Wood flooring and underfloor heating work together — under conditions that are not negotiable. Miss one of them and the floor cups or gaps open up. Here is what actually matters.
Why wood moves at all
Wood is hygroscopic: it absorbs moisture from the air and releases it again, swelling and shrinking as it does — almost entirely across the width of the board. Underfloor heating dries the room air considerably in winter. The floor therefore shrinks, and the wider the board, the more visible that becomes.
Condition 1: engineered, not solid
This is the decisive point. Engineered flooring is built from cross-bonded layers that restrain each other. It moves far less than solid timber, which is why it is the standard answer over underfloor heating.
Solid wood over heating remains the exception and calls for narrow formats and a carefully controlled system.
Condition 2: the right thickness
Wood is an insulator — precisely what you do not want here. The thicker the build-up, the worse it releases heat.
- Overall board thickness: ideally 10 to 15 mm.
- Thermal resistance of the whole build-up — board plus underlay — preferably below 0.15 m²·K/W.
A thick wood-fibre underlay is therefore the wrong choice here.
Condition 3: fully bonded
Floating installation leaves an air gap between screed and flooring, and air insulates. Full adhesive bonding creates direct contact: the heat goes into the room rather than into a void. It is also more stable and much quieter underfoot.
Condition 4: sensible formats
The wider the board, the more any gap shows. Over underfloor heating, widths up to roughly 18 to 20 cm are sensible. Very wide boards remain possible, but they demand stable indoor humidity all year round.
Condition 5: temperature and commissioning
Two figures decide everything:
- Floor surface temperature: 28 °C maximum. Above that the wood dries out too far. The limit is set at the manifold, not at the room thermostat.
- Relative humidity: 45 to 65 %. In winter, heated rooms often fall below 35 % — humidifiers or plants help.
Before fitting, the screed must go through its commissioning heat cycle, documented in writing. The heating is then turned down while the flooring acclimatises and is installed, and brought back up afterwards in steps of about 5 °C per day.
Residual moisture in the screed
The limits are stricter than without heating. Work to 1.8 % for cement screed and 0.3 % for calcium sulphate screed, measured by CM test. No measurement, no installation.
What you can stop worrying about
Gaps that open in winter and close again in summer are normal — wood is alive. It only becomes a problem when they stop closing. And the species matters less than often claimed: oak performs very well, what counts is the construction, the format and the installation.
In short
Engineered board, 10 to 15 mm, fully bonded, sensible width, 28 °C maximum at the surface, controlled humidity. All six together — or not at all.