Underfloor Heating Installation

Wet or electric, and it is not a close call for most jobs

Wet underfloor heating runs warm water through pipe loops in or under the floor, fed from your boiler or heat pump through a manifold. It is the right answer for a whole room, a whole floor or a whole house. Running costs are gas rates rather than electricity rates, which is the whole argument.

Electric underfloor heating is a mat or cable under the floor covering, on a thermostat. It is thin, quick to fit and genuinely useful in one small room, typically an en suite or a bathroom, where you want a warm floor rather than a heat source. Used as primary heating for anything larger it costs a great deal to run, because you are heating with electricity at roughly four times the unit price of gas.

Our honest rule: electric for a bathroom floor as a comfort item, wet for anything expected to actually heat a room. Anyone quoting electric mats to heat a kitchen extension should be asked what the running cost will be.

Screed or low profile, and the height problem

This is where most retrofit projects live or die.

Traditional screed. Insulation, then pipe, then screed over the top. Sand and cement screed needs a minimum of 65mm, liquid screed a minimum of 50mm. With insulation beneath, a conventional build up adds around 100mm to the floor level, which in most existing houses means doors, skirtings, architraves and often a step at every threshold.

Drying is the other constraint people underestimate. Hand mixed screed dries at roughly a day per millimetre for the first 50mm and two days per millimetre above that, so a 65mm screed is around eighty days before it should be commissioned, with four weeks the sensible absolute minimum. Liquid screed is much faster and can be accelerated.

Low profile overlay systems. Purpose made retrofit build ups from about 15mm to 22mm including a self levelling compound. Walkable in around eight hours, flooring down within about seventy-two, rather than the fifty to sixty days a traditional screed wants. You lose some thermal mass and therefore some of the slow steady character, but for a retrofit into an occupied house it is usually the only realistic option.

Between joist systems. Pipe laid between existing joists on aluminium spreader plates or pre grooved panels, adding no height at all. Ideal for a suspended timber floor. The catch is that the depth of your existing joists determines how much insulation can go beneath, and without adequate insulation you are heating the void and the ground rather than the room. BS EN 1264 sets a minimum thermal resistance of 0.75 m²K/W for intermediate floors, and on a ground floor you want considerably better than that.

On a period property there is one more thing worth saying out loud: heritage skirtings and architraves have to be adjusted to accommodate any build up, and on a listed building that is a conversation with the conservation officer before it is a conversation with us.

Controls, and why it is not like a radiator

A screed system holds heat. It takes roughly two to three hours to come up to temperature and it stays warm long after it has stopped. Low profile and spreader plate systems respond much faster because there is less mass to heat.

That changes how you run it. A screed system is not switched on and off like a radiator circuit. It is held at a steady temperature with a gentle setback, and controlled through the manifold with weather compensation where possible. Programming it like a radiator system produces a floor that is warm when you have gone out and cold when you get in.

The manifold carries the pump, the actuators for each loop, flow meters for balancing and, where the system also serves radiators, a blending valve to drop the underfloor circuit to its lower flow temperature. Each room or zone gets its own thermostat and its own loop.

What it costs and how long it takes

Quoted after a survey. The variables are floor construction, build up height available, insulation, floor finish, number of zones and whether the heat source and manifold position work, and a price given without seeing those is a guess.

The honest timescale points: a low profile retrofit into one or two rooms is a matter of days. A screed system is dictated by drying time, not by us, and if somebody promises you flooring down within a fortnight over a 65mm sand and cement screed, they are going to lay flooring over a wet screed.