Underfloor Heating with Carpets and Underlay

Underfloor heating with Carpets and Underlay

Over recent years underfloor heating has become increasingly popular for use in the home this is primarily as the claimed advantages of energy efficiency combined with the the visual advatages of nothaving radiators means it is being specified for many new homes.

Most carpets are suitable for use with underfloor heating systems that operate at the normal operating temperature of 27°C. However not all manufacturers test the thermal resistance of their carpets and provide tog ratings.

What to do if your carpet does not have a tog rating

A carpet will not be unduly effected by a normal underfloor heating system but the higher the tog rating the longer it will take to warm the room initially, but the area will stays warmer for longer.

As a general rule 80% wool 20% nylon carpets have tog ratings between 1.0 and 2.0. 

Carpet and underlays resist the transfer of heat to a lesser or greater degree and this "thermal resistance value" is usually measured as a "tog" value. The general recomendation is that the combined carpet underlay tog value should be less than 2.50.

This thermal resistance value has been the subject of recent tests carried out jointly by the Underfloor Heating Manufacturers Association and the Carpet Foundation.  Their findings have found that carpets perform better than had been expected with underfloor heating when tested in an actual room set condition with carpet open to the surrounding air rather than a laboratory situation. We would conclude from this that the 2.50 tog value should be used as an arboratory value only.

Types of underfloor heating systems

As the use of this type of heating system becomes more popular there are a variety of underfloor heating systems that are available and the manufacturers of the systems may recommend different maximum tog values.  The basic different types are listed below -

Water pipes buried in a sub-floor screed - An underfloor heating system which uses water pipes buried in a sub-floor screed, the maximum acceptable combined tog value of the carpet and underlay is often given as 2.5 tog or even higher. 

Electric heating cables buried in a sub-floor screed or within a suspended timber floor -This type of system generally have a greater tendency to overheat with higher rated tog coverings and for this reason some manufacturers suggest a maximum tog value of 1.5 may have to be adhered to with this type of system.  This low rating is difficult to achieve with carpet and underlay and we would recommend providing the heating manufacturer with the figure and asking for their opinion. 

Electric heating mats set on top of a structural floor - These products are generally only recommended for use with tiled or other hard floors rather than with carpets. If, however, carpet/underlay is installed then suppliers of these types of matting would generally recommend that they should be covered by a flexible adhesive/levelling compound before any floor coverings are laid. This is to prevent the underlay/carpet moulding itself to the levels of the shape of cables in the matting so that they can be seen in the carpet.

Guidelines for carpet and underlay

Choice of underlay

Underlay is perhaps the most important consideration in the whole of this picture.

The underlay needs to allow adequate heat to pass through the carpet and into the room but also gives the carpet a good underfoot feel and adequate performance over its lifetime.  The thermal resistance of carpet underlay is varys over a number of factors, the most important being what material it is made of.  Felt and PU underlays are not normally recommended as they have an inherently high thermal resistance. Usually a rubber underlay performs the best in this situation.

Tog values of underlays vary considerably with a luxury Underlay such as Tredaire Sensation havving a tog value of 2.89 tog and Duralay King has a low value of just 0.80 togs.