Retrofitting Underfloor Heating in Bathrooms

What to Bear in Mind When Installing a Bathroom Underfloor Heating

The advantages of underfloor heating in bathrooms are self-evident. Who doesn't like warm feet when standing in front of the sink, with evenly heated tiles that at the same time also save energy? This is why underfloor heating is often retrofitted when modernising bathrooms. In many cases, low profile underfloor heating provides the best solution because its low construction height enables the existing floor to be retained and thus minimises costly preliminary work.

The low profile underfloor heating is effectively added as a second layer. Additional thermal insulation under the underfloor heating is usually not necessary, as the existing floor often already guarantees this property. A load-bearing floor structure that meets the structural load requirements and is soundproof is nevertheless an absolute prerequisite.

After installing the bathroom underfloor heating, the new surface should be additionally sealed with liquid sealing foil, at least in the areas that can be directly sprayed with water. The tiles can then be laid.

Calculating the Required Heating Surface

Before installing the underfloor heating in the bathroom, you should clarify the arrangement of the sanitary units and the furniture. The heat-active floor area can be minimised by clever planning. It must also be ensured that the underfloor heating pipes are not damaged when later installing and connecting the sanitary units.

Once you have planned the furnishings in the bathroom, you need to draw up a layout plan showing the arrangement of the heating pipes. Important: The individual heating circuits should have approximately the same pipe length. This is the only way to ensure that the floor will warm up evenly later. The installation plan should also take into account the positioning of the control box.

5 square-metre Underfloor Heating for an almost 10 square-metre Bathroom

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

With an average heating water temperature of only 35 °C, underfloor heating with a ceramic covering achieves a heating output of 320 W. If this heat output is insufficient, we recommend installing an additional towel radiator or wall heating. In most cases, however, this will not be necessary.

Example Installation: Underfloor Heating in the Bathroom

“I bought the FLEXIRO underfloor heating system with clip rails for 5 square metres together with the Kompabox Easy RTL 4 for my 8 square metre bathroom on the top floor. Without the corner bathtub and floor level shower, there’s about 6 square metres of heating surface.

My bathroom structure from bottom to top:

  • 200 mm timber beam ceiling
  • 160 mm Isover Ultimate insulation between the supporting beams
  • On top of this is a vapour barrier
  • 2 x 22 mm OSB3 board laid once lengthwise – once crosswise
  • Liquid sealant or liquid foil
  • Thin decoupling mat (made of laminated fleece)
  • Clip rails with FLEXIRO heating pipes
  • Liquid heating screed with at least 10 mm coverage above the pipes
  • More liquid sealant, another decoupling mat, flex adhesive,
  • 8 mm-thick tiles with flexible joints.

Everything is sufficiently decoupled at the edge and provided with corner sealing tapes in liquid foil.

The result:

No cracks or damage have yet been detected after around one and a half years. The RTL valve is set to position 2.5 and, even with very low outside temperatures, people’s feet are pleasantly warm everywhere in the room and the room has an average indoor temperature of 21.6 °C. And I should add that the additional wall heating has been switched off since then!

The energy costs (gas) have noticeably decreased. Exactly as I imagined.”

Mr Sch. from Rheine (NRW)