Underfloor Heating FAQ: Floor types, screed options, the latest products & more

Expert advice on choosing underfloor heating, from which flooring works best with UFH through to whether you should insulate first
by Build It magazine
16th November 2018

Underfloor heating (UFH), both wet and dry, is loved by homeowners 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.

What flooring type should I choose for underfloor heating?

The best flooring for any underfloor heating system 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.

Underfloor heating & tiles

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.

Underfloor heating & timber floors

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.

When it comes to timber flooring and UFH, 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, and 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.

Underfloor heating & laminate flooring

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.

Underfloor heating & carpet

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.

What underlay should I use with underfloor heating?

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 suppliers will take this, and all other relevant information, into account when designing the underfloor heating system.

When it comes to electric underfloor heating, 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.

Can underfloor heating be retrofitted?

Yes! Underfloor heating has come a long way and there is an array of products available that require minimum disruption to your home as they feature low-height build up.

Companies have developed a series of slimline products specially designed for retrofitting underfloor heating. One option is Uponor’s Minitec, offering a build-up height of 15mm, which can be laid on top of existing flooring and has a quick drying time thanks to the thin screed.

“Minitec can be easily customised to fit around obstacles from your existing home,” says says Uponor’s Amar Dharwar. “Its great advantage is that it doesn’t require major adjustments to door frames etc.”

Unscreeded UFH – known as dry (not to be confused with electric systems, also referred to as dry) – comprise panels with the piping inset into the insulation and overlay.

These units work well for suspended floors as they fit in or onto the joists. “Dry UFH is ideal for renovations without space for a new screed,” says Omnie’s Susannah Gardner.

Find out more: Browse underfloor heating systems in our product directory

Dry water-based systems are quick to install, with homeowners able to lay their choice of flooring straight after the UFH fitting. In addition, dry UFH can heat up faster than wet options with thick screed, as the pipes are laid as close to the floor finish as possible.

“Heat emission plates are the best option to use on suspended timber floors,” says Amar. “Homeowners will need to ensure all insulation and necessary battening work is completed before laying the boards.”

Nu-Heat recently launched LoPro Lite, a dry product addition to its retrofit range. “This system was designed to increase ease of installation while maintaining low profiles,” says Emma Brown, from Nu-Heat. “The use of panels means there’s no waiting around for the screed to be ready.”

It’s becoming more popular to install a mix of wet and dry water-based systems to suit the different aspects of a project. “While the right choice of system for new dwellings is highly dependent on the individual build requirements, as a rule of thumb we would suggest utilising a screed system for the ground floors and heat emission plates for a suspended first floor,” says Amar.

Should I choose electric underfloor heating?

There are many great quality electric underfloor heating systems. What’s more, electric underfloor heating is cheaper and easier to install than its water-based counterparts.

Electric UFH consists of looped cables laid beneath the floor finish that are heated as power runs through them. Cabling can be embedded into the screed, although in most cases it’s laid on top for ease of installation.

Although cheaper to install, because this system is powered by electricity, it may not present savings when used throughout large builds. Instead, it’s best placed in one-off rooms such as conservatories and bathrooms where you only need the occasional boost of warmth.

One of electric UHF’s main appeals is its rapid response when turned on, with the temperature difference beneath your feet noticeable almost instantly.

“Electric UFH can actually be more economical to run across smaller surface areas, such as rooms under 22m2,” says Amar. “There’s a low initial setup cost for this option. We recommend installing it within bathrooms, bedrooms and kitchens – the areas where you may often be bare foot.”

How much does it cost to install electric underfloor heating?

If you are fitting UFH in a standard shaped room, then an electric mat may be the solution. Prices for these products start from £16 per m2 of cabling, while the mats themselves cost from £20 per m2 upwards.

Can underfloor heating be used upstairs?

Once again, thanks to the advances in underfloor heating products, there’s no reason why you couldn’t install underfloor heating on your upper floors.

