Short answer: You need insulation under underfloor heating so the heat goes up into the room, not down into the ground or the ceiling below. BS EN 1264-4 sets a minimum thermal resistance under the pipe of 0.75m²K/W over a heated room, 1.25m²K/W over the ground or unheated space and 2.0m²K/W over outside air. On a ground floor, Part L in England usually asks for more: a U-value of 0.18W/m²K for a new floor and 0.25W/m²K for a renovated one.
Insulation is the part of an underfloor heating floor you can't add later. Once the screed or deck is down, whatever is under the pipe stays there. Too little, and you pay to heat the slab, the soil or the room below, and the floor warms more slowly.
This guide explains how much insulation you need, how to turn the figures into a board thickness and where the insulation goes in each type of floor. It covers what goes under the pipe. For the floor systems themselves, see underfloor heating floor systems explained.
Why do you need insulation under underfloor heating?
Heat flows from warm to cold in every direction. Without insulation, a heated floor loses a share of its heat downwards into whatever is below, which costs money and slows warm-up.
- Ground floors: the slab and the ground underneath are cold and very large. They soak up heat for as long as the heating runs.
- Upper floors: heat lost downwards warms the ceiling of the room below. That isn't wasted, but it lands in the wrong room and the floor you want warm responds slowly.
- Edges: heat also escapes sideways into the walls, which is why every UFH floor needs edge insulation too.
How much insulation does BS EN 1264 require?
BS EN 1264-4, the European standard for installing water UFH, sets a minimum thermal resistance (R) for the insulation under the pipe. It depends on what is underneath. These are the figures as summarised by two UFH makers:
| What is below the floor | Minimum R (m²K/W) |
|---|---|
| A heated room | 0.75 |
| Unheated space, cellar or the ground | 1.25 |
| Outside air, 0 to -5°C design | 1.5 |
| Outside air, -5 to -15°C design, or a garage | 2.0 |
These are minimums for the heating to work properly. On a ground floor the Building Regulations U-value usually needs more, so the thicker of the two wins.
How thick should the insulation be?
Thickness equals the R you need multiplied by the insulation's thermal conductivity (lambda, in W/mK). Lower lambda means a thinner board for the same R.
Using lambda values from maker data sheets of 0.038W/mK for EPS and 0.022W/mK for PIR:
| R needed | EPS (0.038) | PIR (0.022) |
|---|---|---|
| 0.75m²K/W | 29mm | 17mm |
| 1.25m²K/W | 48mm | 28mm |
| 2.0m²K/W | 76mm | 44mm |
Worked example. An upstairs bathroom over a heated kitchen needs R 0.75. With EPS: 0.75 x 0.038 = 0.0285m, so 29mm, and you would buy the next standard thickness up. XPS at 0.033W/mK would need 0.75 x 0.033 = 25mm.
Use the lambda printed on the board you actually buy, as it varies between products. Also check the board's compressive strength under screed: one liquid screed maker asks for at least 100kPa.
What does Part L require for floors with underfloor heating?
Part L sets maximum U-values for ground floors, and the floor still has to meet them when it has UFH in it. These are the figures in Approved Document L, Volume 1 (England, 2021 edition):
| Floor | U-value (W/m²K) | Where it applies |
|---|---|---|
| New dwelling, limiting value | 0.18 | Table 4.1 |
| Notional dwelling | 0.13 | Table 1.1, used for SAP |
| New floor in an extension | 0.18 | Table 4.2 |
| Renovated floor, improved value | 0.25 | Table 4.3, if worse than 0.70 |
A floor's U-value depends on the ground and the ratio of its exposed perimeter to its area, not just the board. Your architect, SAP assessor or insulation maker works out the thickness, and building control signs it off. Wales, Scotland and Northern Ireland have their own regulations.
Approved Document L also asks for perimeter insulation at the floor edge that is continuous and at least 25mm thick, so there is no cold bridge where the floor meets the wall.
Where does the insulation go in a screed floor?
In a screed floor the insulation goes on the slab, under the pipe and screed, with an edge strip around the room. Put down the layers in this order:
- Damp proof membrane on or under the slab, as your building design specifies.
- Insulation boards laid tight with staggered joints. Gaps can greatly reduce how well insulation works, according to the Energy Saving Trust.
- A separating layer. One UFH maker asks for a vapour barrier, typically 0.15mm polythene, under screed poured over porous insulation such as polystyrene. Liquid screed makers usually ask for a taped polythene layer as well.
- Edge insulation around every wall and column, to take up screed expansion and stop heat leaking sideways.
- Pipe, then screed, with the pipe fixed by clips, rail or castellated panels.
