How to Install Carbon Underfloor Heating Film, Step by Step

Carbon underfloor heating film is one of the easiest kinds of underfloor heating to put down. There is no screed, no adhesive and no digging up the floor: the film goes on an insulation board, and a floating laminate or engineered wood floor goes straight on top. This guide walks through each step so you know what is involved before you start.

Safety first: laying the board, film and floor is a careful DIY or flooring job. The connection to the mains and the thermostat must be done by a qualified electrician, and in England and Wales a new heating circuit is notifiable work under Part P of the Building Regulations. Always follow the instructions supplied with your kit.

What you need

• Insulation board, such as our 6 mm XPS Thermal Insulation Board
• Carbon heating film, 130 W/m² for most rooms (see why we use 130 W/m²)
• Connection crimps and a ratchet crimping tool
• Bitumen insulation tape and termination discs for the cut ends
• Double insulated cold tail leads
• Polyester vapour barrier and heavy duty cloth tape
• A thermostat with a floor sensor (see our thermostat guide)

A Kit PRO puts the film, crimps, cold tails, tapes and vapour barrier in one box, sized for your room, and costs less than buying the parts separately.

Step 1: Prepare a level, clean floor

The subfloor must be flat, dry, solid and free of screws, nails or grit that could press into the film. Fill dips and sand down high spots. Concrete floors need a damp-proof membrane underneath the insulation.

Step 2: Lay the insulation board

Lay the boards tight together across the whole floor and tape every joint with cloth tape. The board stops heat disappearing into the subfloor, protects the film and acts as the underlay for your new floor.

Step 3: Plan the layout

Sketch the room and decide where the strips will run and where the cold tails will go to the thermostat. Keep these rules in mind:

• Do not lay film under kitchen units, fitted wardrobes, baths or anything fixed to the floor. Heat trapped under furniture is the most common cause of floor damage.
• Leave a small gap between the film and the walls, and between neighbouring strips. Never overlap the film.
• Only cut the film on the marked cut lines.

Step 4: Lay and fix the film

Roll out the strips with the copper strips (bus bars) running along the length, the right way up as shown in the kit instructions. Tape the strips down so they cannot move when the floor goes on.

Step 5: Make the connections

1. Fit a connection crimp onto the copper bus bar at the end of each strip, using the ratchet crimping tool so every joint gets the same pressure.
2. Connect the cold tail leads to the crimps.
3. Cover every crimped connection with bitumen tape on both sides of the film.
4. Seal the cut bus bar ends at the far end of each strip with termination discs.

Neat, fully insulated connections are what make a film system safe and long lasting, so take your time here.

Step 6: Fit the floor sensor

Place the thermostat's floor sensor between two strips, in the area that will be heated, and lead its cable to the thermostat position. The sensor keeps the floor at a safe temperature, so put it where the floor is open, not under where a sofa or bed will stand.

Step 7: Lay the vapour barrier

Cover the film with the polyester vapour barrier, overlapping the sheets and taping the joins. Together with double insulated cold tails, this makes the installation Class II.

Step 8: Electrician connects and tests

Your electrician tests each strip, connects the cold tails and floor sensor to the thermostat, and connects the thermostat to a suitable circuit protected by an RCD. Check the system works before the floor goes down.

Step 9: Lay the floor

Lay laminate or engineered wood as a floating floor straight on top. Carpet and soft vinyl need a firm overlay first, such as the ECOMAX-DUO Dual Board Overlay System. Film is not suitable under tiles or in bathrooms and wet rooms.

How long does it take?

In a typical room the whole job fits into one day once the subfloor is ready: insulation board, film, connections, the electrician's work and the new floor on top.

Common mistakes to avoid

• Laying film under fixed units or where large furniture will stand.
• Cutting the film anywhere other than the marked lines.
• Leaving crimped connections or cut ends uninsulated.
• Putting the floor sensor under a rug or furniture.
• Using high-output film that runs too hot for a wooden floor.

Get the right kit for your room

Send us your room size and floor type and we will tell you exactly which kit and thermostat you need, usually within one working day. Get a free quote, read our underfloor heating film guide, or use the heating calculator. If you would like it fitted, our installation partner Standard Electrical Services can quote for the work.