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The Best Floor Coverings for Underfloor Heating
The perfect underfloor system would have no floor covering whatsoever! Polished screed, for example, is an ideal finish and although unusual, is a very attractive option.
Whatever floor covering is used needs to have as little resistance to heat transfer as possible, as in a perfect situation all the heat would travel from the pipes, upwards into the room. However, the laws of physics dictate that at least some of the heat will be lost downwards. Insulation measures will help minimize this loss but it is almost impossible to prevent it completely.
This implies that the more resistance above the more resistance is required below. The first stage is to minimize the resistance of the floor construction and the second to minimize the resistance of the covering, so the most is being made of the insulation underneath.
The less insulative the floor covering the greater the energy efficiency and the happier the customer. So what are the best floor coverings to use?
Ceramic and marble
Ceramic tiles are excellent both in terms of conductivity and thickness. Marble is also a great conductor, but is thicker and has more thermal mass. In either case, consideration must be given to how the covering is being laid and what it is being laid over.
Both options are good over screed, but may require a decoupling layer (such as Schluter-Ditra polyethylene membrane) which will prevent micro-cracks in the screed from propagating through to the tiles.
Ceramic tiles and marble are possible over a floating floor, but the sub-deck must be completely flat. Nu-Heat’s floating floor construction states that the substructure (concrete slab) must be level and flat to SR2 standards (5mm deviation over 3 metres) to avoid deflection. Insulation must be as incompressible as possible and the deck must be rigid. Plywood or specialist products like Knauf’s Brioboard are ideal decks, although the tiler will probably have a preference. For variations of timber floor, the rigidity must be checked.
Tiles must be laid with a continuous layer of adhesive, not dot and dab, and the adhesive or grout must be flexible.
Stone, limestone and travertine have excellent conductivity, but their thermal mass must be addressed. They are usually laid on a (continuous)
As a rule of thumb, Nu-Heat, for example, suggests the overall thickness of the screed plus the stone should be less than 90mm in total (given that these have similar thermal density). Building Regs advises that if screed is more than 65mm, a setback thermostat is needed due to slower response times. (65mm is the minimum for handlaid screed and therefore a programmable or setback stat will definitely be required).
This is less of an issue for a heat pump system. As heat pumps run all the time, response times are not so critical.
Wood flooring moves with changes in atmosphere and small gaps may be apparent between floorboards as they shrink when the heating is on over the winter months and swell during the summer. The narrower the board (maximum 150mm) the less this is likely to happen. It is important that such flooring is designed to avoid too big or too many gaps appearing and, more importantly, cupping and distortion from occurring.
The best option is engineered board. This is because the board is made up of several layers and is cross-plied, giving greater strength and stability.
For solid wood, the thickness of the wood has to be taken into account (this should be below 25mm to aid conductivity)
The traditional fixing for wood floor coverings is secret nailing. This relies on battens to nail to, or possibly a joist.
For floating floors the covering must be made into a raft. Some boards have an interlocking feature but those that don’t must be glued together. Any glue used for the raft must be flexible.
A wood flooring supplier might suggest large amounts of underlay for a level surface and less footfall, but this is contrary to what the UFH supplier will advise. A foam based underlay – as thin as 3mm – is a better option to ensure good thermal performance. It is helpful to take advice from the wood supplier as to the best finish, such as lacquer or varnish.
Wood and controls
The floor covering will not usually have much impact on the controls – unless it is wood. With wood, any temperature change should be gradual. If the heating has been turned off and is suddenly turned on full, this will shock the flooring. So, for initial start-up and at the start of each heating season, the best method is to put the heating on for an hour, turn it off, put it on for two hours, turn it off and so on and so forth – basically allowing the floor to heat up gradually. At the end of the heating season, it is a good idea to turn the heating off in much the same way, but in reverse.
For wood floors – and other sensitive coverings such as lino or Amtico, it is important that the floor surface temperature is limited (usually to 27°C) or there may be problems with moisture in wood and discolouration of lino. An air sensing thermostat with floor-limiting will sense air temperature, call for heat and cut out if the floor goes above a certain temperature. However, the floor temperature is assessed at design stage and if this is calculated to be significantly below 27°C then this thermostat is not specified.
The key issue with carpet (and bamboo / coir), is the tog value, including underlay, as the lower the tog, the better the heating performance. Undoubtedly underlay improves comfort and protects the carpet, but when used with UFH, it is much better to use one that has been designed with thermal performance in mind. When UFH is partnered with a boiler, carpet and underlay can be up to 2.5 tog total. However, in a heat pump / UFH system, it is not realistic to exceed 1.5 tog. If the tog value is proportional to the quality, it is better to use a superior carpet with the lowest tog underlay available, thus making use of most of the tog quota for the carpet.
The perfect way to fit a carpet used with UFH is the double-stick method, where the underlay is glued down and the carpet is glued on top. However, this is not usual in domestic installations where they will more likely be loose laid.
UFH design leaves 100mm free space around the edge of the room, so if gripper rods are put down, the UFH pipe won’t be damaged. However, the gripper rods should not be put across doorways as they may well pierce the pipe.
One of the implications for a house with UFH is that it is best to avoid rugs. The homeowner might not think that these will have any effect on the heating, but rugs actually create insulation – and if you lift up a rug that has been on top of UFH, the floor will be hot underneath.
The choice of flooring should be made before the UFH is designed, so that any allowances can be made at this point. However, as this is not always possible, Nu-Heat, for example, will assume the worst case (heavy carpet) and design to this. Then, if a lighter floor covering is chosen, the flow rates can be adjusted.
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Nu-Heat is the largest specialist supplier of warm water underfloor heating systems to the UK domestic market, delivering fully designed underfloor systems to housebuilders and installers nationwide.