Tiles and underfloor heating: complete selection guide 7 specifications

By Osman Orman · Tile and technical specialist at Tegelmonsters

Tiles and underfloor heating work together when the tile has low water absorption, the screed meets flatness class 1, the adhesive is classified as C2TE-S1 or higher, and the heating protocol is adhered to. For the architect, the benefit lies in the specification phase.

The combination is technically strong and thermally efficient. The ceramic tile conducts heat quickly, but the detailing must be correct to prevent cracking and hollow spots.

Specification

What rules apply to tiles and underfloor heating in projects?

The European standard NEN-EN 1264 describes the design and installation of underfloor heating systems. The standard specifies an upper limit of a surface temperature of 29 degrees in the living zone and 35 degrees in the edge strip up to one meter from an exterior wall. These limits derive from comfort requirements, not from the tile itself, and are therefore the starting point for the supply temperature and the control strategy.

In addition to the temperature standard, the mechanical aspect is also important. A screed under a tile must achieve flatness class 1 according to NEN 2747, with a maximum deviation of 3 mm under a 2-meter straightedge. For the cyclic operation of underfloor heating, choose an adhesive of class C2TE-S1 or higher according to NEN-EN 12004-1. The grout follows class CG2 WA according to NEN-EN 13888-1:2022. This set forms the working basis for every project specification.

For a broader view of the combination, there is an extensive pillar in our knowledge base about tiles and underfloor heating in conjunction, in which the system design and installation sequence are covered step by step. This blog focuses specifically on the specification phase for architects and project managers.

Material

Which types of tiles are best suited for underfloor heating?

A tile on underfloor heating has two technical tasks: transmitting heat and absorbing cyclic movement. Porcelain stoneware fulfills both. The dense structure with a water absorption of less than 0,5 percent falls under group BIa according to EN 14411 and has a thermal conductivity of around 1,3 watts per meter per Kelvin. As a result, heat passes through quickly and comfort remains high at a low supply temperature.

In addition to the material, thickness plays a role. A floor tile of 8 to 12 mm provides sufficient mass for a long-term heat buffer without inhibiting heat transfer. Tiles thicker than 14 mm are rarely used on underfloor heating because they prolong the heating time. Slabs starting from 6 mm are popular as wall finishes, but for floors on underfloor heating, you typically choose 8,5 to 11 mm.

The wear class depends on the zone. For a residential project, PEI 3 or 4 is sufficient, while for a hotel or restaurant, you need PEI 4 or 5. The PEI value indicates how wear-resistant the tile is under intensive use. In our selection guide for PEI and R-value per room The combinations are detailed per room type. Large-format tiles on underfloor heating have their own specific considerations, which we outline in the guide for large format in projects elaborate further.

Expulsion

How much does a ceramic tile expand on underfloor heating?

The thermal expansion coefficient of ceramic tiles is low. ISO 10545-8 prescribes the measurement method and gives a guideline value of 6 to 8 micrometers per meter per degree Celsius for porcelain tiles. In practice, installers work with 7 micrometers as an average. With a difference of 22 degrees between the switch-off and on position, for example from 5 to 27 degrees in a morning, each meter of tile expands by slightly more than 0,15 mm.

The cumulative effect adds up as the surface area increases. A 25 m² floor measuring 60×120 cm expands by more than half a millimeter along its length. This expansion has to go somewhere, otherwise compressive stress will build up on the joint. Therefore, the installer places an expansion joint every 25 m², at all wall connections and at every subfloor transition. The expansion joint is at least 4 mm wide and is filled with an elastic jointing compound, not cement grout.

For larger formats, movement per element increases. A 320 cm slab expands by approximately 0,5 mm between the on and off. That seems small, but for a row of slabs covering a surface of 30 m², it adds up to several millimeters. In those situations, project teams opt for a wider joint width of 4 mm and a flexible joint mortar from the CG2 class. For those who wish to fine-tune the joint width precisely, this describes our selection guide for joint width in rectified tiling the combinations by size and surface.

Substrate

Which subfloor and screed are suitable for underfloor heating?

The screed determines the final result. Flatness class 1 according to NEN 2747 is mandatory for large formats and highly recommended for medium formats. A screed with deviations of up to 5 mm requires a leveling layer before the tiler can start work. For projects where every workday counts, the contractor sometimes opts for a fast-drying screed with an accelerator additive.

Three types of screed are most common in projects. The choice depends on the construction schedule, drying time, and the installation method of the underfloor heating. Below are the three variants with their typical drying times and properties.

