Thermal Insulation for Radiant Floor Heating Systems

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Thermal Insulation for Radiant Floor Heating Systems

Thermal Insulation for Radiant Floor Heating Systems

Ideally, a radiant floor heating system should direct its entire energy upwards, towards the indoor space being heated. However, the lack of appropriate thermal insulation can allow heat to travel downwards as well, which represents an energy loss.  Where a slab is on the ground then what is happening under the slab can absorb large amounts of heat, particularly during the winter.  This effect can also be significant for the suspended slab of a building where heat can be lost to the area below the slab.

The best way to control heat loss from radiant floor heating systems is to install thermal insulation below the electric heating cables or the hydronic heating pipes.  This way, most of the heat is then directed towards the room, greatly increasing energy efficiency.  Such a system can operate with a lower energy input than one without insulation, reducing its running costs.  In addition, insulation provides a faster warm-up time and allows improved control over the heating to enhance comfort.

This article will provide an overview of the thermal insulation products we recommend at Devex Systems, based on their field-proven performance.


F-Board is a thermal insulation product made from rigid extruded polystyrene sheets with a coating on both sides.  This installation is particularly effective under X mat and tiled floors, which reduces downward heat loss and so provides a faster warm-up time.  F-Board also offers water resistance and compressive strength, while being lightweight and easy to install. Two products are available, each with a different thickness and insulation R-value:

  • F-Board  6-mm, R0.16
  • F-Board 10-mm, R0.29

Although F-Board can be installed under a screed, the reduced thickness of the product diminishes insulation performance.  Other thermal insulation options (such as XPS sheet) are usually better suited when a screed is used.  View the data sheet.

Extruded Polystyrene (XPS) Insulation Sheet

Extruded polystyrene rigid sheets are designed for a broad range of floor constructions and applications:

  • Insulation between an existing slab and a topping slab.
  • Under a screed on a structural slab, complemented with a light steel mesh.
  • Between battens under diffusion plates or timber floors.
  • Freezers, cold storage rooms or other similar applications

The FoamulaR brand of XPS insulation is designed to maintain 90% of its thermal performance after 20 years, and is unaffected by moisture, mould and bacteria.  It is available with various insulation thicknesses and R-values:

  • FoamulaR 25 mm, R0.89
  • FoamulaR 30 mm, R1.07
  • FoamulaR 50 mm, R1.78
  • FoamulaR 75 mm, R2.67
  • FoamulaR 100 mm, R3.57

For maximum thermal insulation performance, XPS insulation should be installed not only over flat areas, but also against slab edges and trenches before pouring concrete.  FoamulaR has compressive strengths that range from 150 kPa to 650 kPa (though FM300 at 300kPa and FM350 at 350kPa are the most common) so the product can adapt to a broad range of applications.

REHAU Tacker Board for Hydronic Installations

REHAU Tacker Board is a system designed exclusively for hydronic floor heating systems embedded in-slab, accomplishing three main functions with a single product:

  • Thermal insulation
  • Acoustic sound attenuation
  • Hydronic piping layout and support

Tacker Board uses expanded polystyrene (EPS) insulated board covered with a fabric reinforced water-resistant polyethylene foil plastic sheet, with a usual thickness of 25 mm.  The overlapping water resistant foil prevents moisture and water from the screed reaching the EPS floor insulation.

Tacker Board complies with the relevant industry standard and is CFC free.  It has a Standard Fire Class F (according to BS EN 131363).

The system ensures that piping is not displaced when concrete slab is poured, while providing a uniform distribution of heat.  REHAU Tacker Board is available in various product versions, where the two or three digits after EPS indicate compression strength in kPa at 10% deformation:

  • Tacker EPS70,   25mm: R0.66
  • Tacker EPS70,   50mm: R1.32
  • Tacker EPS100, 25mm: R0.69
  • Tacker EPS100, 50mm: R1.39
  • Tacker EPS150, 25mm: R0.71
  • Tacker EPS150, 25mm:  R1.43

Quick Heat is another product designed for use under floating timber floors. The panels are 0.5 x 1.0m in size, are just 13mm thin, and are composed of two main layers:

  • thermal insulation, fire-retardant expanded polystyrene layer (EPS), 12 mm.
  • aluminium for uniform heat distribution with grooves for the heating cable, 0.8 mm.

The panels provide insulation and a mounting surface for heating cables in a single product, with a R-Value R 0.26. The grooves on the panels are 100mmm apart and are compatible to the X cables. 

Suspended Timber Underfloor Insulation

Timber floors that are installed with under floor heating between the battens or joists must have significant thermal insulation.  Heat loss through the floor during the winter can easily account for at least 20% of total losses creating a considerable opportunity to save energy with effective thermal insulation. The insulation must be installed so as to block cold air draughts that can leak through the thin spaces between timber boards.  This is usually done with a glass fibre or rock wool blanket that will provide thermal insulation with a minimum R 3.0 value.  The insulation is placed between battens or floor joists under timber flooring with the X cable fixed onto a light steel mesh laid on the insulation, and an air gap between the heating cables to the timber floor above them.  The insulation can sit on an under-floor or on a chicken wire type mesh strung under the joists.  Air should not be permitted to leak past the edges of the insulation.  This method is only used where there is no other option available, because the heat up and response times would be very slow.


The materials selected must suit the radiant floor heating system application.  It is important to check the following

  • user requirements
  • floor construction method
  • whether hydronic piping or heating cables are to be installed
  • heating cables to be appropriate for the application
  • thermal insulation to be appropriate for the floor construction and heating cables

Please refer to our helpfull section on Hydronic Floor Heating instructions.