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 shrinkage. 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. Summary 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: firstname.lastname@example.org Web: www.timber-floor-technology.co.uk We look forward to being of service to you.