The Ultimate Guide to Underfloor Heating

From costs and installation to efficiency, Tim Pullen explains everything you need to know about wet underfloor heating before you specify it for your home

From costs, installation to efficiency, our expert explains all

Underfloor heating (UFH) is a technology that 20 years ago was lost in almost total obscurity; at best, only available to the rich and famous. Now there are not many self-builders or renovators who don’t at least think about it. Anecdotal evidence seems to show that most then go on to install a system. This change has been brought about by a number of factors, principally that UFH is now far more affordable, which in turns leads to greater familiarity and more access.

HOW MUCH DOES UFH COST?

UFH is still more expensive than a comparable radiator system — estimates vary from 20% to 50% more expensive. Chelmer Heating suggests that the system itself is likely to cost £18 to £22/ m2. Nu-Heat suggest an installed cost of around £40/m2. The figures will vary with the size of the property, the amount of heating it needs and whether it is a new build, renovation or conversion.

Retrofit will always be a bit more expensive, but it could be a lot more if existing timber floors need to be taken up or concrete floors excavated. There are low-profile systems, developed specifically for that situation, that can be installed directly onto existing floors that offer significant savings in retrofit situations. Nu-Heat’s Lo-Pro system, for example, was developed for this situation.

UFH is not cheap and never has been. The issue is that the value of houses has increased hugely in the past 20 years so the amount spent on the heating system has fallen as a proportion of the overall build cost.

GETTING THE FLOOR STRUCTURE RIGHT

There is some discussion as to whether the insulation in the floor needs to go on top of the floor slab or below it. There are specific Passivhaus designs that will wrap the whole floor slab in a great deal of insulation on the basis that the slab provides thermal mass, useful in capturing solar heat in the house. There are others who suggest that only about 10% of the heat in a UFH system is lost to the ground so insulation is not a major issue. (It is not clear how that loss is calculated but even 10% might be considered a lot.)

Building Regs require a floor U value of no more than 0.22W/m2. To achieve this, the floor build-up would typically consist of: the damp-proof membrane, concrete slab, at least 100mm of rigid foam insulation, the floor screed with UFH pipes housed in it, and the floor finish.

The issue here is what is known as the reaction time: that is the time taken for the heating system to bring the room to the desired temperature and to cool back down again. This is largely governed by the thickness of the screed.

A standard sand and cement screed would typically be 65mm to 75mm thick and the UFH could take upwards of two hours to heat the room, but take a similar period to cool back down again. This may be fine in the lounge or kitchen as we tend to spend a lot of time there, but might be less acceptable in a bedroom.

If the UFH pipes are housed in the concrete floor slab, which could be 150mm thick, then the time taken to warm the room will be well over four hours, which would dramatically change how the system can be used.

A further option is the same build-up but with a thinner flow-screed, 35mm to 40mm thick, instead of sand and cement. That would bring the reaction time down to 30 to 40 minutes, which is similar to a radiator system.

Which is right, which is best, will depend on the construction of the house itself and how the UFH system is to be used.

UFH AND SUSPENDED FLOORS

A common option for suspended timber floors (ground or upper floors) uses a system where the pipes are installed between the floor joists on foilbacked rigid-foam insulation (Kingspan, Celotex or similar). The insulation itself is supported by battens which are nailed to the inside of the joists.

The pipes are clipped to the insulation board using pipe clips, and the ends of the joists are notched to allow the pipes to pass from one channel to the next. The pipes are then covered with a lightweight aggregate to fulfil the same function as the screed on a solid floor.

An increasingly popular option is to use a spreader-plate. This is an aluminium sheet that both houses the UFH plate and spreads the heat evenly across the floor. Another option is the Lewis Deck system from CDI-ICM.

Specialist UFH systems for joisted floors, such as Nu-Heat’s ClippaPlate system, can either be installed over the joists from above (before the floor deck is fitted), or between joists from below (before the ceiling is in place). “This system is also great for retrofit as with no height build-up, these UFH systems avoid any disruption to fixtures and fittings, and floor coverings can be fitted as soon as the floor deck is in place,” says Emma Brown of Nu-Heat.

CHOOSING A FLOOR FINISH

Tiles, stone or similar are generally accepted as the optimum covering as they absorb and spread the heat rather than insulate, but they are not the only option. Wood is thought to insulate and reduce efficiency but thinner profile (12mm or 15mm) engineered timber flooring has little noticeable impact on heat output.

Carpet is generally considered a no-no but the Carpet Foundation carried out research in conjunction with the Underfloor Heating Manufacturers Association which shows that some carpets can be used with UFH. The research showed that a carpet and underlay with a thermal resistance of less than 2.5 tog does not have a significant impact on efficiency. According to the Carpet Foundation, a typical 80% wool to 20% nylon carpet with a standard underlay will have a tog value of 1.0 to 2.2.

What this ultimately means is that you can use whatever floor covering you want but it is a good idea to tell the UFH designer so that the pipe layout can be properly specified.

GETTING THE PIPES RIGHT

The pipes are obviously a critical component in a UFH system. One of the main concerns is what happens if there is a leak? The answer is that there must not be a leak. Each room or zone will have pipes laid in a single length with no joints. They must accept bends, radiate heat and last at least as long as the house.

UFH pipes come in two forms: single layer and multi-layer. Multi-layer are usually made up of five layers: inner and outer layers being PE-RT (a form of polyethylene), and an oxygen diffusion layer in