What flooring product can be used with underfloor heating?

Underfloor heating used under engineered wood.

Underfloor heating can be installed under tile, slate, stone, carpet, carpet tile, vinyl, LVP, lino, engineered wood, composite wood and composite bamboo. It cannot be used under solid natural timber. Each flooring type needs a different amount of heat per square metre, because each insulates differently and sits at a different depth above the cable.

How much heating per square metre does each floor covering require?

Final flooring	Recommended	Maximum	Minimum
Carpet	100W/m²	180W/m²	100W/m²
Carpet tile	100W/m²	180W/m²	100W/m²
Composite wood	120W/m²	150W/m²	100W/m²
Engineered wood	120W/m²	150W/m²	100W/m²
LVP	120W/m²	150W/m²	100W/m²
Lino	120W/m²	150W/m²	100W/m²
Slate	180W/m²	180W/m²	150W/m²
Stone	180W/m²	180W/m²	150W/m²
Tile	150W/m²	180W/m²	150W/m²
Vinyl	120W/m²	150W/m²	100W/m²

No underfloor heating installation should exceed 180W per square metre. At 180W the cable runs sit about 55mm apart, and underfloor heating cable should never be closer together than 55mm.

Why can't underfloor heating go under solid timber?

Natural timber warps and twists unless it is fixed directly to the floor joists. Nails or screws and underfloor heating are a bad combination. Timber also insulates well, which is the opposite of what a floor covering over heating should do. Underfloor heating cannot be installed under an existing wooden floor without significant cost. Engineered wood and composite wood are the alternatives, and a well-laid engineered floor will look as good and last as long as a natural timber one.

What makes a floor covering unsuitable?

Thermal resistance. The heating cable sits between the final floor covering and the sub-floor. The more it insulates, the more of the heat stays under it.

For carpet this can be measured. The combined R-rate of carpet and underlay should not exceed 0.15, or a Tog of 1.5. The lower the combined figure, the better the system works. Thick wool carpet has a natural level of insulation that does not suit under-carpet heating, and some underlays are designed as insulators, which defeats the system entirely. Ask the carpet supplier for the thermal resistance of both the carpet and the underlay before committing.

The same trap exists with engineered wood and LVP sold with insulation already attached to the underside of the board. That insulation traps the heat under the final flooring.

Is there a maximum floor temperature?

Yes, for the products that can be damaged by heat. Heatwell's maximum recommended temperature under engineered wood, composite wood, vinyl, LVP and lino is 27 degrees Celsius. Above 27 degrees, engineered and composite wood, vinyl, LVP and lino can delaminate and discolour.

Heatwell's HT4 and HT5 thermostats can both be set to a maximum temperature that does not vary between manual and program modes.

Does the thickness of the floor covering matter?

Yes. The thickness changes how deep the cable sits below the surface. Under a 10mm tile, the heating sits 15mm below the surface. Under 20mm stone, it sits 25mm below. The deeper the cable, the longer the floor takes to warm and the more heat per square metre it needs. Heatwell recommends 150W per square metre under 10mm flooring product and 180W per square metre under 20mm flooring product.

It is for the same reason that floors should be levelled before underfloor heating cable is installed. Deeply buried cable will heat differently to cable that is buried shallow.

What about clip-together flooring?

Engineered wood, composite wood and LVP should be the clip-together type. A floor over heating expands and contracts, and planks that are not clipped together develop gaps. Cheaper products are generally not designed for that movement and will crack between planks over time. Full sheets of vinyl or lino are fine — they still move, but no cracks develop.

LVP and lino planks can be used. However, they must be designed to minimise expansion and contraction. You must ask your final flooring supplier if your LVP or lino planks are suitable for underfloor heating. Please also make sure you understand what the maximum temperature is for your final flooring choice.

Is bamboo flooring suitable? 

Yes, provided it is a composite. It cannot be 100% bamboo.

Can carpet tiles be glued down over underfloor heating? 

Yes, onto heating that has been screeded in, on a clean and prepared surface. A foil mat system cannot be used under glued carpet tiles.

Does engineered wood need to acclimatise first?

Poorer quality product often does, and probably should not be used. High quality engineered or composite wood can usually be laid without an acclimatisation period, but the screed must be 100% dry first, otherwise moisture can be trapped under the floor. You must ask your final flooring supplier if your choice is suitable for underfloor heating and if there is any acclimatisation required.