Retrofitting Underfloor Heating in the Bathroom

What to consider when retrofitting underfloor heating

The benefits of underfloor heating in the bathroom are obvious. Who doesn't like warm feet when standing at the sink, evenly heated tiles and energy savings to boot? As a result, underfloor heating is often retrofitted when bathrooms are modernised. Thin-layer underfloor heating is often the ideal solution because the low installation height means that the existing floor can be retained, minimising the need for costly preparatory work. Connection is made via a control box on the return of the old radiator. A control box with additional connections on the front of the housing is ideal for operating a towel rail radiator in conjunction with the underfloor heating.

The underfloor heating is installed as a second layer. Additional insulation under the underfloor heating is therefore generally not required, as the existing floor often already provides this property. However, it is essential that the floor construction is strong enough to withstand the static loads and is decoupled from impact sound.

Once the underfloor heating has been installed, the new surface should also be sealed with liquid sealant, at least in the areas directly exposed to splashing water. Finally, the tiles can be laid.

Calculating the required heating area

Before starting to install underfloor heating in the bathroom, it is important to consider the layout of the sanitary units and furniture. Clever planning can minimise the area of floor that will be heated. It is also important to ensure that the underfloor heating pipes are not damaged during the subsequent installation and connection of the sanitary units.

Once the bathroom has been designed, a layout plan is drawn up to show where the heating pipes will be located. Important: The individual heating circuits should have approximately the same pipe length. This is the only way to ensure that the floor is heated evenly. The installation plan should also take into account the position of the RTL box.

5 m² of underfloor heating for a bathroom of almost 10 m²

In the sketch, the floor area of the bathroom is 9.62 m². However, only 5 m2² of underfloor heating is required. The areas under the sanitary installations are not thermally activated.

With an average hot water temperature of only 35°C, the underfloor heating with a ceramic covering achieves a heat output of 320 W. If this heat output is not sufficient, the installation of an additional towel radiator or wall heating is recommended. However, in most cases this will not be necessary.

Example of underfloor heating installation in a bathroom

"I bought the FLEXIRO underfloor heating with clip-on rails for 5 square metres together with the Kompabox Easy RTL 4 for my 8 square metre upstairs bathroom. With the corner bath and floor-level shower removed, the heating area was about 6 square metres.

My bathroom layout from bottom to top: 

  • 200mm wooden beam ceiling
  • 160mm Isover Ultimate insulation between the joists
  • Vapour barrier on top
  • OSB3 2x 22mm one length - one width
  • Liquid seal or liquid foil
  • Thin decoupling mat (laminated fleece)
  • Clip rails with FLEXIRO heating pipes
  • Liquid heating screed with at least 10mm overlap over the pipes
  • Liquid waterproofing again, decoupling mat again, flexible adhesive, 
  • 8mm tiles with flexible joint.

Everything is sufficiently decoupled from the edge and fitted with corner sealing tapes in liquid foil.

The result:

After about 1.5 years there are no cracks or damage. The RTL valve is set to position 2.5 and even at very low outside temperatures the feet are comfortably warm throughout the room and the room has an average room temperature of 21.6°C. Note that the additional wall heating has since been switched off!

The energy costs (gas) have dropped considerably. This is exactly what I had in mind".

Herr Sch. from Rheine (NRW)