Q&A: Underfloor Heating

Expert advice on choosing underfloor heating, from which flooring works best with UFH through to whether you should insulate first
by Build It
28th November 2012

Underfloor heating (UFH), both wet and dry, is loved by self-builders for its even heat distribution, energy efficiency and compatibility with green methods of heating. But while the idea is simple, the practicalities might not be quite so straightforward. What are the implications for flooring? How long does it take? Can I extend the system later?

Here we answer reader’s questions about installing UFH in new build or renovation projects.

Q: Can UFH be used on any type of floor substrate? What are the considerations?

A: Underfloor heating, both wet and dry, can be installed on most floor constructions, as long as there is enough space to build up the floor, taking into account doors, stairs, etc, and the floor can take the weight. The key to success is to choose the right floor-heating system for the construction. For example, on screeded floors, pipes can simply be clipped on top of insulation, then covered with screed, or on a joisted floor a plated system can be used, as long as it is well supported and has no movement.

Floating floors are a popular choice for renovation projects, as they have little height buildup. One suitable product is Nu-Heat’s heat diffuser plate, designed to fit into a high-density polystyrene base layer, which can be put down on top of the standard insulation layer or existing deck, and tubing simply walked into it. The chipboard or final timber floor deck is laid directly on top of the panels.

Floating floors can also be installed in new builds, where there is 50mm allowance between the foundation slab, or beam-and-block floor, and the door linings. The insulation required is 77mm plus final floor covering over heated areas, or 117mm plus final floor covering over unheated areas.

On suspended timber floors, ‘plate’ or ‘foil’ systems can be used, whereby an aluminium heating plate is installed on top of the joist, and below the deck – it’s easiest to do this from below. However, as James Garrod of Continental Underfloor Heating points out: “Any screeded system will provide a higher W/m2 output than a plate or foil system. Ensure your contractor or supplier gives you detailed heat-loss calculations and CAD designs, and ensure that the product is insured and backed up by manufacture warranties.”

William Scott-Malden at Ebeco says: “Electric UFH can be used on top of any floor substrate, although we recommend a limit of 135W/m2 on wooden sub-floors. There is no limit on concrete.”

Q: What do I need to take into account when determining the type and depth of screed?

A: The first consideration is getting the right amount of pipe-work into the floor and in the right areas to provide between 40W/m2 and 55W/m2 of heat. Too much can overheat the floor, and cause the thermostat to overshoot, whereas too little would mean the system could be running constantly, increasing your heating bills.

When it comes to the screed, thicker screeds will take longer to warm up, but will then retain the heat for longer. The minimum British Standard is 65mm of hand-mix (3:1 sand/cement) or 50mm of liquid (or self-levelling) screed. Such a thickness will take about one hour to heat up and cool down.

But as James says: “Providing that UFH is run correctly (allowing the system to monitor and maintain the temperature), the only time you’d really notice the difference in warming and cooling times is when you’d return from a holiday and the heating has been off for the duration.”

Liquid screed’s density makes it a good heat conductor, maximising heat transfer and enhancing efficiency. This makes it particularly good for use with heat pumps, according to Heather Oliver at Nu-Heat, as it maximises the output of the UFH, allowing the heat pump to work at a lower temperature, optimising its efficiency.

Something else to look out for, according to Paul Mee of Robbens Systems, is that self-levelling screeds can need additional pipe fixings to add rigidity and stop the pipes from lifting. “Thinner liquid screeds can also give the end user the feeling of ‘tracks’ in the floor rather than an even temperature. He also says that, sometimes, additional pipe-work needs to be integrated.

Q: How long does the screed need to dry before flooring is laid on top?

A: Both types of screed generally dry at a rate of one day per millimetre for the first 50mm, and two days per mm after that. You can turn the UFH on – at a low temperature – 30 days after laying a standard 3:1 sand/cement, screed. No flooring should be laid until the screed is completely dry, otherwise the floor covering could be ruined over the first few months.

Heather Oliver of Nu-heat adds: “If the floor covering is impermeable, the moisture content should be assessed, either with a hygrometer, or by covering an area of the floor with plastic sheeting overnight and seeing if there is condensation under it in the morning.”

Drying times will be much less with a liquid screed – which will be ready in seven days – not only because it’s thinner, but also because it has better drying characteristics, and can be dried using dehumidifiers.

Q: Is insulation needed beneath the UFH? If so, how much?

A: Insulation under your UFH is always recommended. It ensures that as much heat as possible goes upwards, into the house, and as little as possible goes downwards, into the ground. Insulation below the pipes should therefore be much greater than the insulation created by the floor covering.

The position of the insulation within the floor structure is important, too. After changes to Building Regulations in 2002, all new builds need to have insulation below the concrete slab. But with UFH, more insulation needs to be added above the slab, directly below the pipes, to reduce the mass of material to be heated, improve response times, and reduce downward heat losses.

