A question that is asked a lot, is what underlay for underfloor heating can you use? Although you can lay carpet and underlay on top of certain Underfloor Heating products there are limitations and hazards that you must be aware off. We do not recommend a system for directly under the carpet as the heat output would be too high, therefore we recommend a water system or electric system that is incorporated into a screeded floor that allows the heat to dissipate.
The issue with carpet and underlay for underfloor heating, it can act as an insulator and stop the heat penetrating to the surface, therefore, the combined TOG rating of the underlay and carpet must not exceed 2 togs (thermal insulation resistance) and the temperature we would not recommend exceeding 27 degrees Celsius.
According to the carpet supplier AE Burt ltd 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 varied 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.