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How Well UFH Works With Different Finishes

THE commonest forms of underfloor heating (UFH) all involve circulating water through high-performance plastic pipe which has been set within the structure of the floor.

UFH has become the preferred form of heating throughout the residential sector and is also often specified for schools and medical facilities, for offices, factories, boats and even zoos. It is the most comfortable form of heating because it has a high radiant component. It is safe because it creates no hot surfaces. It is unobtrusive, more hygienic and more energy-efficient.

With UFH, in order to heat a newly constructed building, the floor surface only needs to become about as warmas the palm of your hand (25-27degC). Despite what some people still fear, it won’t incinerate your shoes!
The right form of UFH can achieve this surface temperature using water which is much cooler than even over-sized radiators require and, if your heating system can be effective while using cooler water, then not only
is this more energy-efficient but it also opens up the prospect of this lower temperature water coming from renewable energy sources.

UFH has the potential to wean a building off any future dependency on continued availability/affordability of fossil-fuels.

Despite all of its advantages, some flooring professionals continue to be concerned about how UFH works with different floorcovering, so let’s consider how it works with each type.

Tiles/Stone: UFH ismost effective with this type of floor finish simply because tiles and stone are thermally-conductive and this better conductivity allows the required heating power to be generated while using the lowest water temperature.

Tiles/stone laid over UFH set within a screed/concrete structural floor work best but tiles/stone laid over the right type of timber floor UFH can be very nearly as effective.

The only issue for a floorlayer is to check that floor shrinkage or floor deflection will be insufficient to cause the tiles/stone to crack, and this can be addressed by the correct use of a de-bonding substrate and/or flexible
adhesives and grout.

Vinyl: Several years ago, when British buildings required 100W/sqm or more of heating power because of poor construction U values and high ventilation heat loss, UFH had to achieve floor surface temperatures as hot
as 29-30degC in order to generate this amount of heating power. Such temperatures were just too high for vinyl because it can soften and then stretch when it reaches temperatures above 27-28degC.

Nowadays, a new construction that is built in accordance with current Building Regulations typically requires only about 50W/sqm heating power and this requires an UFH floor surface temperature no warmer than 25-26degC, which is fine formost vinyl.

Timber: There are still some hardwood floor specialists who remain concerned that somehow UFH can damage a timber floor covering but this is amistaken belief. In fact, good UFH can be the best way of looking after a timber floor covering.

Some people believe that timber expands and contracts as its temperature changes. This may be true but only to aminiscule extent, particularly over the range of temperature change involved with UFH – say between 6degC
when the heating has been off for a long period and 29degC which is the top limit for UFH.

Instead, timber expands and contracts according to changes in its moisture-content. As it absorbs moisture, timber swells, and it shrinks as it dries.

The important issue is to ensure the moisture-content of the upper and lower surfaces of the timber remain the same. If they stay the same, the timber will remain completely flat. If the upper surface dries out relative to the lower surface, the timber will cup. Conversely, and it will crown.

In order to maintain consistent moisture content, there are two main requirements. The first is to ensure that the timber is installed with a moisture-content of 8-10%. The second is to ensure that the heating system controls
turn on the heating system very gradually when the floor is first installed and there after at the beginning of each heating season.

Any timber floorcovering will absorb atmospheric moisture at the end of the summer and during the onset of autumn. By using programmable room thermostats and then leaving the heating system to come on automatically, at the beginning of a heating season, the heating will come on initially perhaps for half and hour every two/three days and, a week later, perhaps for one hour per day.

This gradual increase in heating time gives the timber floor finish the opportunity to naturally respire any atmospheric humidity it might have absorbed while the heating system has been off, and itsmoisture-content is given time to become even throughout its thickness.

The worst possible scenario, and the commonest reason for timber floors to be damaged, is to keep the heating system turned off for weeks ormonths on end, then wait for the first seriously cold day, and then turn the
heating systemon full-bore. If this is allowed to happen, the timber floor has no chance to stabilise and it will react by splitting, cupping or crowning. Such a scenario is completely, totally and easily prevented.

UFH generally transfers its thermal energy through the complete thickness of the timber floor finish, and it can be a far better way of maintaining constant moisture-content than is possible with any formof heating which relies on heating the air in a room, which can very easily dry out just its upper surface.

Carpets: The general guidance is for the thermal resistance of any carpet/underlay set over UFH to be limited to a maximum of 1.5Tog. However, practical experience with UFH in this country over many years indicated that some carpet/underlay combinations with higher Tog values than this can work well over UFH.

Consequently, a study was undertaken in the laboratories of BSRIA during 2006 which compared the performance of 24 different combinations of carpet and underlay when installed over UFH. This study was undertaken in association with the Carpet Foundation.

The tests confirmed that there are combinations with thermal resistance much greater than Tog 1.5 which work well with UFH and the tests enabled a table to be drawn up which set down the Published Tog ratings of
different carpet/underlay combinations alongside the Effective Tog ratings of the same combinations when they are used over UFH.

In general, the tests showed that the performance of the underlay is key, and since the tests were completed themanufacturers of underlay have developed a range of underlays which specifically suit UFH.
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This article has been reproduced from the Contract Flooring Journal website. You can find them online at