Underfloor Heating Insulation – What Is Worth Knowing?

Underfloor heating needs thermal insulation that can bear foot traffic, waterproofing against the ground, and an edge insulation strip along the walls. Under a heated floor we suggest 10-15 cm of EPS, XPS or mineral wool sub-floor insulation. Without proper insulation, underfloor heating loses heat towards the ground and, with a covering temperature of 20-25 degrees, may not be able to heat the house.

Why is underfloor heating not efficient without good insulation?

With radiator heating, if we feel cold, we simply turn the heating up. With underfloor heating this reserve does not exist: the ideal surface temperature of the covering is 20-25 degrees, and it must not and should not be set warmer, because walking on a hot floor is unpleasant. The missing output therefore cannot be made up with a higher temperature.

With wet underfloor heating, heating fluid at 35-40 degrees circulates in the pipes. In a poorly insulated house it can happen that the system can hardly, or not at all, heat the rooms sufficiently with this. If there is no budget for insulation, it is worth looking for another heating solution – or putting the insulation in order first.

The floor on the ground: the sixth side of the house

In a family house without a basement, besides the facade walls and the attic or slab, the floor lying on the ground is the sixth bounding side of the building. If we insulate the other 5 sides but not the floor, the floor becomes the weakest point of the thermal insulation, and the heating energy escapes there. This is why the principle holds for family houses that heating cost can be reduced significantly only with thermal insulation of the whole building. A full underfloor heating system is only worth building on a properly insulated sub-floor.

What layers are needed under a heated floor?

Thermal insulation in the sub-floor

The thermal insulation of the sub-floor prevents the screed concrete from taking up the heat produced and conducting it towards the ground. Good-quality EPS (expanded polystyrene) that can bear foot traffic, XPS (extruded polystyrene) and mineral wool insulation are all suitable.

For a heated floor, we suggest 10-15 cm of sub-floor insulation. For a new building in Hungary, the energy requirements are currently set by Decree 9/2023 (V. 25.) ÉKM (the earlier regulation lost effect in November 2023); what this means exactly in terms of insulation for a given building is calculated by the energy designer. If there is no thermal insulation in the sub-floor, or it is inadequate, we do not recommend building a full underfloor heating system.

Reflective foil: a supplement, not instead of the insulation

A thin reflective foil does not replace thermal insulation that can bear foot traffic: in the floor, heat loss is reduced by the thickness and quality of the insulating layer. With heating film underfloor heating, however, the reflective foil is part of the layer structure – it goes on the thermal insulation, directly under the heating film. This order is also shown by the layer structure of our installation in Sitke, with laminate parquet.

Waterproofing and technological foil

Whether wet or electric underfloor heating is being built, the thermal and waterproofing insulation must work together. The technological foil prevents the thermal insulation from getting wet and losing its insulating ability; it is worth placing it both under and over the insulation that bears foot traffic. With a floor on the ground, ground vapour and ground moisture can enter the structure, so bituminous or welded PVC waterproofing is needed there in every case. When building the sub-floor of buildings with underfloor heating, special attention must be paid to following the layer structure exactly.

Edge insulation strip

Also called an expansion tape. The temperature of a heated floor changes, and the sub-floor expands with the heat. So that the expansion does not press on the walls, an edge insulation strip must be placed along the walls. The strip also prevents the heated screed concrete from being in direct contact with the walls and conducting heat away through them (thermal-bridge-free design). In a lightweight-frame building this is especially important, because there the steel supporting structure can conduct heat even down to the foundation slab.

With electric underfloor heating: a separate heating circuit for each room

With electric heating, every room is an independent heating circuit with its own thermostat, so a combined system can also be built. In thermal terms, underfloor heating is a very comfortable solution, but whether it is the right choice in a given room is also influenced by the covering, the furnishing and personal needs.

Carefully executed sub-floor insulation, with any underfloor heating solution – infrared heating with heating film, heating mat or heating cable – reduces heating cost in the long run and gives more even warmth.

Checklist for floor insulation

• Is there thermal insulation that can bear foot traffic in the sub-floor, and is it thick enough for a heated floor?
• Does a technological foil protect the thermal insulation from below and above?
• For a floor on the ground, will bituminous or welded PVC waterproofing be done?
• Does the edge insulation strip run around along every wall?
• With heating film, is the reflective foil there between the insulation and the film?
• Will the heating be controllable room by room?

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