How much more insulation is added depends on how much, if any, insulation is in the slab, and the height allowance of the floor. A rule of thumb is to just use as much as possible. Your UFH supplier will be able to advise on and provide any insulation you need. Edge insulation provides extra coverage by allowing expansion of the screed as it heats, and additional insulation should be used against external walls.

Q: What flooring is best for UFH? Are there any special laying requirements?

A: The best flooring for any UFH system is solid, such as ceramic tiles, slate, or stone flags, as they have the best thermal conductivity, and will provide the best heat output. But bear in mind that a screeded construction will expand and contract with the heat. In the screed, this is absorbed by an expansion gap and edge insulation, so any tiles fitted on top – which may well expand at a different rate to the screed – should also be laid with an expansion gap around the perimeter of the room (the bigger the room, the bigger the gap should be). This gap can usually be covered by a skirting board. Flexible adhesive and grout must be used, or you’ll be taking up your tiles again in six months.

Suspended timber floors, by their nature, are more prone to movement than screed, but tiles can still be used by covering the joists with a minimum of 18mm (although 22mm is the recommended measurement) staggered chipboard or ply panels with glued joints screwed down securely. Once again, you’ll need to use flexible tile adhesive and grout.

Wood flooring also goes fine with UFH, providing that it is less than 22mm thick, the boards have a width-to-thickness ratio of no more than 4:1, it has a moisture content of six to eight per cent, and that the floor temperature does not exceed 28°C. The most suitable type is engineered board, which is made of several layers laminated together, often with the wood grain running in opposite directions in each layer, finished with a thick hardwood surface. As it is an inherently stable material, expansion and contraction will be minimal.

Solid timber is also suitable, although narrower boards are recommended as they expand and contract less with atmospheric changes than wide boards. Thin laminate flooring is less suitable, and will need a chipboard or ply sub-deck to ensure stability. Good-quality laminate floors and engineered boards will probably have their moisture content corrected before lamination, but care should always be taken to acclimatise any timber floor. To avoid excessive movement of the finished floor, the screed, timber flooring and internal construction (wall and ceiling plaster) must all be moisture-free, which may mean leaving boards unpacked in a dry room for several weeks to acclimatise.

Despite popular misconceptions, it is perfectly possible to fit carpet over underfloor heating; it’s just a question of choosing the correct combination of carpet and underlay. To get the best performance from your UFH, the combined tog value of the carpet and underlay should not exceed 2.5. The typical tog value of an 80 per cent wool/20 per cent nylon carpet will be between 1.5 and 2.0, and as the tog value of underlay varies enormously, it’s possible to source a combination to suit your needs. Quality manufacturers will have technical information on their products available to customers.

Q: Should I use an underlay?

A: Both wood flooring and carpet suppliers usually prefer underlay to be fitted with their products, while UFH suppliers will want as little underlay as possible to minimise thermal resistance. All good UFH supliers will take this, and all other relevant information, into account when designing the underfloor heating system. When it comes to electric UFH, most materials can be used, with tiles once again the best choice. Cork and rubber flooring should be avoided, except for non-heated areas, such as beneath fridge and freezers or any cooler areas for pets to retreat to.

Q: I may want to extend my UFH system later. What should I install at this early stage?

A: It’s always easier to factor any planned extension into the original installation by allowing for future feed pipes and the extra ports on the manifold (the main distribution centre). But some manufacturers, such as Continental, offer zone extension kits and ‘additional’ bolt-on manifolds. However, the key consideration is the placement of the manifold at the outset – you don’t want to tear up the screed to relay pipe-work travelling to an extension.

As Paul Mee at Robbens points out: “Extending the manifold can be the easy part – laying new pipes through an existing kitchen could be a major issue.” One solution, he says, is to lay ‘dummy-loops’ into the floor, which could be connected later, should one decide to extend the underfloor heating system. Electric UFH is more flexible than wet UFH, and can be laid room by room without the worry of extra manifolds, plumbing, etc – all it needs is an electricity supply in the room that is to be heated.

Q: How can I avoid hotspots, eg where many pipes pass through a doorway?

A: One solution is to insulate alternate pipes to restrict the temperature in that particular area. British Standards require that surface floor temperatures must be below 29°C, so most UFH suppliers and contractors will conduct heat loss calculations as part of their design process. You do not tend to get hotspots with electric UFH, as the elements are laid with equal distances between the centres.

Q: What if I don’t want the disruption of laying wet UFH? What flooring goes with electric UFH?

A: Most electrical ‘floor warming’ systems are geared towards tile heating, although increasingly wood floors are also being used – it’s worth checking with each supplier as there are many different types on offer.

Main image: Worcester Bosch‘s Greenfloor underfloor heating can be used in tandem with a range of floor coverings

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