How underfloor heating (UFH) works?

In its simplest term Underfloor heating or UFH is a method of warming the building by using the floor as a radiator.  However, unlike the radiators that you may be used to, the floor is operated at a much lower temperature.  The reason for this is that the floor is a far larger area than a radiator and, therefore, does not need to run at such a high temperature to heat the room.

The Romans had a form of UFH that used warm air from a fire drawn through ducts in the floor.  Today we use warm water to through pipes in the floor to achieve the same result.  Vast improvements to the thermal insulation of our modern buildings makes UFH the best option for heating. 

Heating Pipes are pipes laid in the floor at the time of construction or during renovation work.  Warm water is circulated through these underfloor heating pipes that are either embedded within a concrete screed or within a suspended timber floor (see floor types).

radiator heat profileunderfloor heating profile

The water is distributed to the pipes through manifolds at between 35-45°C and heats the floor surface to a temperature in the range of 22-29°C.  Heat is emitted into the room as approximately 60% radiant heat, 20% convection, and 20% conduction.  

Each room or area would have its own circuit and be individually controlled by a room thermostat.  When the room reaches its pre-set temperature the thermostat sends a signal to an electrically operated valve on the manifold, closing that particular circuit. The floor effectively becomes one large heat store giving off a gentle, even, radiant heat to the whole room.

Any type of boiler can be used to provide heat although the condensing boiler option or a heat pump offers far greater energy savings.

Can I fit Underfloor Heating (UFH) in to my building?

UFH can be used in any type of building and in most types of floor structures. What best describes your project - New Build & Complete Renovation, Partial Renovation, Retro-fit.

New Build & Complete Renovation Plan the use of UFH from the very start of the project.  Decide what you are going to use to generate the heat because this can affect the way in which UFH is used. 

For instance, if you plan to use a heat pump then you will need to put in more pipe and also use one of the floor types that has good thermal mass.  This will ensure that the heat pump always works with a high level of efficiency.  It is also better to have all suspended floors in block and beam rather than timber.  Block and beam has a number of advantages over timber such as excellent sound suppression, improved fire prevention, good thermal mass and the ease of putting in UFH.  However, thermal mass can still be used with a timber suspended floor.

If you intend to use a conventional boiler (modern oil or gas condensing type) then you can use any of the floor types.  The pipes can also be spaced a little further apart which will keep the installation cost down. 

The building itself can be of conventional brick or stone, timber frame, structurally insulated panels (SIPS) or architectural glass and steel.  It is only the floor structure that needs to be carefully considered when deciding to use UFH.

Partial Renovation This is similar to a Complete Renovation although the levels of insulation may not be able to be quite so high. 

If the ground floors are to be removed and replaced to overcome damp or structural defect then UFH is the obvious choice in the new solid floor.  If the ground floor is the suspended timber type and is structurally sound, UFH can still be fitted.  The floor boards have to be lifted to expose the timber joists.  The floor is then insulated between the joists and pipe fitted to this insulation (see floor types, suspended timber floors, thermal mass).  If the joists are strong enough to take a thin layer of screed then this should also be applied.  Thin screed of around 20 – 25mm depth and adds an additional weight to the floor joists of around 25kg/m2.  The prime benefit of this is to add thermal mass to the floor which will give an improved performance for the UFH.  Lifting the floor boards is also an ideal opportunity to treat the joists with preserving agents to give long term protection.

For suspended first floors that are structurally sound the choice is quite limited.  You either have to take up the floor boards, take down the ceiling or fit a floating floor system with a new floor over the top.  The latter is very expensive and adds around 25 – 50mm extra height onto the existing floor.  For reasons of cost you may therefore decide to use radiators instead of UFH.

Retro-fit Removing a radiator and fitting UFH in an existing solid ground floor room is never straightforward.  The UFH pipes have to be placed in a new floating floor (see floor types) and this requires a minimum space of 25mm plus the floor board.  This means the total floor build up is around 44mm over the existing floor.  If this is not what you want then the existing floor will have to be taken up to gain enough space for the insulation, pipe and the finished floor.

The normal depth of screed is typically 50mm and this can often be very easy to break up and remove.  If the floor already has insulation below the floor slab (oversite concrete) then Radiant has a special 4mm thick insulation foil to lay over the exposed slab.  This foil reflects the heat from the pipes up into the new screed.  A reinforced screed of 46mm can then be applied to bring the floor back to the same level.

The method for suspended timber floors is the same as described in Partial Renovation.

Floor Insulation – what type and why?

On all ground floors and even raised floors where the space below is unheated, floor insulation must be used.

It is important to put down a good thickness of insulation when fitting Radiant Radfloor underfloor heating (UFH) pipe. 

In a standard concrete floor expanded polystyrene (EPS) is placed below the concrete sub-base.  This sub-base is often referred to as the oversite concrete base.  This EPS insulation will be just enough to comply with current building regulations.

However, with UFH the insulation should be to a higher thermal value because the floor will now be heated.  With normal heating such as radiators the floor temperature is around 19°C whereas with UFH the floor temp can be as high as 29°C.  This higher temperature leads to what is known as greater down losses.  In other words the floor is much warmer than the base below and so more heat will be lost downwards.

To prevent this waste of energy a better quality or thicker floor insulation is used.  Furthermore the floor insulation is now positioned above the oversite concrete base.  The reason for this is to reduce the overall amount of thermal mass to a manageable level.  If the mass is too great (too thick) the floor will take a very long time to both 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).  Both of these are supplied in ridged sheets or boards.

For floor heating another requirement is the compressive strength of the board.  When laying the pipe for the UFH the insulation boards are walked over many times.  Weak boards such as normal ESP and some types of PU will crush making the floor uneven.  Some are even too weak to hold in the pipe clip or clip rail in place.

