Underfloor heating in a bathroom: water or electric, wet rooms and towel rails

A warm bathroom floor is one of the most popular uses of underfloor heating. It is also the room with the most to plan in the least space: water, electrics, tanking, fixtures and a very small heated area. This guide covers which system to choose, how to check the output, wet rooms, membranes, sensors, towel rails and zoning.

For a room-by-room overview, see which rooms can have underfloor heating.

Should I use water or electric underfloor heating in a bathroom?

Both work. The right choice depends mostly on what heating the rest of the house has, and whether the floor is coming up anyway.

Because a bathroom floor is small, the higher running cost of electric matters less than it would across a whole house. If you are fitting a wet system to the rest of the floor, adding the bathroom as one more loop is usually the better long-term choice. See water vs electric underfloor heating and underfloor heating running costs for the full comparison.

For electric, choose from electric heating mats for regular shaped floors or loose cable for awkward shapes. See electric loose cable vs mats. For water in an existing floor, overlay boards keep the build-up low.

Will underfloor heating be warm enough in a small bathroom?

This is the main thing to check. A bathroom floor loses a lot of its area to the bath, shower tray, WC and vanity unit. You only heat what is left, called the free floor area. A floor can only give so much heat per square metre before it gets uncomfortably hot to stand on.

BS EN 1264, the European standard for water underfloor heating, sets maximum floor surface temperatures. Danfoss design data gives 29°C for normal living areas, 35°C for edge zones, and room temperature plus 9°C for bathrooms, which is 33°C in a bathroom heated to 24°C. A floor 9°C warmer than the room gives roughly 100W/m² in typical design data.

Worked example, with round figures:

• Bathroom 2.4m x 1.8m = 4.3m²
• Bath 1.7m x 0.7m = 1.2m², plus WC and basin areas of about 0.5m²
• Free floor area: about 2.6m²
• At about 100W/m², the floor gives about 260W

If a heat loss calculation says the room needs more than that, the floor alone won't keep it at temperature in cold weather. That is common in bathrooms with an outside wall and a window. The fix is usually a towel rail as well, covered below.

For electric, MaxHeatUFH mats come in 100, 150 and 200W/m². The 150W/m² mat suits most bathrooms. A 100W/m² mat can suit a low heat loss en-suite. Pick a mat to the free floor area, never larger, as the heating cable can't be cut. For water, closer pipe spacing raises output. See pipe spacing and heat output.

Can underfloor heating go under a bath or shower tray?

No. Don't lay heating under baths, shower trays, WCs, vanity units or anything fixed to the floor. Heat trapped under them can't escape, which wastes energy and can overheat the cable or the fixture. Electric heating maker instructions also warn against fixed furniture over heated areas.

• Plan the layout with the bathroom suite positions fixed first.
• Keep heating cable or pipe clear of the WC pan fixing points and waste pipes. Maker instructions typically ask for a gap between electric cable and other pipes.
• Note where the cable or pipe runs, and photograph it before tiling, so nobody drills into it later.

Wet rooms and shower formers

A wet room has a level-access shower area. The floor falls to a drain, often formed with a pre-made shower former, which is a shaped tray board that gives the fall and holds the drain.

• Don't run water pipe or electric cable through the former or near the drain body.
• Never fix through a former or through the tanking.
• Whether heating can go in the walk-in shower area itself depends on the heating maker's and former maker's instructions. Check both before you plan it, and talk to us if you are unsure.

Heating the floor around the shower area helps the wet room floor dry faster after use, even if the shower area itself is left unheated.

Do I need tanking or an uncoupling membrane?

They do different jobs, and some bathrooms need both.

• Tanking is a waterproof layer, as a sheet membrane or a painted liquid, on floors and walls in wet areas. It stops water getting into the floor and wall structure. Wet rooms need it across the floor and up the walls.
• An uncoupling membrane sits between the floor and the tiles and takes up small movements in the subfloor, so tiles and grout don't crack. Our uncoupling membrane also holds loose heating cable at the right spacing. It is designed for floors that move, such as timber, new screed or mixed substrates.

Systems differ on whether electric heating goes below or above the tanking layer. Use a tanking system that is compatible with underfloor heating and follow both the tanking maker's and the heating maker's instructions on the order of layers. Use flexible tile adhesive throughout.

Insulation under the heating makes a big difference to warm-up time. A cement-coated XPS tile backer board cuts heat loss downwards and gives a tiling surface. See underfloor heating floor coverings for tile and stone.

Why does bathroom underfloor heating need a floor probe?

