Home / Guides / The 2.5 TOG combined limit explained

The 2.5 TOG combined limit explained

The Underfloor Heating Manufacturers Association guideline puts the combined TOG of floor covering plus underlay at or below 2.5 for a system to run efficiently.

What the 2.5 figure actually covers

The limit applies to the total thermal resistance sitting above the heat source, which means the floor covering and its underlay taken together. It is a suitability guideline from the Underfloor Heating Manufacturers Association, not a legal maximum and not a guarantee. It reflects the point beyond which most systems struggle to push enough heat into the room at a reasonable flow temperature.

Because it is a combined figure, neither the carpet nor the underlay is judged on its own. A modest carpet paired with a thick insulating underlay can breach the limit just as easily as a very warm carpet on a thin underlay.

Carpet TOG and underlay TOG are added

The arithmetic is simple addition. If a carpet is rated at 1.2 TOG and the underlay beneath it is 0.9 TOG, the combined figure is 2.1 TOG, which sits under the limit. Swap in a 1.7 TOG underlay and the same carpet now totals 2.9 TOG, which is over. Nothing about the carpet changed, only the layer beneath it.

This is the single most useful thing to understand: you cannot judge either layer in isolation, and a comfortable underlay chosen without checking the sum is the usual reason a combination fails.

Two levers you control

To keep the total under 2.5 you have two things to adjust. The first is the underlay, where choosing a low-TOG type designed for heated floors frees up headroom for the carpet. The second is the carpet itself, where a lower-TOG covering leaves more room for a slightly thicker underlay if comfort matters to you.

In practice most people fix the carpet they love first, then pick the underlay that keeps the pair legal. A low-TOG underlay around 0.4 to 1.0 TOG will keep a wide range of carpets inside the limit.

The installer still has the final say

The 2.5 rule tells you whether a combination is broadly suitable. It does not set your flow temperature. Your underfloor heating installer commissions the system and decides how hot the water runs, and they should confirm your intended covering before you order. The rule keeps you in the right ballpark so the installer is not left trying to overcome an insulating floor.

Related

Sources: Underfloor Heating Manufacturers Association (UHMA) · Cormar datasheets