Radiant always recommend using a very ridged board such as Kingspan TF 70 or similar.  If we are designing the UFH scheme we will state the required thickness and type of insulation board in our quote.

When using a heat pump it is very important to use a minimum thickness 0f 70mm (100mm is recommend if practical).  This is because the water temperature of the UFH pipes is very low and down losses have to be virtually eliminated.

A typical build up of the floor from ground up is shown in the solid floor section of floor types

For a suspended timber floor with an unheated space below the insulation boards are fitted between the joists.  The UFH pipes are fixed to the insulation with clips or in
a heat diffusion plate.  When using clips the space between the pipes and theunderside of the floor boards can be filled with a very weak screed mix to give thermal mass.  The additional weight of this weak screed is around 25kg/m2 which is easily accommodated at the design stage. 

Radiant also use a special foil and clip system for suspended timber floors where the space below is heated.  This system has no thermal mass so is best suited for traditional boiler systems (see floor types).

Floor Coverings
wood floor

Hardwood flooring and wood laminates Underfloor heating is actually very good for wooden floors because it maintains a constant even temperature over the surface of the floor.  It is good practice to lay the natural wood flooring out loose to allow it to acclimatise to the room in which it is to be finally laid. This acclimatisation should be for a minimum period of 10 days but only once all moisture is removed from the building.  Don’t forget, building, plastering and screeding introduces lots of water into the fabric (walls and floors etc) of the building.  This water has to dry naturally so that the internal humidity of the building is reduced to a normal level.  You don’t want your beautiful new wood floor to absorb all this moisture as it will warp and be permanently damaged.

You can have natural hardwood rather than engineered, if you wish, but the surface temperature will need to be limited to around 27°C.  If the floor is too warm them damage to the wood may occur once the heating comes on.  Close control of floor temperature is achieved by fitting a floor sensor.  Before laying a wood floor the concrete must be thoroughly dried out.  As a rule of thumb a concrete screed dries at approximately 1 day per 1mm thickness.

There are various ways to lay hardwood but in every instance you should follow the manufacturer's guidelines.  Some woods require a nail fixing, and for this it would be necessary to fit battens into the screed to facilitate this, or fix directly into the joists, depending on the floor construction.  Most hardwoods are either glued directly to the dried screed with (flexible) mastic adhesive or floated over it.  If floating, the wood is clipped or glued together and it then lays over a proprietary hardwood underlay. This allows the entire floor to expand and move as all natural materials do.

carpet floor

Carpet The carpet and underlay should not have a combined tog rating greater than 1.5.  Radiant’s radfloor underfloor heating can be used in conjunction with most types of carpet although natural fibres are best.  Carpets will act as an insulator so the output from the floor will be slightly reduced.  Most reputable carpet manufacturers have approved carpets and underlays for underfloor heating.  If you are in any doubt please contact us with your questions.

natural stone floor

Stone or Tiles Any type of hard stone or ceramic tile is an excellent floor covering for Radiant’s radfloor underfloor heating.  The thermal resistance of a hard material (concrete, stone, clay, rock or ceramic) is extremely low.  Heat therefore passes through will virtually no losses at all.  This makes a hard floor finish perform better than any other type of floor finish.  It is therefore the idea choice when using low temperature heat sources such as a heat pump.

Are there any drawbacks to Underfloor Heating?
  1. Some consideration should be given to the type of carpet to be used.  Natural fibre carpets give a better performance than man made fibre.
  2. The type of underlay used should not be the closed cell foam type as this is an insulator, but crumb rubber based underlay.
  3. Underfloor Heating can be much slower to warm up than radiators.  It is, therefore, not practical to have the UFH on for short periods as you would with radiators.  The UFH should be selected to run for between 4 to 12 hours a day.  This may seem extravagant, but the reality is that once the fabric of the building has been heated the UFH system requires little energy to maintain an appropriate level of heat.
  4. The possibility of putting a nail through the UFH pipe is no more likely than for any other type of pipe.  Remember that Multi-Layer Pipe is a plastic/metal pipe and is easily detected with a metal detector.  If the UFH pipe is buried within screen then there is at least 50mm of screed above the pipes.  For a timber floor the pipes will be directly below the timber so great care should be taken.  In the unlikely event that a continuous length of pipe is damaged, a simple connection piece is all that is required to make a permanent repair.
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A thin flat metal plate with one or two pipe channels pressed in to it. These plates are either 1 or 2 metres long and are placed directly below wooden floor boards. The plate effectively increases the surface area of the pipe which improves output. Used on both floating and suspended timber floors. Not recommended for use with Heat Pumps.
Thermal mass (also called heat capacity) is the capacity of a body to store heat. This is achieved in a building by placing the UFH pipes in a concrete or gypsum screed. A further benefit is that this mass can have a lower temperature input without loss of performance. It is therefore ideal for low temperature UFH using heat pump systems.
Thermal mass (also called heat capacity) is the capacity of a body to store heat. This is achieved in a building by placing the UFH pipes in a concrete or gypsum screed. A further benefit is that this mass can have a lower temperature input without loss of performance. It is therefore ideal for low temperature UFH using heat pump systems.
Thermal mass (also called heat capacity) is the capacity of a body to store heat. This is achieved in a building by placing the UFH pipes in a concrete or gypsum screed. A further benefit is that this mass can have a lower temperature input without loss of performance. It is therefore ideal for low temperature UFH using heat pump systems.
When using a heat pump the average temperature of the water in the UFH pipes is 35°C. This is cooler than the UFH water from a conventional boiler. In order to get the same output from the floor the pipes must be installed closer together. On average a heat pump system will have 30% more pipe in the floor than a conventional boiler system.