Under Floor Heating

Screed or low-profile: the floor height decides it, not the budget

In a new build or a full slab replacement, pipes are clipped to insulation and buried in a screed of fifty to seventy millimetres. The screed is the point: it holds heat, spreads it evenly and releases it slowly, which is what makes underfloor heating comfortable. The cost is inertia — a screed floor takes hours to come up to temperature and hours to come down, so it is controlled by leaving it on at a steady temperature rather than by switching it on and off.

In a retrofit, that build-up is usually impossible: adding sixty millimetres to a floor means the doors no longer open and the bottom stair becomes the wrong height. Low-profile systems exist for exactly this, in panels of around fifteen to twenty millimetres, and they respond much faster because there is far less mass to heat. They also store far less, so they suit a room used in bursts rather than a whole house. Which of the two you can have is decided by measuring the floor build-up and the door thresholds, and that is a survey, not a phone call.

Insulation underneath is not optional

A heated floor emits in every direction. Without proper insulation beneath the pipes, a substantial share of what you pay for goes into the ground or into the ceiling below, and the room above never quite reaches temperature on a cold day. This is the most common fault we find in underfloor heating that somebody else installed and the customer describes as underpowered — it is not underpowered, it is uninsulated.

It is also the one thing that genuinely cannot be corrected later. Every other part of the system is accessible: the manifold, the actuators, the controls, the blending valve. The insulation is under a floor that has been laid over it. Getting it right at the point the floor is open is the difference between a system that works for thirty years and one that is a permanent disappointment.

Why underfloor and radiators need different water, and different controls

Radiators are designed around water at sixty to seventy degrees. A floor cannot take that — the surface temperature has to stay comfortable underfoot and within limits set by the floor covering, so underfloor circuits run at something closer to thirty-five to forty-five degrees. A house with both needs a blending valve and a pump on the underfloor manifold to take hot water from the boiler and mix it down to the floor's temperature.

The controls then have to respect that the two behave differently in time. A radiator responds in minutes; a screed floor responds over hours. Running both from one programmer means either the floor never catches up or the radiators overshoot. Separate zones with their own thermostats, and a floor set to hold a temperature rather than to chase one, is what makes the combination work. It is also the part most often left out of a cheap installation, and the reason people say underfloor heating was not worth it.

One practical note that saves arguments later: the floor covering matters. Tile and stone conduct well and suit underfloor heating best; engineered timber is fine within the manufacturer's temperature limit; thick carpet and heavy underlay insulate the room from the very floor you are heating.