by Norman Carless
NBS Technical Author
As underfloor heating grows in popularity, more and more people are finding that installing finishes over this form of heating is not quite the problem they thought it would be. Indeed, with a little careful consideration and the right advice, most conventional finishes can be utilized.
The following notes cover some of the factors that must be taken into account when selecting a finish over underfloor heating. In all cases the advice and recommendations of the heating system and flooring manufacturers or suppliers should be followed.
Textile floor finishes generally are very good insulators, especially those made from natural fibres and those with rubber or foam backing, and will reduce the output from the underfloor heating. The supply temperature in the system can be raised to compensate for this, but this is likely to have a detrimental effect on the carpet or underlay, and will also increase running costs.
The thermal resistance of the carpet and underlay should be as low as possible. Thick carpets and felt underlays should be avoided. Most carpet manufacturers publish tog values for their products – a tog is a metric unit of thermal resistance used to express the insulating properties of a fabric (1 tog = 0.1 K•m2/W = 0.1 R-value) Heating system manufacturers vary in their advice regarding the maximum recommended tog value for a carpet/underlay combination. Figures quoted vary between 1.5 and 2.5 tog.
Carpets can be loose laid, or bonded to concrete, cement based screeds or wood based board substrates. The heating should not be used for 48 hours before and after installing the carpet, and then should be brought up to full working temperature gradually over period of seven days.
This is perhaps the material that gives specifiers most concern when used over a heated floor. The movement characteristics of wood are generally well known, but are not always fully understood. The basic principle is that wood shrinks as it dries and expands as its moisture content increases. Tangential movement is greater than radial movement; longitudinal movement can normally be ignored. The amount of movement varies between different species.
During the summer, or when the heating is turned off for long periods, the higher relative humidity of the air will produce an increase in moisture content of the wood flooring. If the floor is tightly jointed and expansion provision has not been allowed, there is a high risk of the floor lifting. Conversely, wood flooring laid with a high moisture content will shrink when the heating is turned on, producing gaps between the strips or boards.
Thus the choice of species and moisture content at time of laying is critical. BS 8201:1987 Code of practice for flooring of timber, timber products and wood based panel products recommends using species with low movement – see BS 8201, table 8. Solid wood strips should not exceed 75 mm in width, and should be kiln dried to a moisture content of 6–۹%. The flooring should not be brought onto site until excess moisture has been removed from the building fabric. It should be acclimatized by stacking or laying loose in the room where it is to be laid, with the heating on, for at least seven days before installation. Expansion provision around the flooring edges must be provided Construction that permits an air space directly below the flooring should be avoided, as this can cause undesirable temperature fluctuations.
Wood is also a good insulator, and the thickness of material will affect the heat output from the floor. However, in this case, as increasing the supply temperature of the system is not normally an option (because of the risk of damaging the floor), flooring manufacturers will usually tailor their products to suit the normal system operating temperatures. A typical waterfilled system will have a maximum average water temperature of 50°C (55°C flow, 45°C return), giving a maximum floor temperature of 27°C.
Engineered boards (boards made up of wood layers laminated together), and other proprietary, wood based laminated flooring systems are generally more stable than solid wood floors. However, manufacturer’s recommendations on conditioning and laying should be followed strictly.
Ceramic tiles, natural and synthetic stone
Having low thermal resistance values, these materials are particularly suitable for use with underfloor heating. Normal provision should be made for thermal movement.
Linoleum and vinyl tiles or sheet
Most products are suitable. It is important with this type of material to ensure that concrete or screed substrates are dry. Any trapped moisture could condense on the underside of the flooring and cause bubbling or delamination. The flooring should not be laid until a hygrometer test – carried out by the method described in BS 8201, Appendix A – gives a reading of not more than 75% relative humidity.
Finishes to avoid
Most underfloor heating system manufacturers claim that their systems are suitable under most commonly used floor finishes. However the following materials should be avoided – very thick carpets, felt underlays, cork tiles and sheet, some softwoods (see BS 8201),and some foam-backed laminate flooring (consult manufacturers).