Underfloor Heating

Good underfloor heating (electric or water-borne) is an ideal heating method, and underfloor heating combined with a wood floor gives optimal domestic comfort. The laying instructions for each floor type also apply when laying over underfloor heating.

The following instructions summarise what is specific to wood floors laid over underfloor heating.

Special requirements:

-	The floor construction must have a heat distribution layer that provides a very even temperature over the entire floor area, in order to avoid localised excessively high temperatures.

-	The whole floor area must be heated. However, this does not apply with comfortheating systems, which complement ordinary heating. The temperature with these systems is considerably lower than the permitted maximum of 27°C.

-	The floor covering must have a low resistance to thermal transmittance.

-	It must be possible to control and limit the surface temperature with great accuracy.

-	The surface temperature of the finished floor must never exceed 27°C at any point. This also applies under rugs, etc., and furniture. Provided that rugs are not too thick, or there are not too many, a reasonable temperature for exposed floor areas is 23°C, giving a room temperature of approx. 21°C. This of course assumes that the room has normal heating requirements and normal standards of draught-proofing, insulation, window areas, etc.

-	A vapour barrier must be built into the floor construction, as close to the wood floor as possible. If the joists are thick or heavy, it is particularly important that the vapour barrier is close to the wood floor. The vapour barrier must never be underneath the joist structure.

-	Ensure that there are no air gaps between the wood floor and the surface beneath, as this could cause the wood to dry out.

Choice of wood floor:

Ideal choices are Kährs 15 mm multi-layer parquet floor or Kährs 7 mm Linnea, with the woodloc joint. This reduces the risk of cracking.

Kährs parquet can be laid over underfloor heating, which must distribute the heat efficiently and evenly across the floor.

The surface temperature of the floor must never exceed 27°C. This also applies near radiator pipes and under any rugs, etc.

Beech and Hard Maple multi-layer parquet floors expand and contract more than other wood species. Underfloor heating causes the wood to dry out more, which results in additional shrinkage. In a cold, dry climate (e.g. in Sweden), cracks between strips and slightly concave boards are to be expected.

Installation:

The working temperature when laying should be at least 18°C. This applies to boards, subfloor and the room air temperature. Relative humidity (RH) of the air must be less than 60% before, during and after laying the floor. Flooring paper should be the first choice as the interlayer, because both expanded polythene foam and cork paper have a higher resistance to thermal transmittance. Kährs multi-layer parquet or Kährs Linnea with woodloc must not be glued.

Remember that a floor laid over underfloor heating is more susceptible to moisture than an unheated floor, because of the greater difference in floor's moisture ratios in its driest and most moist states.