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Underfloor heating

Underfloor heating has become increasingly popular in recent times. The claimed advantages of energy efficiency combined with this obviously comfortable heat source means it is being specified for many new homes, particularly in the self-build sector and in many commercial developments.

Can carpet be used with underfloor heating?

The Carpet Foundation carried out some research in conjunction with the Underfloor Heating Manufacturers Association (UHMA) and this proved conclusively that most carpet can be used over underfloor heating systems without impairing the performance of the system. The research showed that a carpet/underlay with a combined thermal resistance of less than 2.5 togs allows underfloor systems to operate efficiently. However we always recommend the information on the tog values (see below) for your choice of underlay/carpet is given to the heating manufacturer so they can ensure that the heating system can perform correctly.

Suitability of Brintons carpets

Brintons carpets are suitable for use with underfloor heating systems at the normal operating temperature of 27°C, it must however be appreciated that all carpets and tiles act as thermal insulation materials and this means that carpeted flooring takes slightly longer to get warm initially, but stays warm longer. The choice of underlay and installation method is also critical for best performance from the heating system. Carpets and underlays resist the transfer of heat to a lesser or greater degree and this "thermal resistance value" can be measured as a "tog" value. 

Typical Brintons 80% wool/20% nylon carpets have a tog value between 1.0 and 2.2. We can offer test reports in accordance with BS 4745 2-plate test from BTTG Shirley, Manchester. Actual copies of these reports are available on request. The tog value is displayed within the specification section on each of the carpet pages. Alternatively you can see a complete list of our ranges and their tog values here.

Types of underfloor heating systems

There are a variety of underfloor systems available and the manufacturers tend to recommend different maximum tog values. Brintons always recommends that you consult with the heating manufacturer to ensure that the carpet/underlay combined tog value is suitable for the system. 

  • Water pipes buried in screed

With an underfloor system which uses water pipes buried in screed, the maximum acceptable combined tog value of the carpet and underlay is often given as 2.5 tog or even higher. Brintons carpets are generally suitable for use over this type of system (Check with heating manufacturer).

  • Electric heating cables buried in screed or within a suspended timber floor

These systems have a greater tendency to overheat with thicker floor coverings and for this reason a maximum tog value of 1.5 may have to be adhered to (Check with heating manufacturer).

  • Electric heating mats set on top of a structural floor

These products are generally only recommended for use under tiled or other hard floors. 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. The maximum tog value quoted by the system manufacturer should be adhered to.

Guidelines for installation of carpets and underlay

As a general rule Double Stick installation systems are recommended to provide close contact between the floor, underlay and carpet allowing better heat transfer and to avoid uneven heating pockets.

Choice of underlay

There is a balance to be sought in the choice of underlay. The underlay used should allow adequate heat to pass through the floor covering into the room space above, whilst also giving the carpet adequate support to enhance the carpet's performance over its lifetime.

Thermal resistance values of underlays

The thermal resistance of underlay is dependant on a number of factors, the most important being what it is made of and its thickness. Felt, which is not normally recommended, has a greater thermal resistance than crumb rubber. Generally thinner rubber products perform best. Tog values of underlays vary considerably. Typical tog values for felt underlays (which are not normally suitable) would be in the order of 2.5 to 2.9 togs. System Ten (Duralay) has a value of 1.55 togs, but is generally considered to be too thick to perform satisfactorily with underfloor heating.

Durafit (Duralay) is 1.09.

Roma and Silver Lining (Ball & Young) have values of 0.8 and 1.5 togs respectively,

Duralay have a range of underlays specifically designed for underfloor heating called Heatflow, for domestic and commercial use. The tog values are 0.75 and 0.92. The 0.75 tog domestic product is for stretch installation and the 0.92 tog product is for stretch or Double Stick installations.

Two of our preferred underlays both from the point of view of carpet performance and for low tog values are Gates Technics 5 (tog value of 0.5) and Technics 6 (tog value 0.63). Both are Double Stick underlays.

Adhesives/tackifiers

Normal tackifiers and adhesives such as F Ball F41 & F3, are suitable for use at the normal average surface operating temperature of 29°C. However, it is important to ensure that the heating is switched off for 48 hours prior to installation and for 48 hours afterwards. In addition, after installation the heating should be brought up to full working temperature over a period of 7 days.

Wooden subfloor

Where the carpet is due to be laid on a wooden subfloor, it is very important to ensure that adequate time has been allowed for the wood to condition whilst the underfloor heating is running, to avoid variability in dimension from changes in moisture levels. We have heard of carpet fitted by the Double Stick method on floating wooden floors where there has been considerable problems with bubbling. Brintons cannot be held responsible for rucking or shrinkage of carpet and underlay caused by movement in the underlying floor surface. We advise that on floating floor installations, it would be better to fit by a traditional stretch and fit method so that the carpet can be easily taken up and restretched or refitted if dimensional changes occur.

This information is given as a service to our customers only. Suppliers of the flooring timbers, underlays and adhesives together with the underfloor heating manufacturer should be made aware of the combined tog values of the carpet/underlay so that the correct settings and calculations can be used.

For further information please telephone 01562 635446 or e-mail technical enquiries.

Gates Technics and Durafit Information - www.interfloor.com

Ball & Young Information - www.underlay.com

F Ball & Co - www.f-ball.co.uk


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