Underfloor heating in concrete floor

The power requirement varies with the type of room to be heated. Normally, heating cables are installed in the lower part of a 30-50 mm thick layer of screed (concrete), atop thermal isolation with high insulation properties. It is always advisable to install insulation under the concrete floor, to avoid heat loss downwards.

In renovation projects or when, for other reasons, it is necessary to keep the building height of the floor at a minimum, special low profile screeds, which provide a thickness down to 10-15 mm, are used.

Floor thickness
Traditional/conventional screed thickness in a floor is about 5 cm. Nexans normally recommends a maximum thickness of 6 cm, as going beyond this gives an extremely thick floor with a high degree of thermal storage (meaning slow adjustment of temperature).

The lower part/layer of concrete/screed in thicker floors can also be quite challenging to compact when being poured, which in turn can lead to reduced thermal conductivity. This may, consequently, mean an even further reduction of the possibility to adjust the temperature of the floor. In the worst case, a layer of concrete with insufficient thermal conductivity may result in overheating of the heating cable and ultimately damage to and malfunction of the cable.

Pouring of concrete/screed
In all types of concrete floors, it is important with good mixing, proper mixing ratio (follow the vendor’s user manual carefully), and good compacting of the screed to avoid air pockets and to ensure that the screeds thermal conductivity is as good as possible.

Screeds of concrete shall dry and cure naturally, and the heating cable can be connected and carefully put to use after 6-8 weeks (normally).

Temperature control and adjustment
To achieve the most energy-efficient floor, it is recommended to use an electronic, programmable thermostat, which allows for day and night setbacks. The degree of efficiency will depend largely on the installed power and the thickness of the floor (in combination with the quality of the screed used).

Underfloor heating combined with waterborne heat
Heating cables can be combined with waterborne underfloor heating. This can be beneficial in the summertime, where some heating is desirable e.g. in the bathroom.  

Often, it will not be appropriate to operate the centralized heating system just to heat one or two bathrooms. In a floor construction like this, the heating cables should be placed over the water pipes, and not directly next to the water pipes.