Wood Floors with Under-Floor Heating

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					                                                                               Contract Floors, April 2005

                            Wood Floors with Under-Floor Heating
When we look at the popularity of under-floor heating in countries like Switzerland and Germany we can
see that the under-floor heating market here is still in its infancy, with demand only looking likely to
increase. This demand has been partly fueled by the popularity of wood, stone, ceramic and laminate.
With increasing demand and so many potential combinations of wood flooring and heating system,
contractors and specifiers are looking for answers regarding compatibility. Will the flooring product
perform well with the heating system, and of concern to the specifier, will the heating system perform
with the flooring?
Hardwood flooring requires of the heating system a suitable maximum temperature, quoted by some
flooring manufactures as 27 C; this is to ensure there is not excessive moisture loss and accompanying
dimensional change. The UFH must also provide an even distribution of heat which allows any
movement in the flooring to occur evenly, and this does vary between systems.
What is needed of the flooring to perform well is mainly good stability, which is affected by the product
construction, profile, moisture content, timber specie and product dimensions. Many of the well
manufactured engineered products available can be a good choice, with some manufacturers
recommending fully bonding with flexible adhesive, whilst others prefer floating installation. For large
areas of engineered flooring a fully bonded installation with a flexible adhesive can be a good option, and
is said to have some capacity to restrain movement, whilst avoiding the potential for cumulative
Wood flooring laid over under-floor heating reaches quite a low moisture content depending on floor
temperature and humidity of the environment, and this must be kept in mind when selecting suitable
products. BS 8201: 1987 recommends 6-8% for use with UFH. Many suppliers recommend solid wood
flooring for use with UFH at levels above 8% with some success depending on numerous factors, not
least of all board width and installation method.
When it comes to solids especially, board width plays a significant role in performance and this is
especially so if a product is used with moisture above 8%. If a solid wood product looses moisture and
shrinks, the degree of shrinkage will be relative to the amount of moisture loss and the width of the board.
BS 8201: 1987 recommends a maximum width of 75mm on solid strip flooring with UFH, which is
obviously erring on the side of caution. Some brands of solid flooring in widths over 75mm perform very
well due to low moisture content, good equalization which provides an even moisture content throughout
the board, and in some cases a balancing membrane or finish on the underside of the product.

The product width is also relevant to the stability of the board. The narrower the board the more stable is
will be in terms of staying flat when small changes in moisture content occur. If a very wide board is
needed, a well balanced engineered board installed by fully bonding with flexible adhesive is a safer
option. Some of the engineered products, especially the all-hardwood structured boards are impossible to
tell from solids when installed this way.

The timber specie is also a factor in how much movement will occur in a solid product with moisture
change; this is expressed as the dimensional change coefficient specific to each hardwood. This figure can
be used to predict the approximate change in dimension of solid hardwood strip and plank of a specific
width, with a given moisture change.
With the potential for some shrinkage in any solid wood flooring installation, customer expectations are
important and this is even more so with installations over UFH. Along with explanations regarding
natural variations and grain characteristics, a simple explanation of timber movement should be
incorporated into sales contracts.
What the heating system needs from the flooring is a suitable level of thermal resistance. Some
manufacturers give a figure of the maximum thermal resistance recommended for their system. If that
figure is exceeded the time taken to heat the floor to the desired temperature will the increased. If an
underlay or polythene layers are involved their thermal resistance must also have to be taken into account,
with thick underlay best avoided for this reason. BS8201: 1987 recommends, “construction which
permits an air space directly under the floor should be avoided as this can cause undesirable temperature
fluctuations.” Fully bonding with a suitable adhesive is said to greatly improve heat transfer.
If growth forecasts for UFH are to be believed, considerations of compatibility will arise more frequently,
and although UFH does add another consideration to an installation, those who adapt to the changing
needs of the market will doubtless benefit from the trend.


The Sub-Floor
   • Ensure the sub-floor is thoroughly dry. Beware that surface readings taken with electrode devices
      may give false low readings due to a moisture gradient. Carry out an equilibrium relative humidity
      test in accordance with British Standards.

The UFH System
   • Maximum floor temperature must not exceed level recommended for the wood flooring product.
   • UFH System must provide an even distribution of heat.
   • UFH System must have a high degree of controllability.
   • Check what the maximum thermal resistance of floor covering is recommended by the UFH
      manufacturer (if provided.)
The Wood Flooring.
  • Check moisture content of flooring material is correct for the installation.
  • Install using the flooring manufacturers recommended method for UFH.
  • Ensure client has been informed in writing about seasonal movement, and shrinkage.
UFH Operation
  • If flooring is installed by adhesive bonding, adhesive must be fully cured before heating system is
     switched on following the installation.
  • Following the installation the floor temperature must only be raised very gradually from a low
     starting point over a period of several weeks.

Contact Us

Tel:                  01255 224776.
Email:                info@timber-floor-technology.co.uk
Web:                  www.timber-floor-technology.co.uk

                             We look forward to being of service to you.

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