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What TOG means for underfloor heating
TOG is a measure of thermal resistance, and over underfloor heating a higher TOG works against you because it blocks the heat trying to rise into the room.
TOG is thermal resistance, nothing more
TOG stands for Thermal Overall Grade. It is a number for how well a material resists the passage of heat. A duvet uses the same scale, and there a high TOG is what you want because you are trying to keep warmth in. Under a heated floor the job is reversed. You want the heat the pipes or mats produce to travel up through the covering and into the room, not to be held back by an insulating layer sitting on top.
This is why a carpet and underlay that feel luxuriously warm underfoot can be the wrong choice over underfloor heating. The very qualities that make them cosy in an unheated room, thickness and trapped air, are the qualities that resist heat flow. The warmer a covering feels to stand on, the more likely it is to have a high TOG.
Higher TOG makes an underfloor system work harder
When the covering above the heat source insulates too well, the room warms slowly or never reaches temperature at a sensible flow temperature. Installers can compensate by running the water hotter, but that wastes energy and, with some coverings, risks running the surface hotter than the material is rated for. Keeping the TOG low is the simplest way to let the system deliver its heat efficiently.
The figure that matters is not the underlay alone or the carpet alone but the two added together, because heat has to pass through both before it reaches your feet.
What counts as low, and what counts as high
As a rough guide, many standard carpets sit somewhere around 1.0 to 1.7 TOG. An underlay chosen for underfloor heating is often in the region of 0.4 to 1.0 TOG. A thick crumb-rubber or dense foam underlay chosen purely for comfort can reach 1.5 TOG or more on its own, which is usually too much once a carpet is added on top.
Treat every published figure as a manufacturer datasheet value for a specific product. The safe move is to look up the actual TOG of the exact underlay and carpet you are considering rather than assuming a type.
Where the limit comes in
The Underfloor Heating Manufacturers Association publishes a working limit for the combined TOG of covering plus underlay. Staying at or below that limit is what keeps a system efficient. Our tool takes the guesswork out by adding the two figures for you and giving a pass or fail against that rule, then pointing you to a low-TOG underlay if the pair you picked comes out too high.
Check your carpet and underlay before you buy
Open the underlay TOG toolAdd carpet and underlay TOG and get a pass or fail against the UFH limit.Find low tog underlay underfloor heating on eBayaffiliateRelated
Sources: Underfloor Heating Manufacturers Association (UHMA) ยท Interfloor (Duralay/Tredaire)