The short answer: every Termofol electric underfloor heating system, whether it is a heating mat or heating film, should be laid on a layer of insulation that covers the whole heated area. Insulation reduces heat loss into the subfloor, so more of the heat goes up into the room. Which product you need depends on the floor finish. Under tiles and a heating mat, use the 6 mm Insulation Tile Backer Board, which can be tiled over. Under laminate or wooden flooring with heating film, use the 3 mm or 5 mm foil-faced insulation underlay. Under floating vinyl, use the 6 mm Supertherm insulation board. None of these is included with a mat or a film kit, so order the insulation at the same time as the heating.
This guide explains why the insulation layer matters, what each of our insulation products is for and how to work out how much to order.
Why insulation goes under electric underfloor heating
A heating mat or heating film warms whatever is on both sides of it. Without an insulating layer underneath, part of the heat is lost into the concrete or timber subfloor instead of warming the floor surface and the room. The insulation layer limits that downward heat loss and directs more of the warmth upwards. Our underfloor heating installation guide therefore asks for insulation across the entire floor area, not just under the heated strips, and both our mat and film product pages say the same thing.
The insulation also does a second job. The subfloor must be level and dry before any heating goes down. The installation guide asks for a subfloor moisture level of no more than 2 per cent, and if the floor is damper than that, a damp-proof insulation layer is recommended. Our tile backer board, the Supertherm board and the foil underlays are all described on their product pages as resistant to water and moisture, and the underlays also act as a vapour retarder.
Which insulation for which floor
We stock four insulation products, and the choice comes down to the floor finish and the heating system that goes with it.
| Product | Thickness | Material | Use it under | Format |
|---|---|---|---|---|
| Insulation Tile Backer Board | 6 mm | XPS foam with a glass fibre reinforced cement coating | A heating mat under tiles, mosaic or natural stone; bathrooms, kitchens and other wet rooms | Boards 600 mm x 1200 mm, pack of 2 |
| Insulation underlay 5 mm | 5 mm | Closed-cell cross-linked polyolefin foam (XLPE) with a foil face | Heating film under laminate and wooden floors; also suitable under carpet | Roll 1 m wide, sold by the metre |
| Insulation underlay 3 mm | 3 mm | The same XLPE foam, thinner | The same floors, where floor height must stay as low as possible | Roll 1 m wide, sold by the metre |
| Supertherm Insulation Board | 6 mm | XPS foam | Heating film under floating vinyl flooring | Boards 80 cm x 120 cm, 10 boards per pack (9.6 m²) |
If you are still deciding between a mat and film, our guide heating mat or heating film: which suits your floor? explains the difference. The short version is that a mat is laid in tile adhesive under tiles, and film lies on insulation under a floating floor.
Under a heating mat: the tile backer board
Our electric underfloor heating mats are laid in flexible tile adhesive under tiles and stone, so the insulation beneath them has to be something you can tile over. That is what the Insulation Tile Backer Board is for. It is a 6 mm board with an extruded polystyrene core, covered with a polymer resin mass and a glass fibre mesh, and the heating mat can be laid directly on it.
Points from the product page worth knowing before you order:
- Thermal conductivity of 0.033 W/(mK), with a slim 6 mm profile, so it adds very little height to the floor build-up.
- Resistant to water, moisture and humidity, with a closed-cell structure, so damp conditions do not affect its insulation properties. This is why it suits bathrooms and other wet rooms.
- Compressive strength of at least 300 kPa, fire resistance class E, and resistant to fungi and mould.
- Suitable for tiles, mosaic and natural stone, and lightweight and easy to cut.
- Each board is 600 mm by 1200 mm and the boards are sold in packs of 2.
Measure the free floor area you plan to heat, the same area you used to size the mat, and order enough boards to cover it. If you have not sized the mat yet, start with how to choose the right heating mat size.
Under heating film: foil underlay or the Supertherm board
Heating film is laid dry on top of the insulation, under a suitable floating floor such as laminate, engineered wood or vinyl. Our underfloor heating film kits include the film, connectors, tapes and a vapour membrane, but not the insulation, which must cover the whole heated area. There are two options, depending on the floor finish.
