Tiles & underfloor heating: answers to the most frequently asked questions

Tiles and underfloor heating have one thing in common: both offer building owners numerous advantages in terms of living comfort and therefore form an ideal combination. Nevertheless, the topic repeatedly raises uncertainties and questions in the run-up to the event. You will find the most important answers in this article.

Which floor covering is best suited for underfloor heating?

Whether tiles or stone, parquet or laminate, vinyl or carpet – anyone who wants cosy warmth without radiators can in principle draw on the full range. In principle, any floor covering can be laid on underfloor heating.

The difference, however, is how quickly the heat from the underfloor heating system is transferred to the floor, or what flow temperature is necessary (and possible) to control the room temperature as desired. In other words, the different materials differ in their thermal conductivity. This in turn has an impact on energy efficiency and thus ultimately on heating costs.

This is precisely why tiles are the best choice for underfloor heating. Due to their dense surface, they score points with a very high thermal conductivity between 2.3 and 2.8, which is about 5 times higher than, for example, underfloor heating under hardwood flooring or underfloor heating under vinyl flooring. In addition, the heat is stored in the tiles, which means that the floor not only heats up quickly, but also stays warm for a long time – thus saving additional energy.

Can all tiles be used in combination with underfloor heating?

There are essentially no restrictions when using tiles on underfloor heating. Both natural stone and stoneware and porcelain stoneware are equally suitable for underfloor heating. To achieve the most efficient heating result, tiles with a maximum thickness of 20 mm are recommended. Thicker coverings are also possible, but the heating time may increase slightly.

Regardless of the heating system, when selecting the floor tiles, attention should also be paid to the abrasion group of the tiles and the anti-slip class of the tiles. This ensures that the floor meets the requirements of the respective area of application in the best possible way.

What surface temperature can tiles for underfloor heating withstand?

A great advantage of tiles is that they can withstand even high temperatures without damage. In contrast to many other floor coverings, they can therefore be heated to a surface temperature of 29°C and more without any concerns. This results not least in the high heat output of up to 200W/m2 with simultaneously low energy consumption.

Tiles on tiles & floor heating – is that possible?

Particularly in the case of renovations, the question often arises as to whether the new tiles can be laid on top of the existing tile covering. The answer to this is quite clear: Yes, on underfloor heating this is no problem. The combination of tile on tile – underfloor heating works perfectly and without affecting the heating result: Due to the extremely high thermal conductivity of the tiles and the correct tile adhesive as an additional heat conductor, the heat of the underfloor heating is transferred from layer to layer without loss.

Only condition: The old flooring must be perfectly laid and intact and in the spatial conditions the higher floor construction must not cause any problems (e.g. stripes of doors etc.).

What should be observed when laying tiles on underfloor heating?

Although tiles and underfloor heating harmonise perfectly, there are a number of points to consider before and during installation to prevent any difficulties from the outset.

Type of underfloor heating

There are basically two options for underfloor heating: They can be operated either with water or with electricity to generate heat. In principle, both water-guided and electric underfloor heating can be installed under tiles. Which heating system is most suitable depends rather on the respective application.

Warm water underfloor heating systems work via plastic or copper pipes, which are inserted into the screed as a wet or dry system and through which heated water circulates. Because the installation effort is relatively large and a certain installation height is also necessary, hot water underfloor heating systems are primarily used in new buildings.

In contrast, with electric underfloor heating systems thin heating mats ensure the correct room temperature. These can also be laid on the screed at a later date with relatively little effort and require a lower installation height. They are therefore also suitable for the renovation of existing buildings or can be retrofitted as a supplementary heating system in rooms with high heat loss (e.g. conservatories).

Condition of the substrate

An optimal substrate is the basic prerequisite for a durable and flawless tiled floor. Therefore, tiles should not only be laid on a substrate that is as smooth as possible, but especially on a completely dry substrate. Because too much moisture under the tiles can otherwise sometimes cause the tiles to come off or other damage later.

Particularly when laying on newly erected and wet laid heating screeds, care must therefore be taken to ensure that they are ready for laying. Normally, a screed needs about 4 weeks to harden completely – in the meantime, however, special quick binders are increasingly used to accelerate the process. In general, it is advisable to heat the screed in a controlled and gradual manner using underfloor heating. On the one hand, the drying phase can be shortened and, on the other hand, both the heating and the screed can be checked for function and condition.

The screed is finally ready for laying the tiles when the residual moisture determined by means of a suitable measuring method (e.g. CM method) has fallen below a certain limit. As a guideline, a maximum of 2 % for cement screeds and a maximum of 0.3 % for calcium sulphate screeds are to be taken into account, whereby the laying instructions of the manufacturer are to be observed for the exact values.

Protection against moisture

In principle, tiles can be laid directly on the screed. However, in order to protect the substrate from penetrating moisture from the tile adhesive, it is recommended to apply a sealing primer or tile primer before laying. Otherwise, there is a risk that the screed will soften and the flooring will no longer adhere properly.

Suitable tile adhesive and tile mortar

Although underfloor heating generally heats tiles very evenly and constantly, temperature fluctuations still occur. This leads to slight expansion of the material and corresponding stresses on the surface. To compensate for these movements – and thus prevent stress-induced cracks in the tiles – highly flexible and temperature-resistant materials should therefore be used as tile adhesives or tile mortars. In some cases, a decoupling mat can also be laid under the tiles.

During installation, care should also be taken to ensure that these are applied over as much of the surface as possible. This allows the tile and adhesive to bond better, which is particularly advantageous in the case of surface tension. And besides, the heat transfer is additionally optimized.

Expansion joints

A no less important aspect in connection with the temperature-related expansion of the materials is the interaction expansion joint – tiles – underfloor heating.

Expansion joints should be taken into account, especially in edge areas, and should allow for a minimum of 5 mm room for movement. In addition, it is essential to create additional expansion joints in those places where the screed also has them. Otherwise, the tile and substrate may expand differently, which may lead to cracks or fractures.

When can the underfloor heating be put into operation after tiling?

A freshly tiled and grouted floor must not be heated up immediately. This could cause the adhesive and joint sealant to dry too quickly and become brittle. The general recommendation is to wait about 28 days until the underfloor heating is activated. The temperature should then be increased continuously in 5-degree steps over a period of several days until the desired flow temperature is reached.

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