The Millimetres Decide: Why Retrofit Underfloor Heating Is a Build-Up Problem, Not a Heat Problem

The constraint is the ceiling, not the radiator

When people picture underfloor heating they picture the output: a warm floor, an invisible emitter, no radiators eating wall space. In a renovation, though, the decision almost never turns on how much heat the floor can give. It turns on a single fixed number, the height between your existing subfloor and everything that cannot move, the bottom of a door, the nose of a stair, the finished level of the room next door. In a new build the designer sets that stack from scratch. In an existing house it is already spent, and every millimetre a heating system adds has to be found somewhere.

That is why retrofit underfloor heating is really a build-up problem wearing a comfort costume. A traditional screeded system, with pipes cast into 65 to 75mm of sand-and-cement, is wonderful and, in most refurbishments, simply impossible without digging the floor out and dropping the whole level, which drags in new insulation, damp-proofing and often the doors. The retrofit market exists precisely to dodge that excavation, and it splits cleanly into two families, each chosen not by taste but by what your floor is made of.

Two ways in, set by the floor you already have

If you have a solid floor, concrete or an old slab you are keeping, the answer is a low-profile overlay. These are grooved insulation or cement-based boards that sit on top of the existing surface and carry a slim 12 to 16mm pipe, usually finished with a thin self-levelling compound. The whole assembly can come in at roughly 15 to 20mm including that levelling layer. That is little enough to absorb into a floor-finish change in many rooms, though it still eats into door clearances and needs edge trims and thresholds thought through.

If instead you have a suspended timber floor, joists with a void beneath, you can put the heat where there is already space, between the joists, and add essentially no height at all. Pipes are held against the underside of the boards by aluminium spreader plates, thin 0.5mm diffuser sheets that grip the pipe and smear its heat across the deck so you do not feel stripes. The elegant part for a retrofit is that these plates can be fitted from below, working up into the joist bays from the room underneath or a cellar, so the floor finish you are keeping never comes up. The price of that zero build-up is access and insulation: you need to get under the floor, and you must pack the void beneath the plates with a serious layer of insulation, around 100mm of mineral wool as a sensible minimum, or the heat simply falls into the ground instead of rising into the room.

Why the millimetres decide the response, too

The build-up you choose does not only fit or fail to fit; it sets how the system feels to live with. Thickness is thermal mass, and thermal mass is lag. A deep screed stores a great deal of heat and releases it slowly and evenly. It is superb under a heat source that runs gently all day, but it is sluggish, taking hours to come up and hours to back off, which suits a well-insulated open plan and frustrates a bathroom you want warm for an hour in the morning. A low-profile overlay, with far less material between pipe and foot, does the opposite, and so do spreader-plate systems: a slim assembly can bring a room up to temperature in roughly twenty to thirty minutes and respond to a thermostat like something you can actually control. So the same millimetres that let a system fit a renovation also decide whether it behaves like a slow, steady background or a quick, room-by-room emitter, matched to how each room is used rather than chosen once for the whole house.

The real prize: a low flow temperature

None of this is why underfloor heating is worth the trouble in 2026. The reason is what it does to the water. Because a whole floor is an enormous emitter compared with a radiator, it can deliver the same warmth with far cooler water, typically a flow temperature of 35 to 45C against the 55 to 70C an old radiator system was designed around. That gap is the whole game for a heat pump. A heat pump gets dramatically more efficient the cooler the water it has to make; run it into a floor at low temperature across the season and it will commonly return three to four units of heat for every unit of electricity, whereas forcing it to feed hot radiators claws that back toward the cost of the resistance heating it was meant to replace. Underfloor heating is the emitter a heat pump actually wants, and a retrofit floor is often the cheapest way to give a heat-pump conversion the large, low-temperature surface it needs without re-plumbing every radiator to twice its size.

Getting the specification right

A few things separate a floor that works from one that disappoints. Insulate under the pipes, always, because heat is lazy and will go down as happily as up if you let it. Match pipe spacing and flow temperature to the room and its finish: a stone or tile floor passes heat freely, while a thick carpet and underlay can throttle output enough to matter, so the covering belongs in the calculation. Expect a realistic output around 70 to 90 W per square metre from a low-temperature system; if a poorly insulated room needs more than that at heat-pump temperatures, fix the fabric first rather than turning the water up. And zone it honestly, so rooms used at different times are not tied to one slow loop.

When it is not worth it

Retrofit underfloor heating stops making sense where the build-up you can afford collides with the response you need, or where the fabric is so leaky that the floor cannot meet the load at a heat-pump-friendly temperature. A room with no spare height, a floor finish you refuse to lift, and no access from below has run out of options that do not mean excavation. But in the two common cases, a solid floor with a finish change already planned, or a suspended timber floor with a reachable void, retrofit underfloor heating is one of the few genuinely elegant renovation moves: it removes the radiators, warms the surface you stand on, and above all hands your heat pump the low flow temperature that makes the whole low-carbon system pay.