Companies continue to innovate to meet homeowner’s needs, whatever their project and lifestyle. Omnie’s Torfloor RdB, for instance, offers acoustic insulation within the panels, making it a popular product for households with children – as it can minimise noise and vibration coming from the upper floors.

“This panel incorporates acoustic insulation, structural floor decks and underfloor heating all in one,” says Susannah, from Omnie. “It’s not only quick and easy to install – just as fast as any other structural floor deck – but it also reduces build-up.”

However, if your build is well insulated it may be sufficient, and cos effective, to install underfloor heating on the ground floor and use radiators for the rooms upstairs.

Underfloor heating vs radiators

One of the most popular reasons to choose UFH is because, as the name suggests, it is hidden beneath your floor covering. This allows for clean lines and an open flow throughout your home that isn’t interrupted by radiators.

So, with underfloor heating you can enjoy large expanses of glass and you won’t have to worry about where to fit your furniture. Freeing up wall space will give you the feeling of extra room, too.

Having said that, modern radiators now come in beautiful designs that nearly resemble art pieces, bringing real wow factor to your home interiors.

How should I distribute UFH and radiators?

Underfloor heating is great for use in large rooms – so it’s ideal for open-plan spaces. The gentle warmth of underfloor heating also makes it a good choice for bathroom spaces.

One potential downside is that UFH takes a long time to get up to temperature, so it needs to be on fairly constantly during the heating season (albeit at a low temperature, which leads to good efficiency and cost-savings in use).

When fitting radiators, you ideally need to have one installed every 4m in a room, so they aren’t ideal for large open-plan kitchen extensions, for example. But they work well in bedrooms, which tend to be smaller in size.

Whether you’re self building, renovating or extending, selecting the right heating system is a major decision. From design considerations through to energy efficiency, running costs and other practical concerns, there’s a lot to wade through when choosing between underfloor heating (UFH) and radiators.

Here’s what you need to know…

Read the complete low-down: Radiators or Underfloor Heating?

Underfloor heating vs radiators: energy efficiency

UFH can be extremely eco-friendly and affordable to run, as long as it’s installed properly and partnered with the correct flooring material so it can conduct heat. Stone, ceramic, engineered timber or vinyl flooring are best, but always check with your supplier before buying.

Wet UFH can be around 25% more efficient than radiators when paired with a modern condensing boiler; and up to 40% more effective when running from a heat pump.

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

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.

Georgia_Betts_manifold_underfloor_heating

Georgia is renovating and extending her home in West Sussex. You can read her journey here

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.

Can UFH be used on any type of floor substrate?

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 flooring, 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.”

How do I choose the type and depth of screed?

The first consideration ahead of specifying the screed 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.

How long does is takes for the screed to dry?

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.

Underfloor heating without screed: What are the options?

Non-screed systems are known as dry underfloor heating. The terminology can get confusing here, as manufacturers of water-based underfloor heating refer to products that don’t require screed as ‘dry’ – not to be confused with electric systems, which are also known as ‘dry’, because they don’t carry water.

“Plumbed-in dry UFH systems are best suited to properties that are reasonably well-insulated,” says Nu Heat’s Rachel Roberts. This is because they have a lower heat output than screeded options. Nu-Heat’s LoPro 10 is a pre-routed gypsum board that holds the UFH tube in place, adding just 15mm to floor heights. “Most types of floor covering can be fitted immediately, directly over the top,” says Rachel.

Other options include Omnie’s LowBoard, which comes in 15mm or 22mm thicknesses, and includes a foil layer to promote heat diffusion. Alternatively, VarioComp’s 18mm panels claim to give the highest heat output for specialist retrofit UFH systems.

Expanded polystyrene panels such as ProWarm are also available, but their lightweight composition means that they are not suitable for tiling over.

Find out more: Browse underfloor heating systems in our product directory

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

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.

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

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.

Main image: This professional is installing underfloor heating from Robbens Systems

One Answer

  1. gmando says:

    Hello wondering if someone can help- i have electric underfloor heating and replacing the carpet in the bedroom with laminate. It has concrete floor. Do i need to put a polythene layer down before the underlay?

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