Tests prove that underfloor heating and carpets can make perfect partners

Published:  18 March, 2007

Carpets and underlay with a combined rating of 2.5 Tog can be used with underfloor heating without impairing its efficiency, according to tests carried out by the Underfloor Heating Manufacturers’ Association. The results are said to prove that the old maximum of 1.5 Tog is too low.

The findings are said to confirm what UHMA members have known from long years of practical experience.

Rex Ingram, UHMA chairman, organised the testing at BSRIA. He explains, ‘The tests were devised to clear up one of the last myths about underfloor heating. BS EN 1264 advises a maximum thermal resistance of just 1.5 Tog for any floor covering laid upon a heated floor. However, all experienced underfloor heating specialists know that it is possible to achieve full comfort conditions — and good response times — with the higher levels of resistance offered by some combinations of carpets and underlay.’

The tests are said to constitute the most exhaustive research into the subject ever. They were carried out with the Carpet Foundation and BSRIA engineers in a test room built to the requirements of BS EN 442-2:1997. An underfloor heating system topped with a 22 mm floating chipboard deck was constructed in the room, with sensors and measuring devices configured to effectively create ‘single-plate’ test conditions.

This system was used to test the thermal conductivity of 19 combinations of carpet and underlay at flow temperatures of 40, 50 and 60°C. Air temperature in the test enclosure was controlled to maintain 20°C at the central inner room reference point 0.75 m above the floor.

Rex Ingram explains, ‘Our tests showed that the tested combinations of carpet and underlay functioned as if they had a resistance of about 1.0 Tog less than their combined published Tog values. As a consequence, it has been demonstrated that some of the carpet and underlay combinations with published Tog values of up to 2.5 Tog have “real-life” resistances that are within EN 1264 guidelines.’

He continues, ‘However, it must not be assumed that the tests validate any carpet plus any underlay combination above a 2.5 Tog combined resistance. The nature of their construction may determine their “real-life” resistance over a heated floor. Some carpets and underlay have air layers and cells within their structure and this may increase the thermal resistance, because air is a poor conductor. Radiant heat can pass easily through these air spaces, and this could explain the difference between the “real-life” and published resistances.

‘Practical experience also suggests that as carpet wears in with age, with its fibres being compacted by foot traffic and as manufacturing fluff is vacuumed out, the resistance decreases even further. These factors, combined with the improved connectivity achieved by using double-sided tape or spray adhesives to fix the carpet, make possible even greater reductions in “real-life” Tog ratings.’

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