Sound Advice on Underfloor Heating with Timber Floors

Bill McConnell, Technical Director of Continental Underfloor Heating, gives sound advice about fitting timber floors over underfloor heating.

Introduction

The modern age in which we live means we are spoilt for choice of floor coverings and using them with underfloor heating (UFH) is a popular and cost effective option. Whilst different floor coverings have different requirements, specification of UFH does not need to be complicated or technical and a good UFH supplier will give you sound advice on what is required. In our experience, it’s UFH with timber floors that raise the most questions - and some concerns - which is surprising considering timber flooring has been used with underfloor heating for over 20 years in Europe and Scandinavia.

Choice of Timber

Timber is not a homogenous material and therefore reacts to changes in humidity. Whilst most timber will expand only laterally, there are some exceptions that will also expand longitudinally. Different varieties of timber also have different thermal resistances and in general, hard woods are better conductors than soft woods. It can therefore be difficult to calculate the exact level and direction of expansion when the timber is heated. Timber must be specifically prepared for use with underfloor heating as the way the timber is cut and the moisture level of the wood will effect the installation. It is therefore essential to consult with the wood supplier, as they will give advice on selecting the most appropriate wood to use with UFH.

Moisture Levels

If straight wood (for example oak boards) are used, then it is generally advised that the maximum width of board is not more than 100mm - some suppliers will recommend up to 150mm but our experience is that this requires a greater thickness of wood to reduce the risk of the wood ‘cupping’. At Continental we recommend that the moisture level of the wood is between 6-8%. Most kiln drying will only reduce the moisture content down to about 10-12%, therefore the timber will require a secondary drying process.

Once the underfloor heating pipe is in place and the screed is laid over the top it is important to ensure that the screed has completely dried. Typically the screed will dry at a rate of approximately 1mm depth per day without heating, although this will vary depending on ambient conditions. Gentle heat can be applied via the UFH system to assist with this after the initial screed curing process.

When the screed has dried the timber flooring can be laid but it should not be secured to allow it to acclimatise to the under floor heating and that air can circulate around.

Most timber floor suppliers will advise that the maximum floor surface temperature does not exceed 27ºC. This will give a heat output of approximately 70W/m2 - more than the required output to heat a standard room. As this maximum floor temperature is slightly less than with other floor coverings, timber floors are more suitable in houses where heat loss is between 50 and 70 W/m2.

Although timber floors present a challenge, they can work effectively with UFH, and the points raised will limit the risk of shrinkage and expansion, and therefore prevent excessive gaps appearing. Natural fluctuations will still occur due to the moisture levels, but you get this with radiators too, therefore a minimum 10mm gap around the room should be allowed to accommodate movement.

Laminates are a good alternative as they are more stable because the wood grain runs in both directions. The finish on some of these is as good as real timber with a final finish 3 or 4mm thick rather than just a thin veneer

Conclusion

We always advise our customers to consult with their flooring supplier to ensure the product they are buying is the most suitable. Also be aware that the heat output will be severely reduced if the flooring is covered with thick rugs or carpet.

go back to main news page