The performance of your underfloor heating system performs depends on how your floor is constructed. This determines its thermal mass, or the floor’s ability to absorb and store energy, and how fast it releases it.
For example, a concrete floor is ‘high mass’. It will act like a giant storage heater, staying warm for many hours without any heat source.
On the other hand, a timber-based floor is relatively ‘low mass’. An underfloor heating system laid on this floor type will therefore tend to heat up and cool down more quickly, rather like a conventional radiator system.
Floor types at a glance
Download our handy PDF for an ‘at a glance’ look at how underfloor heating is installed with different floor types. Or read on for full details and advice on choosing the right insulation.
Most of the underfloor heating systems we supply are installed under screed, just before the contractor screeds the floor. This type of heating is a direct-acting system. Screed or concrete floors are ideal for underfloor heating as they offer high thermal mass. This enables them to deliver the highest heat output and lowest running costs of any floor type.
Installation is fast and simple as you just staple or clip the rail system directly to the insulation. You then use either traditional or liquid screed to cover it.
- No increase in floor height.
- Compatible with low temperature heat sources such as heat pumps, saving more energy and further reducing costs.
- Pipe spacing is flexible; you can reduce or increase it depending on the building’s heat loss levels and the water temperature supplied by the heat source.
The usual process is as follows:
- Install the damp proof membrane.
- Lay the insulation for the underfloor heating, then fit a vapour barrier on top.
- Use a clip gun to fix the pipes to the insulation using barbed clips.
- Install the manifold, then pressure test this and the pipework.
- Lay the screed. If you’re using liquid screed, take care to make sure the screed doesn’t run between the boards and through gaps in the vapour barrier. Your screed supplier will always advise you of any specific requirements for their products.
See diagrams A to D for alternative installation methods that might be used in solid floors.
Suspended timber floors use joists to support the floor deck. The underfloor heating system either straddles these joists or lies between them. This kind of floor is best suited to well-insulated buildings using oil or gas-fired boilers. It’s also commonly used with first floor installations.
Suspended timber floors require a water flow temperature of around 40 to 60oC, although this will depend on how well-insulated the building is.
- Quick and easy to install, due to the use of aluminium plates.
- Fast response time when used with a high temperature flow of 45oC
- Can be used with both new and existing buildings.
This floor type can’t be used above any uninsulated areas such as a garage unless the correct thickness of insulation is installed to comply with building regulations.
See diagrams E, G and H for alternative installation methods that might be used in suspended timber floors.
A floor with good thermal mass enables an underfloor heating system to operate at lower temperatures (30 to 40oC) whilst still giving high energy performance. This means you can use an air or ground source heat pump as the heat source.
To achieve good thermal mass, the ridged insulation boards must be 60 to 100mm thick for ground floors (or any floor where the space below is unheated) and 25mm for upper floors. You’ll need to leave a space of 20 to 25mm between the top of the insulation and the underside of the floor boards for the pipes and the sand/cement filling.
The sand/cement filling is a just a weak semi-dry mix with no structural strength. However, it will increase the weight load on the joists by around 25kg/m2, so you’ll need to design the floor joists to accommodate this.
- If you’ll be using a gas or oil-fired boiler as the heat source, you won’t need a sand/cement filling.
If you do use sand/cement, you’ll need to make sure this is completely dry before fitting the floor boards. Otherwise, moisture from the cement could distort and damage the boards.
A floating floor is simply a floor that doesn’t need to be nailed or glued to the subfloor. This kind of floor tends to be used when underfloor heating is retrofitted to an existing floor that’s already insulated, or where the space below is heated.
The term ‘floating floor’ actually refers to the installation method but is also used to describe floor coverings such as laminate, floating tile systems and vinyl flooring in a domestic context.
A ‘sprung floor’ is a special type of floating floor designed to enhance sports or dance performance. The term is also used to describe a floor type that reduces noise or vibration.
The installation process for floating floors requires a special type of underfloor heating system consisting of grooved insulation panels and pipework with a foil heat diffusion layer. The insulated panels are available in different thicknesses for floors with higher or lower insulation levels.
Domestic floating floors
Domestic floating floors can be built over a subfloor or an existing floor. You can use a glass fibre, felt or cork layer for sound insulation with neoprene pads holding up a laminate floor. A gap is left between the floating floor and the walls to decouple them and enable expansion. This gap is then covered with skirting boards or mouldings.
Floating floors can add appeal to your home as they’re aesthetically pleasing. However, they’re not recommended for areas that can get wet, such as kitchens, bathrooms and next to external doors.
Floor build-up for floating floors
- Existing floor (concrete or timber) in good sound condition
- Insulation panels of 25 to 50mm thickness, including pipework
- Sub-floor of either timber floor boards or specialist solid floor panels
- Floor covering of stone, tile, timber, laminate, or carpet on top of chip board flooring
Commercial floating floors
Floating floors used in sound studios are either larger versions of the domestic variety or much larger constructions that use concrete slabs to keep the resonance frequency down. The manufacture of integrated circuits uses massive floating floors with hundreds of tons of concrete to avoid vibration affecting the mask alignment.
Floating floors are one of the requirements for the THX high-fidelity sound reproduction standard. This is used for movie theatres, screening rooms, home theatres, computer speakers, gaming consoles and car stereo systems.
You’ll need to install floor insulation on all ground floors and any upper floors where the space below is unheated. It’s important to fit a good, thick layer of insulation to get the best performance from your underfloor heating system.
For standard concrete floors, the usual practice is to place a layer of expanded polystyrene (EPS) below the concrete sub-base. (This sub-base is often referred to as the ‘oversite concrete base’.)
Whilst EPS insulation complies with current building regulations, you should install floor insulation with a higher thermal value when using underfloor heating. With a conventional heating system such as radiators, the floor temperature is normally around 19oC. However, with underfloor heating, it can be as high as 29oC.
This higher temperature causes greater down losses, which means more heat will be lost downwards as the floor is much warmer than the base below. A thicker or higher quality floor insulation will help reduce this.
The floor insulation should also be placed above the oversite concrete base as this will
reduce the floor’s thermal mass to a manageable level. If the mass is too high, the floor will take a long time to warm up and cool down.
There are two main types of floor insulation for solid concrete floors: high density expanded polystyrene (HDPS) and foil or linen faced polyurethane (PU). You can obtain both as ridged sheets or boards.
If you’re using a heat pump, it’s very important to use a minimum thickness of 70mm (100mm if possible). This is because the water temperature in the pipes will be very low, so down losses must be virtually eliminated for the system to provide enough heat.
Another consideration with underfloor heating is the compressive strength of the boards used. When laying the pipework, the insulation boards will be walked over many times. Weak boards such as normal ESP and some types of PU will be crushed as a result, making the floor surface uneven. Some are even too weak to hold the pipe clips or clip rails in place.
At Radiant Heating Solutions, we recommend using a very ridged board such Xtratherm or similar. If we’re designing your underfloor heating system, we can advise on the thickness and type of insulation boards to be used.