Hardwood Floors on Underfloor Heating?

It is feasible to install engineered flooring onto a conventional hot water or electric heating systems. However, if it is not controlled properly, it can cause problems.

The majority of manufacturers who make wood flooring deemed suitable for underfloor heating, do so on the proviso that the sub-floor surface temperature must not exceed 27 degrees C. Exceeding this temperature rises the risk of excessive shrinkage, opening linear splits and cracks, and possible de-lamination of the hardwood wear layer.

Another equally important factor is the relative humidity (R.H.) of the room. A healthy room temperature of 18-20 degrees C should always accompanied by a by a relative humidity of 50-65%. There are many methods of maintaining correct humidity levels, the most accurate being electric humidifiers.

These are about the size of a shoebox, are re-filled with tap water, and release moisture into the atmosphere automatically until the correct pre-set humidity level is reached.

However sometimes just ventilating or keeping plants in the room will have the desired effect. In extreme cases where the R.H. of the room drops below 40%, a bucket of water in a corner of the room will absorb into the air within seven days or so.

Always remember that it is humidity that carries the heat in a room. If you have a really dry room, it will need a higher floor temperature to maintain a 20 degrees C air temperature than if you have correct R.H. It therefore follows that keeping a correct R.H. enables your heating system more economical to run.

Another potential problem is the volume of air within the room that your system is trying to heat.

A room of approximately 20 sq meters with reasonable standards of insulation, and a 2.5 meter ceiling height can be safely heated to up to 22 degrees C with your underfloor heating running at 27 degrees C

The same room with a 3.6m height would require a far higher below floor temperature to achieve the same room temperature. As we now know, this is not possible below wood flooring, so therefore an ancillary heating will also be needed. The same also applies to rooms with inadequate insulation and high heat loss.

Choosing The Flooring

The initial moisture content of the wood floor is an important factor in minimizing movement. For always on heated flooring, wood moisture content of between 6-8 degrees C is recommended. However bear in mind that if the heating is not being used in the warmer more humid months of summer, there will be far greater movement (expansion) of the flooring.

If hardwood with higher moisture content is to be used- say 9-10%, then it is advisable to maintain the humidity at between 55-65%

The width of the boards is relative to its stability. The narrower the board in relation to it's thickness, the more stable it will be, making it more resilient to movement with changes in humidity.

The make up of the board is also an important factor. A 20mm thickness flooring comprising of 14mm of plywood with a 6mm hardwood wear layer is extremely popular today as once installed it is indistinguishable from solid wood planks.

My personal opinion though is that there are two potential problems using this type of flooring.

Firstly a 20mm thickness flooring makes an extremely good insulator, preventing a proportion of the heat transfer, which in turn means that there is a tendency to increase the below floor temperature to compensate for this. Secondly, plywood is an extremely stable base layer. The expansion and contraction of plywood is minimal and is far less than the hardwood wear layer that it is bonded to. Therefore movement in the hardwood layer caused by moisture loss is resisted by the stability of the plywood.

However, this only works up to a point. I have seen cases where the strength of movement in the veneer can make it peel from the edges, or in extreme cases delaminate from the base.

My view is that a 10mm or 14mm thickness engineered floor is far better suited to efficient lower temperature underfloor heating. The makeup of 3.6mm or 4mm hardwood wear layer with 1mm base layer and inter spaced softwood core running at right angles is optimum. Whilst the softwood core is not as stable as plywood, it will allow movement broadly in line with the hardwood wear layer, helping to minimize de-lamination.

Precautions

Always have a floor sensing temperature probe installed below the wood flooring, and set this to limit the temperature to 27 degrees C (unless a different temperature is specified by the wood flooring manufacturer).

Don't rely on the temperature stated by a hot water heating manufacturer. In the case of Polypipe installations, if pipes in certain areas are closer to each other, a far higher temperature will be achieved at this position.

Likewise lower screed levels over parts of the floor will cause the temperature to be exceeded. A responsible heating installer will always check the sub-floor temperature on completion of the work. Always ensure that the moisture content of the sub-floor is below 2% before the wood floor is installed.

Installation

Hardwood flooring should be acclimatized within the room with the heating turned on for a minimum of 5 days prior to installation. While wood flooring can be fully bonded to the sub-floor, there is always the chance that moisture content in the screed could rise at some point, which could cause the adhesive to fail. Our preference is to install over a moisture retardant underlay using the 'floating floor' method.

The heating should be turned down to 18 degrees C while the floor is being installed and then for four days after.

Final Considerations

1. Increasing or decreasing temperature levels should be done at 5 degrees C per day sequences.

2. Rugs should not be used over heated wooden flooring. These have the effect of increasing the wood temperature to unacceptable levels and will cause failure.

3. Opening joints cannot be avoided when using heated sub-floors, and the end user must accept far greater seasonable movement.

4. The carbon film electric underfloor heating is the safest and most economical periodic type of wood floor heating, as this is a lower more even heat with little likelihood of below floor temperature levels being exceeded.

5. Please be aware that are many thousands of installations of wood flooring installed above all types of floor heating giving supreme user comfort and no problems whatsoever.

David is a director at Birbek Floors Ltd, . UK distributors of hardwood flooring since 1982. Birbek have been assembling and distributing made to plan electric underfloor heating, for wooden floors in their own dedicated workroom since 1998 flooring

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