In short: add the carpet tog to the underlay tog. For underfloor heating, keep the total at 2.5 tog or less. With a heat pump, aim for 1.5 tog or less, as the water runs cooler. Most nylon carpets are around 1 tog, and many sponge underlays are 0.5 to 0.7 tog.
Your result
What is tog?
Tog measures how well a material holds back heat. Duvets use the same scale. For underfloor heating you want the opposite of a warm duvet: a low tog lets the heat through into the room.
The limits
- 2.5 tog combined for carpet and underlay is the widely used maximum for underfloor heating, based on research from the Underfloor Heating Manufacturers Association.
- 1.5 tog or less is the target with a heat pump, because the water runs cooler.
- Underlays of less than about 1.3 tog are usually recommended on their own.
Typical tog values
| Material | Typical tog |
|---|---|
| Nylon pile carpet | about 1.0 |
| Wool mix twist carpet | about 1.2 to 1.3 |
| Thick wool carpet | can be over 2.5 |
| 5 to 6mm sponge underlay | about 0.55 to 0.7 |
| 8mm waffle rubber underlay | about 1.1 |
Always use the tog figure for your exact carpet and underlay. Avoid thick felt and foam underlays that are not made for underfloor heating. Check that any carpet gripper and adhesive are suitable too.
Frequently asked questions
What tog carpet can I use with underfloor heating?
Keep carpet and underlay to 2.5 tog or less combined. With a heat pump, aim for 1.5 tog or less.
What underlay is best for underfloor heating?
A thin, low-tog underlay made for UFH, ideally well under 1 tog. Many 5 to 6mm sponge underlays are about 0.55 to 0.7 tog.
Does carpet stop underfloor heating working?
Not if the tog is low enough. A thick carpet and underlay over 2.5 tog holds back the heat, so the room warms slowly and costs more to heat.