Bathroom underfloor heating

Electric or wet, for one room

For a single bathroom being retiled, the electric mat wins on nearly every practical count. For a room where the floor is coming up to structural level anyway, or where the rest of the house is on a wet system and the heat source is efficient, a wet loop is worth pricing – see underfloor heating.

What underfloor heating in a bathroom is actually for

It is worth being clear about this, because it affects the specification.

• Comfort underfoot is the primary reason, and a floor that is merely not cold achieves most of it.
• Heating the room is a secondary question. A mat can be the primary heat source in a small, well-insulated bathroom, and frequently it cannot – particularly where a large share of the floor is occupied by a bath, a shower tray and a vanity unit, none of which can be heated under.
• Drying the floor between uses, which is a genuine benefit in a family bathroom.
• It does not dry towels. That is a towel rail’s job, and it is why most bathrooms end up with both – see towel rails.

The calculation that matters is the heated area, not the room area. Measure what is actually left once the bath, the shower tray, the WC and any fitted furniture are deducted. In a small bathroom that can be less than half the floor, and it decides whether the mat can carry the room or whether a radiator or towel rail is still needed.

Build-up, and why it matters more than anything

The floor height is the constraint people discover late.

1. Decoupling or insulation board under the mat, which is what stops the heat going downwards into the joists or the slab and makes the system respond quickly. Skipping it is the commonest way to end up with a floor that takes an hour to warm and costs more to run.
2. The mat itself, which is thin.
3. Tile adhesive, which must fully encapsulate the cable with no voids – an air pocket over a heating cable is a hot spot and a failure point.
4. The tile.

Added together that is a real dimension, and it has consequences: the door may need trimming, the threshold to the landing changes, and on a timber floor the deflection matters – a floor that moves cracks tiles and can damage a cable. Suspended timber floors frequently need additional noggins or an overboard before tiling.

On a solid floor, insulation board underneath is even more worthwhile, because a concrete slab is an enormous heat sink that will happily absorb everything the mat produces.

Installation, and the two tests

1. Survey and calculateHeated area after deductions, heat loss for the room, and whether the mat can carry it or is a comfort layer alongside another emitter.
2. Check floor construction and levelsDeflection on timber, and the build-up the door and threshold will tolerate.
3. Insulation or decoupling boardBonded down properly.
4. Test the mat before layingResistance and insulation resistance, recorded.
5. Lay the matTo the planned layout, keeping clear of the areas that will be under fixed items – a cable under a bath or a fitted unit cannot dissipate its heat and will fail.
6. Set the floor sensorIn a conduit, between cable runs, not touching a cable, and routed so it can be replaced from the thermostat position if it fails. This is the single most useful detail on the whole job and it is routinely skipped.
7. Test againAfter laying, before adhesive.
8. TileFull encapsulation, no voids.
9. Test a third timeAfter tiling, before commissioning. A cable damaged during tiling is found now rather than in six months.
10. Electrical connection and thermostatBy an electrician, to the requirements for a bathroom, with the thermostat sited outside the room or in a permitted position.
11. Commission graduallyAdhesive and grout need their full cure before any heat is applied – rushing this cracks the floor.

Controls and running it sensibly

• A programmable thermostat with a floor sensor is the standard, and the floor sensor is what protects the floor covering as well as controlling the room.
• Set a floor temperature limit appropriate to the covering. Tile and stone tolerate more than engineered timber or vinyl, and exceeding the covering’s limit damages it.
• Timed to the times you use the room, with a warm-up period. Adaptive thermostats learn how long the floor takes and start accordingly.
• Do not run it continuously on the assumption it is cheaper. With insulation board underneath the response is quick enough that timed operation is both cheaper and perfectly comfortable.
• Keep a record of the mat make, model, wattage and layout, and the test results. If it ever needs fault-finding, that paperwork is what makes it possible.

Electrical work in a bathroom is subject to its own requirements around zones and protective measures, and a thermostat generally cannot be sited inside the room unless it is a type permitted there. That is an electrician’s call and it is part of the design rather than an afterthought.