Anhydrite requires extra attention. The substrate must be sanded before tiling to remove the calcium sulfate layer on the top surface. Additionally, a primer compatible with cement-based tile adhesive must be applied. Skipping this step risks insufficient adhesion between the tile adhesive and the screed for cyclic thermal loading. For anhydrite, a C2TE-S2 adhesive is generally chosen instead of S1, precisely because of the extra deformation required by the combination.

Glue and grout

Which adhesive and grout do you choose for tiles on underfloor heating?

The adhesive is the first link that accommodates cyclic movement. According to NEN-EN 12004-1, the letter C stands for cement-based, the number 2 for improved bond strength above the basic class, T for anti-sagging in wall applications, E for extended open time, and S1 or S2 for deformable and highly deformable, respectively. For underfloor heating, C2TE-S1 applies as the minimum, and C2TE-S2 for heavier loads or large formats.

The grout follows the same logic. According to NEN-EN 13888-1:2022, CG stands for cement-based, the number 2 for improved properties, W for low water absorption, and A for wear resistance. A CG2 WA grout meets all four criteria and maintains its structure over its lifespan on underfloor heating. For areas subject to chemical stress, such as a commercial kitchen, choose a reaction resin-based RG grout with higher chemical resistance.

The joint width depends on the calibration and the edge finish. A rectified large-format tile has a joint of 3 to 4 mm, while a tile with a natural flame edge has 4 mm or more. For underfloor heating, 3 mm is a lower limit to allow the grout room for micro-movement. A joint smaller than that width will yield to cyclical action and may show micro-cracks after a few heating periods.

Temperature

What surface temperature is safe and comfortable?

The temperature limit is derived from comfort requirements, not from the tile product. A surface temperature above 29 degrees feels unpleasant to bare feet and increases the risk of dry air and dust in the living zone. NEN-EN 1264 therefore uses 29 degrees as the upper limit and 35 degrees for the narrow strip of one meter along an exterior wall, where the system sometimes works harder to compensate for heat loss.

For the supply temperature, the controller regulates based on the desired surface temperature. In a properly dimensioned system, the supply temperature is between 35 and 45 degrees in a modern low-temperature installation. A higher supply temperature is possible but is discouraged, as it increases the cyclic load on adhesive and grout without measurably improving comfort. For the energy performance of underfloor heating systems, the Building Decree refers to the applicable performance requirements, which can be found at Building Decree Online.

For their control strategy, project teams are increasingly opting for weather-dependent control with an outdoor sensor. In this system, the supply temperature follows the outdoor temperature via a heating curve. The effect: a smoother heating pattern, less cyclical operation under the tiles, and a lower energy bill over the heating season. For commercial projects with varying occupancy, zone control per room works best, with a separate manifold and thermostat for each zone.

Warming up

How does the warm-up and shut-off protocol work after tiling?

The heating and shutdown protocol is the area where planning often becomes a bottleneck in projects. The standard requires two phased heating cycles: a first before tiling to remove residual moisture from the screed, and a second after tiling to allow the tile adhesive and grout to complete their final curing. The site manager records the heating curve in a heating report, which is included with the handover.

The initial heating starts three to four weeks after the cement screed has been poured. The supply starts at 25 degrees and increases by 5 degrees daily until it reaches the maximum permitted 45 degrees. This is followed by a holding period of two to three days at that maximum value. The cooling proceeds downwards in the same steps. For fast-drying screeds, the schedule is shorter but still phased.

After tiling, the second heating takes place. The grout requires at least seven days to harden; installers often wait 21 days to allow for a generous margin. After that, the supply is increased again in stages. Heating too quickly, for example directly to 35 degrees, creates compressive stress on the joint and can result in micro-cracks that become visible in year two as fine lines along the joint.

The shutdown at the end of the heating season also takes place in stages. A supply temperature that suddenly drops from 40 to 18 degrees causes a thermal shock to the grout. In the projects managed by us, we see that a cooling period of 5 to 7 days, in daily increments of 4 degrees, keeps the grout and tile adhesive in excellent condition. This is less of an issue for commercial projects with permanent climate control, because the system continuously maintains a ground temperature.

Sample package

How do you put together a sample pack for your underfloor heating project?

On the page floor tiles Filter for free by zone, size, color, and finish. For a underfloor heating project, set the filter to porcelain, size 60×120 or larger, and rectified edges. This immediately displays the collections used in underfloor heating projects. For each zone, choose the application page that matches the focus of the design.

What project managers often tell us: the first filtering round takes place online, followed by a physical assessment in our showroom in Utrecht. The showroom is accessible to professionals by appointment only and offers full-size samples alongside additional sample areas. Every autumn, our manufacturers from Spain and Portugal showcase new collections at the Cersaie trade fair, which we add to our range shortly thereafter, including the technical datasheets you need for specifying underfloor heating projects.

FAQ