Underfloor heating warms a room from the floor up, using either warm water flowing through pipes laid under the floor (a "wet" system) or electric heating mats. For extensions, conversions and new builds, a wet underfloor system is usually the practical choice because it runs at low temperatures, spreads heat evenly and works well with most modern boilers and heat pumps. This guide explains how these systems are put together and what to weigh up before committing.
What a wet underfloor system actually is
A wet underfloor system circulates warm water through a network of plastic pipes laid in loops across the floor area. The water is heated by your boiler or heat pump, then sent out to the pipes through a distribution unit called a manifold. As the water flows through each loop, it gives up its heat to the floor surface, which radiates warmth gently into the room.
Because the whole floor acts as the emitter, the water only needs to be around 35–45°C, compared with 60–70°C for traditional radiators. That lower operating temperature is why underfloor heating pairs so well with heat pumps, which are most efficient when producing lower-temperature water. The trade-off is that the system responds more slowly than a radiator, since the floor and any screed above the pipes must warm up first.
Where it suits a project and where it doesn't
Underfloor heating warms a room from the floor up, using either warm water flowing through pipes laid under the floor (a "wet" system) or electric heating mats.
Underfloor heating is at its best when it can be designed into a project from the start. New builds, extensions and full conversions all allow the floor build-up to be planned around the pipes, so this is where a wet system is easiest to fit and most cost-effective.
It works particularly well with hard floor finishes such as tile and stone, which conduct heat efficiently. Carpet and thick underlay can be used, but they insulate the floor and reduce output, so the total covering should generally be kept below a tog rating of around 2.5. Rooms with large glazed areas or open-plan layouts also benefit, because the heat is distributed across the whole floor rather than coming from a single point.
It suits a project less well where floor height is tightly constrained, as in some conversions, or where you cannot lift an existing floor. Retrofitting into an unaltered room usually means either low-profile systems that add minimal height or accepting disruption to dig out the existing floor. In a single small room being added to an otherwise radiator-heated house, the cost and complexity may not justify it, and a radiator can be simpler.
How a manifold and zones are arranged
The manifold is the control hub of the system. It is a pair of metal or composite bars, one supplying warm water to the loops and one collecting the cooled water returning from them. Each loop connects to the manifold, and small flow gauges or valves let an installer balance how much water goes to each circuit so that every area heats evenly.
A single loop should not be too long, typically no more than around 100 metres, or the water cools too much before it returns and the far end of the room stays cool. Larger rooms are therefore split into several loops that each run from and back to the manifold. The manifold is usually mounted in a central position, such as a cupboard or utility area, to keep the loops a sensible length.
Zone controls let different rooms or areas be heated independently. Each zone has its own thermostat, and a small actuator on the manifold opens or closes the relevant loops in response. This means a bedroom and a living area can be set to different temperatures and times, which improves both comfort and running costs. A pump and a mixing valve, often built into the manifold assembly, blend the hot water from the heat source down to the lower temperature the floor needs.
Screed, floor build-up and response times
Screed is the layer of material that sits over the pipes and forms the surface the finished floor is laid on. Most wet systems embed the pipes in screed because it holds and spreads the heat well. A traditional sand-and-cement screed is typically laid 65–75mm thick over the pipes, while liquid (flow-applied) screeds can be thinner, often 40–50mm, because they flow tightly around the pipes and conduct heat more readily.
The full floor build-up matters as much as the screed. Insulation must sit beneath the pipes so heat travels upward into the room rather than down into the ground or the floor below. Skipping or under-specifying insulation wastes energy and slows the system. Above the insulation sit the pipes, then the screed, then the floor finish.
Thicker screed stores more heat, so the system is slow to warm up and slow to cool, which suits homes that want steady background warmth. Thinner build-ups, such as low-profile boards with grooves for the pipes, respond faster and add little height, which helps in conversions where every millimetre counts. Newly laid screed must be allowed to cure fully and then be brought up to temperature gradually before a floor finish goes down, which can add weeks to a build programme.
What underfloor heating costs to install
The cost depends heavily on the floor area, the type of system, the screed and how much the floor needs to be built up or dug out. As a broad guide, supplying and fitting a wet underfloor system in a new build or extension where the floor is open is far cheaper per square metre than retrofitting into a finished room, because there is no demolition and the layout can be planned cleanly.
Beyond the pipes and manifold, the budget should account for insulation, the screed and its drying time, the thermostats and wiring for zone controls, and any work to connect the system to the heat source. Larger areas tend to cost less per square metre than small ones because the manifold and controls are a fixed cost spread over more floor. Anyone comparing quotes should ask exactly what is included — particularly insulation, screed, controls and commissioning — since these are easy to leave out of a headline figure.