be shown against 0.18 W/(m²·K), or when the electric heating is a system of mat, insulation, and controls rather than a single board. Also stop if the floor is lime or earth, or if you are about to replace most of a ground-floor thermal element. The listing cannot do that calculation for you, and a missing lambda makes it impossible.
Related pages
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.
- Approved Document L 2026, GOV.UK (retrieved 22.10.2026)
- Scottish building standards handbook 2022, section 6.2 (retrieved 22.10.2026)
- Ban on combustible materials, GOV.UK (retrieved 22.10.2026)
- Circular letter 02/2022, combustible materials (retrieved 22.10.2026)
- Recticel Eurothane GP datasheet (retrieved 22.10.2026)
- Polyfoam XPS: ground floor insulation (retrieved 22.10.2026)
- IMA: PIR floor insulation guidelines, November 2024 (retrieved 22.10.2026)
- HSE cellular plastics, HSG92 (retrieved 22.10.2026)