Underlay over Underfloor Heating: Tog and the 0.15 Limit
What tog means, why a hard floor and its underlay together stay within 0.15 m²K/W, how to add the figures up, and why carpet works to a different number.
Over underfloor heating the underlay is part of the heating system, whether anyone planned it that way or not. Every layer between the pipes and the room holds some heat back, and the underlay is the layer most often chosen last and cheapest. This guide explains the unit its figures come in, the ceiling they have to stay under, and how to add them up before the order goes in.
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Tog and m²K/W are the same quantity
Thermal resistance is given either in square metre kelvin per watt, m²K/W, or in tog. The Contract Flooring Association spells out the relationship: one tog is a tenth of one m²K/W, so 0.1 m²K/W is one tog, and a tog figure divided by ten gives the m²K/W figure. A packet that says 0.5 tog and a data sheet that says 0.05 m²K/W describe the same underlay.
The ceiling for hard floors
Floor coverings over a water-based heating system are normally limited to a thermal resistance of 0.15 m²K/W, which is 1.5 tog, and Uponor, for one, designs its systems in dwellings for that worst case. Quick-Step, a large laminate maker, sets the same 0.15 m²K/W for its laminate over heating and over cooling. The figure covers everything above the pipes: the board and the underlay together.
Adding it up
The arithmetic is addition. Junckers, which publishes figures for its own wood boards and underlays, works the example of a 14 mm board at 0.08 m²K/W on its PolyFoam underlay at 0.07: together 0.15, exactly at the ceiling. Its thinner Foam underlay is 0.04. On the same figures a 20.5 mm board, at 0.12, leaves 0.03 for everything beneath it.
So the question to ask of an underlay for a heated floor is not how thick or how soft it is, but what its thermal resistance is, in tog or m²K/W, and what that leaves for the floor on top. If the packet gives no figure, ask for one before buying it for a heated floor.
What the underlay still has to do
A low figure is not the only job. The MMFA, the European association for multilayer modular floors, groups what an underlay under a floating floor does into three: levelling small unevenness so the floor installs properly, protecting the floor against wear and against rising residual moisture from the building, and improving impact sound, walking noise, heat insulation and comfort. Over underfloor heating, heat insulation is the one property you want least of, and on a screed the moisture protection is still needed. An underlay made for heated floors is the compromise between them.
Some manufacturers rule out particular underlays for their own floors. Quick-Step allows all its laminate over low-temperature heating except with its Thermolevel underlay, and recommends its Silent Walk underlay for the best results. That is a rule for Quick-Step products; for any other floor, the instructions in the box say which underlays the maker accepts.
Floors that bring their own underlay
The 36 rigid vinyl floors in our range arrive with their underlay backing already attached, and all are listed as compatible with underfloor heating. The backing is part of the board as the manufacturer supplies it, so the underlay question is largely answered before the box is opened; read the installation instructions before putting anything else beneath it.
Carpet works to a different number
Carpet is measured against a different ceiling. BEAMA, the UK underfloor heating trade association, says most carpets are suitable for water-based underfloor heating as long as the total tog of all materials, including any underlay, does not exceed 2.5 tog, a figure from research with the Carpet Foundation. That is above the 0.15 m²K/W, or 1.5 tog, that heating makers such as Uponor design every covering to, carpet included, so a carpeted room needs to be in the heating design from the start.
Common questions
What tog underlay do I need for underfloor heating?
For a hard floor, the board and the underlay together should stay within 0.15 m²K/W, which is 1.5 tog. Take the floor's own figure off that, and the underlay has to fit into what is left: on Junckers' example a 14 mm board at 0.08 leaves 0.07 for the underlay.
How do I convert tog to m²K/W?
Divide by ten. One tog is 0.1 m²K/W, so 1.5 tog is 0.15 m²K/W and 0.5 tog is 0.05 m²K/W.
Can I use any underlay over underfloor heating?
No. Use one with a stated thermal resistance that keeps the build-up within the limit, and check the floor's instructions: some makers exclude particular underlays, as Quick-Step does with its Thermolevel underlay over heating.
Does rigid vinyl with an attached underlay need another one?
The rigid vinyl floors in our range come with their underlay backing attached and are listed as compatible with underfloor heating. Read the installation instructions before adding anything beneath them.
What tog is allowed for carpet over underfloor heating?
BEAMA's guidance is that the carpet and its underlay together should not exceed 2.5 tog. Heating makers such as Uponor design every covering, carpet included, to 1.5 tog, so a carpeted room needs to be planned into the heating design.
Underlay goes on the same delivery as the floor, to the towns listed area by area. Browse by town and region.
What the published standards say
Who publishes this guide
- Samples £1.99 each
- UK-wide delivery
- Advice before you order