A floor probe is a temperature sensor laid in the floor between the heating cables or pipes. It lets the thermostat read the floor itself, not just the room air. In a bathroom it matters for two reasons:

• Comfort: many people want a warm floor even when the room air is warm enough.
• Protection: it limits the floor temperature, to protect tiles, adhesive and the heating.

The Heatmiser neoStat-e and Slimline-E for electric UFH both come with a floor sensor. The neoStat-e can run in air, air and floor, or floor only mode. Lay the probe in a conduit so it can be replaced without lifting tiles, and keep a 10K floor probe in mind as a spare. See how to install an electric underfloor heating mat for where the probe goes.

What are the electrical safety rules for bathroom underfloor heating?

BS 7671, the UK wiring regulations, treats rooms with a bath or shower as special locations. At a high level:

• The space is divided into zones. Zone 0 is inside the bath or shower tray. Zone 1 is above it, typically up to 2.25m from the floor. Zone 2 extends 0.6m beyond zone 1. Each zone limits what electrical equipment may go there.
• All circuits serving the bathroom need additional protection by an RCD (residual current device) rated at no more than 30mA. That includes electric underfloor heating, lighting and electric towel rails.
• Thermostats and connection points must go where the electrician says they are allowed. Heating makers often advise putting the thermostat outside the bathroom.

Electric underfloor heating in a bathroom must be connected by a qualified electrician. In England, a new circuit, or work near a bath or shower, can be notifiable under Part P of the Building Regulations. A registered electrician can self-certify it, otherwise building control must be told before work starts. Rules differ in Wales, Scotland and Northern Ireland. This guide does not give wiring instructions.

Do I still need a towel rail with underfloor heating?

Usually, yes. Underfloor heating runs at a low surface temperature, which is what makes it comfortable, but it is poor at drying towels. A towel rail does that job and adds heat when the free floor area is too small to heat the room alone.

The bigger question is what feeds the towel rail:

If the house is on a heat pump at low flow temperatures, a wet towel rail will run cooler than on a boiler. An electric element gives dry towels all year. Any electric element or electric towel rail in a bathroom must be connected by a qualified electrician.

How should I zone and control bathroom underfloor heating?

Give the bathroom its own zone with its own thermostat. Bathrooms are usually run warmer than bedrooms, and people often want the floor warm at set times, such as mornings and evenings. A shared zone with a bedroom rarely suits both rooms.

Many people don't want a thermostat on a tiled bathroom wall, and the electrician may not allow one in the zones. Two common answers:

• Thimble sensor: the Heatmiser thimble sensor is a small wired room sensor. It can be up to 20m of cable from the thermostat, which then goes outside the bathroom, such as on the landing. It works with thermostats that have a remote sensor input, such as the DS1-L and neoStat V3.
• Floor sensing: a floor-sensing thermostat placed outside the room, reading the probe in the floor.

For a water system, the bathroom thermostat links to the manifold through the wiring centre in the usual way. See underfloor heating thermostats explained and zoning and wiring.

Products for this job

• Electric Underfloor Heating Mat 150W/m²
• Electric Underfloor Heating Mat 100W/m²
• Electric Loose Cable, 10W/m or 20W/m
• Uncoupling Membrane (1m wide roll)
• Heatmiser neoStat-e
• Heatmiser Thimble Sensor
• Cement-Coated XPS Tile Backer Board
• All electric underfloor heating

Frequently asked questions

Is electric underfloor heating safe in a bathroom?

Yes, when it is installed to the maker's instructions and connected by a qualified electrician with 30mA RCD protection, to BS 7671.

Can I fit an electric mat myself?

You can lay the mat and tile over it, following the instructions and testing the mat as you go. A qualified electrician must make the final connection.

What output mat do I need for a bathroom?

150W/m² suits most bathrooms. A well insulated en-suite with little heat loss can use 100W/m². Size the mat to the free floor area.

Can underfloor heating replace a bathroom radiator?

Sometimes. If the free floor area can meet the room's heat loss, yes. In many small bathrooms it can't, so a towel rail is kept for extra heat and drying towels.

How warm can a bathroom floor be?

BS EN 1264 design data allows room temperature plus 9°C in bathrooms, which is 33°C in a 24°C bathroom. Wood and some vinyl floors have lower limits, so check the floor maker's figure.

Should the bathroom be its own zone?

Yes, ideally. It usually runs warmer and on a different schedule to the rooms around it.

Sources: Danfoss UFH design guide (BS EN 1264 limits), Voltimum: BS 7671 Section 701, Planning Portal: electrics and Part P, ProWarm electric mat instructions, Heatmiser neoStat-e sensing modes