Retrofitting Underfloor Heating

Delivering an even, ambient warmth that is comfortable underfoot and creating room layouts uncluttered by radiators, underfloor heating (UFH) has become the emitter of choice for many new builds. However, these systems can be suitable for retrofitting into older homes as well.

Electric UFH is relatively easy to install in a renovation project, but has greater running costs than water-based systems. Yet the latter are traditionally more complex to install. Here’s what you need to know to make your choice.

Wet underfloor heating systems

At one time, retrofitting underfloor heating only really made sense if you were working on a major renovation project, as it can be a fairly disruptive job – skirting boards usually need to be removed and doors shortened or replaced.

Many manufacturers of water-based UFH have developed lower-profile systems that minimise disruption and height build-up in retrofit situations.

They can usually be used alongside a regular range of products that sit within a deeper level of screed (those that are typically installed in new builds). This is handy if you’re tackling the refurb of an existing house and an extension as part of the same scheme, and want to mix and match.

With suspended timber floors – which you’ll find in most older houses – you have the choice of taking up the floorboards and laying the tubes between them (atop a layer of mineral wool or cut-to-fit insulating board) or adding everything on top of them (sometimes referred to as overfloor heating).

“Ensure that you consider the full height build-up, including any structural decks that may need to be laid over the UFH before your choice of floor covering can be fitted,” says Rachel Roberts, Nu-Heat’s technical account manager.

UFH works at a lower temperature than radiators. As such, it’s a fine match for well-insulated, airtight homes – but installing it in a draughty house with single-glazed windows and no loft insulation will leave you feeling chilly. Some form of barrier within the floor structure is obviously critical to prevent warmth from escaping, too: suspended timber setups can have insulation installed between the joists, and solid concrete ones will need a layer of insulating board (obviously, this will add to any height build up).

If your central heating combines UFH with radiators, you need to be able to control them separately, since they heat up at different speeds. Ideally, the UFH should be warming a different zone to the radiators, and you can split them up at the boiler so that the UFH has its own feed, passing through a blending valve to lower the temperature of the water.

Option 1: Water-based, non-screed systems

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 Rachel. 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.

Option 2: Water-based, screeded systems

This setup has more in common with the type of installation you’d see in a new build, where pipes are sealed into a layer of screed rather than covered over. But where you might expect to have 70mm or so of screed in a new house, low-profile systems require significantly less. Omnie’s ClipPlate Compact uses 14mm panels installed within a 20mm screeded floor, whereas Nu-Heat’s LoPro Max adds no more than 22mm of height.

There’s another advantage to thinner screeds, as Omnie’s Alex Chalk points out: “ClipPlate Compact aims to minimise the drying time compared to higher build-up setups. Depending on the screed makers’ specifications, the total drying time is approximately 24 hours compared to thicker setups, which can take up to two months to dry completely.”

Self-levelling compounds also even out any discrepancies in floor height. “This makes wet UFH setups ideal for running between the original property and a new extension,” says Nu-Heat’s Rachel. With screeded setups, the chosen floor finish can be installed directly on top.

Economy of scale is a big factor in pricing, as costs per m2 drop substantially if you’re installing over a larger area. A Nu-Heat LoPro Max kit (including panels and heating tubes, manifold, self-levelling compound and the thermostat etc) is £2,789 for a 60m2 kit, yet costs £789 for the 4m2 kit.

Electric underfloor heating systems

This kind of arrangement is cheaper and easier to install than wet UFH. The cables that deliver the heat add barely any height compared to pipes that carry water, and a competent DIYer could fit them. While installation costs may be lower, running the system tends to be more expensive. Kits that give you everything you need for a specific-sized room are simple to fit, and typically include the correct-sized mat, a thermostat, floor primer and the tubing required to install the temperature sensor.

A suspended timber floor presents a variety of options. “You can take the floorboards up and heat the void below. Or leave them down and use an arrangement that caters for how stable they are and what covering you want to put on top of them,” says Andrew Dye, director of Floor Heating Systems, which supplies Varme’s products.

The company offers a range of setups for areas that are going to get wet (eg bathrooms) and for different floor finishes, so the spec very much depends on your individual setup. For example, carbon heating film is recommended for laminate or engineered wood boards, with the film and insulation board taking up less than 7mm depth.

Control is by digital thermostat. “We tend to break them into zones – you can do more than one room on a single thermostat, depending upon the electrical loading. Most people manage each individual zone with one controller,” says Andrew, who adds that smart thermostats, which can be remotely operated via an app, are becoming more popular.

Find out more about electric systems

Things to consider

If you are renovating your property or would like to add underfloor heating to a new extension, it’s important to scrutinise which type of setup will best suit the space. Working at lower flow temperatures means that a wet underfloor heating system may take longer to heat a room, so it is vital to combine it with an advanced programmable timer.

Pairing your UFH with thermostats will help establish controllable heat, and you can make each room a separate zone varying its output according to your location within the house and the time of day. Advanced controls offer features such as weather compensation, too, which can improve efficiency.

Water-based underfloor heating works particularly well with heat pumps, which operate efficiently at a low temperature of 35°C-45°C, whereas a traditional boiler system typically heats water at 60°C. So the former’s output is conveniently aligned to the required level for the UFH to function well (around 30°C-35°C). If you want to combine electric UFH with renewable tech, the strongest option will be solar photovoltaic (PV) panels.

Remember to choose your flooring carefully as some coverings are known to work better with this system than others. For example, engineered wooden boards will be less likely to warp than solid timber, while stone makes sense as it’s a great conductor of heat.

If you want the feeling of soft carpet underfoot, ensure the tog rating of the underlay and carpet combined is no higher than 2.5 – otherwise it will be too insulating, reducing the warmth that is available to the room.

Image: Nu-Heat