Laminate and wooden floors: the 3 mm or 5 mm underlay
The 5 mm insulation underlay and the 3 mm insulation underlay are the same material in two thicknesses: a closed-cell cross-linked polyolefin foam (EXPEL PETM, also written XLPE) with a reflective foil face. The product pages describe them as reflecting up to 90 per cent of heat radiation back into the room, 100 per cent waterproof and moisture resistant, tear-proof, non-toxic and resistant to fungus and bacteria. They are suitable under laminate, carpet and wooden floors, and both work as a vapour retarder.
Which thickness? Choose the 5 mm underlay as the standard option. Choose the 3 mm underlay when the floor height must stay as low as possible, for example in a renovation where the new floor has to meet an existing threshold or door. Both rolls are 1 m wide and you select the length you need on the product page, so a room 4 m by 5 m needs 20 m of underlay from a 1 m wide roll, plus a little extra for trimming. The underlay can be cut with scissors and fixed with suitable tape.
Floating vinyl: the Supertherm board
For vinyl floating flooring, use the Supertherm Insulation Board instead of the foil underlay. It is a 6 mm XPS board developed specifically for vinyl floating floors, and it does two extra jobs that matter with vinyl: it compensates for subfloor irregularities of up to 0.88 mm, which gives the thin vinyl planks a smoother base, and its high density helps to quieten footfall in the room. The product page lists a thermal conductivity of 0.035 W/(mK), a compressive strength of 300 kPa and compliance with the CEN/TS 16354:2012 standard. Each board is 80 cm by 120 cm, and a pack of 10 boards covers 9.6 m².
What about the vapour membrane?
The vapour membrane is often mistaken for insulation, but it is a different layer with a different job. It is a 2 m wide polyethylene film laid over the heating film, between the heating layer and the floor finish, and secured with tape. It protects the film and keeps the build-up clean. One is included in every film kit, and it is also sold on its own in cuts from 2 m x 10 m up to a full 2 m x 50 m roll. It does not replace the insulation underneath; you need both.
How much insulation to order
- Cover the whole heated area. Insulation goes everywhere the heating goes, and it is simplest to cover the whole floor of the room, including the strips between runs of film or mat.
- Tile backer board: each board is 600 mm x 1200 mm, sold in packs of 2. Divide your floor area by the area of one board and round up, allowing for cuts around the edges.
- Supertherm board: 80 cm x 120 cm boards, 10 per pack, 9.6 m² per pack.
- Foil underlay: 1 m wide, so the number of metres you order equals the floor area in square metres, plus an allowance for trimming.
- Order with the heating. Insulation is not part of any mat or film kit. Our guide what’s in an electric underfloor heating kit? lists exactly what is and is not in the box.
The electrical connection must be made by a qualified electrician, and we recommend not booking your tiler, fitter or electrician until your delivery has arrived. Delivery is free to the Republic of Ireland and Northern Ireland.
Frequently asked questions
Can I lay a heating mat or film without insulation?
We do not recommend it. Both our installation guide and the product pages call for insulation across the entire heated area. Without it, part of the heat is lost into the subfloor.
Can I use the foil underlay under a heating mat and tiles?
No. The foil underlay is for floating floors such as laminate and wood. A heating mat is laid in tile adhesive, so it needs a board that can be tiled over: the Insulation Tile Backer Board.
My laminate already comes with an underlay. Do I still need the insulation underlay?
Our film kits are designed to be laid on our insulation underlay, which has a reflective foil face and acts as a vapour retarder. If your flooring comes with its own underlay, check with the flooring manufacturer whether it is approved for use with electric underfloor heating, and see our guide which flooring is compatible with infrared heating film?
How much height does the insulation add?
The 3 mm underlay adds 3 mm, the 5 mm underlay 5 mm, and both boards 6 mm. The heating film itself is only 0.33 mm thick, and a heating mat sits inside the tile adhesive layer.
Order your insulation
Browse all insulation materials, or go straight to the tile backer board for a mat under tiles and the 5 mm insulation underlay for film under laminate. If you are not sure which product suits your floor, contact Termofol Ireland with your floor finish and room size and we will help you choose.