Our edge insulation is a 100mm x 5mm foam strip in 25m and 50m rolls. Check the edge strip thickness your screed maker asks for. For pouring liquid screed, see castellated panels and liquid screed.
Do overlay boards need insulation underneath?
Many overlay boards are insulation boards themselves, with the pipe grooves cut in, so they cut heat loss downwards as well as holding the pipe. They are not usually thick enough to bring an old floor up to Part L on their own.
- MaxHeatPanel is routed XPS insulation board, 18mm or 25mm thick, and you tile direct.
- MaxHeatDeck is routed XPS with a cement face on both sides, 20mm thick.
- MaxHeatPro is 400kPa EPS with a 100 micron foil face, 16mm or 20mm thick.
If you are lifting an old floor anyway, that is the time to add insulation below. Insulating on top of an existing solid floor raises the floor, which can mean trimming doors and moving skirting and sockets. Read retrofitting underfloor heating for older homes.
How do you insulate a timber floor for underfloor heating?
In a joisted floor the insulation fits between the joists, under the spreader plates or routed chipboard. Mineral wool or rigid boards are the usual choices.
- Cut rigid boards to fit snugly between the joists. Mineral wool can be held up with netting from below.
- On a suspended ground floor, never block the air bricks. The void needs ventilation to prevent damp and rot.
- Upstairs, insulation over a heated room needs R 0.75 at least, which is about 29mm of EPS.
Our guide to underfloor heating in joisted timber floors covers how the insulation, plates and deck fit together.
What insulation goes under electric underfloor heating?
Under an electric mat on a concrete floor, use a cement-coated XPS tile backer board. It cuts heat loss into the slab, so the mat warms the floor faster and uses less power to do it.
Our cement-coated XPS backer board comes in 6mm, 10mm and 20mm, at 1200 x 600mm. Bond it with flexible tile adhesive and fix it with washers, then lay the mat in the tile adhesive on top. A thicker board insulates better if you have the height.
Worked example. A bathroom with 4m² of free floor on a concrete slab: 4 ÷ 0.72 = 5.6, so 6 boards of 10mm XPS backer board at £9 each, £54. Add a pack of 100 washers (£7) and a 4m² 150W/m² mat (£104). Prices inc VAT, October 2026.
Is reflective foil enough insulation for underfloor heating?
No. Our reflective foil roll is made to reflect heat upwards and act as a moisture and vapour barrier under screed or overlay boards. Use it alongside insulation boards, not instead of them. The insulation thickness for Part L and BS EN 1264 comes from board thickness and lambda.
Lap foil joints by 50 to 100mm and seal them with aluminium foil tape. Don't use foil tape with every liquid screed: one calcium sulphate screed maker says never to use foil-backed tape with its product, so check the screed data sheet.
Products for this job
- Edge Insulation Foam Roll 100mm x 5mm
- MaxHeatPanel Routed XPS Board
- MaxHeatPro Foil-Faced EPS Panel
- Cement-Coated XPS Tile Backer Board
- Overlay Panel Washers
- Reflective Aluminium Foil Insulation Roll
- Insulation and adhesives
Design it free with MaxHeatUFH ProLoop. Add your rooms and floor build-up, and get a loop design and full kit list with prices, including boards and edge insulation. Start your design
Frequently asked questions
Can I lay underfloor heating without insulation?
It will work, but a share of the heat will go down instead of up, so it costs more to run and warms up slowly. BS EN 1264-4 sets minimum insulation under the pipe for this reason.
How thick should insulation be under underfloor heating on a ground floor?
Thick enough to meet Part L, which is usually more than the BS EN 1264 minimum of R 1.25. The exact thickness depends on the floor's size, shape and the board's lambda, so get it from a U-value calculation.
Do I need insulation under underfloor heating upstairs?
Yes. Over a heated room, BS EN 1264-4 asks for at least R 0.75 under the pipe, about 29mm of EPS or 17mm of PIR.
Is PIR or EPS better under underfloor heating?
PIR insulates better per mm, so it is thinner for the same R. EPS is usually cheaper. Check either board's compressive strength suits the screed and loads above.
What is edge insulation for?
It lets the screed expand as it heats and stops heat leaking sideways into the walls. Fit it around every wall, column and door frame before the screed, and trim it after the floor finish.
Do I need insulation under an electric heating mat?
It is strongly worth it on a concrete floor. An insulated backer board stops the slab soaking up heat, so the floor warms faster and the mat runs for less time.
Sources: Approved Document L, Volume 1 (gov.uk), Danfoss UFH design guide (EN 1264), Uponor (BS EN 1264-4 insulation table), Timoleon staple system data sheet, NBS Source: PIR board data, Gypsol preparation for liquid screeds, Gyvlon flowing screed data pack, Energy Saving Trust: floor insulation