Underfloor heating 4 min read

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.

Guide type

Underfloor heating

Reading time

4 min

1 tog equals

0.1 m2K/W

Max. resistance over UFH

0.15 m2K/W

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 that is also the worst case the heating is designed for. 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. That figure is for carpet and its underlay together. It does not carry over to wood, laminate or vinyl, where the ceiling is 1.5 tog.

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. That figure is for carpet only; hard floors work to 1.5 tog.

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

Each entry is quoted from its source and linked to it. The full set is in the standards reference.

Tog and m²K/W — the same quantity, two scales

One tog equals 0.1 m2K/W, so a tog value is divided by 10 to give thermal resistance in m2K/W.

1 tog	0.1 m2K/W
1 RSI	1 m2K/W

“The basic unit of insulation coefficient is the RSI, which is equal to 1m²K/W. One tog is one tenth of one RSI, therefore one tog is 0.1m²K/W”

Source: CFA Guidance Document — Beyond Installation: Guidance on Underfloor Heating (Contract Flooring Association)

Thermal resistance of the floor covering

Floor coverings over a warm water underfloor heating system are normally limited to a thermal resistance of R(λ,B) ≤ 0.15 m2K/W, which is also the worst permissible case used for system dimensioning in dwellings.

maximum thermal resistance of floor covering (R lambda,B)	0.15 m2K/W

“The following types of floor coverings can be laid on Uponor surface heating/cooling systems if a maximum thermal resistivity of Rλ,B ≤ 0,15 m²K/W is observed and approved by the manufacturer”

Source: Uponor underfloor heating/cooling — Planning information (Technical Information TI-1186660)

Thermal resistance of wood boards by thickness

Junckers gives the thermal resistance of its wood floorboards as 0.08 m2K/W at 14 mm, 0.09 at 15 mm, 0.12 at 20.5 mm and 0.13 at 22 mm, adds the underlay to the board, and works the example of a 14 mm board on its PolyFoam underlay (0.07) to a total of 0.15 m2K/W.

14 mm floorboards	0.08 m2K/W
15 mm floorboards	0.09 m2K/W
20.5 mm floorboards	0.12 m2K/W
22 mm floorboards	0.13 m2K/W
Junckers PolyFoam underlay	0.07 m2K/W
Junckers Foam underlay	0.04 m2K/W

“22 mm floorboards: 0.13 20.5 mm floorboards: 0.12 15 mm floorboards: 0.09 14 mm floorboards: 0.08 Intermediate layer: Junckers PolyFoam, incl. extra 0.20 mm PE membrane: 0.07 Junckers Foam: 0.04 … Example: 14 mm clip system laid on PolyFoam, the total insulation is: md = 0.08 + 0.07 = 0.15 m² °K/W.”

Source: Junckers — Underfloor Heating, General Information (technical data sheet E 4.0)

Laminate over underfloor heating: one manufacturer's rules

Quick-Step allows its laminate over low-temperature underfloor heating or cooling with a maximum contact temperature of 27 °C and a total thermal resistance of no more than 0.15 m2K/W, always floating, and excludes its Thermolevel underlay.

maximum contact temperature	27 °C
maximum total thermal resistance	0.15 m2K/W

“This means that the maximum permitted contact temperature (= contact surface of the floor with the feet) is 27°C. … Please ensure that the total thermal resistance does not exceed 0.15 m² K/W. … make sure that the laminate is always installed floating.”

Source: Quick-Step — Can I use Quick-Step laminate in combination with underfloor heating? (FAQ)

What an underlay is actually for

The MMFA groups the benefits of an underlay under floating multilayer modular flooring into three headings: ensuring a satisfactory installation by levelling unevenness, permanently protecting the floor against wear and against rising residual building moisture, and improving the floor's properties in terms of impact sound, walking noise, heat insulation and walking comfort.

“The requirements for suitable underlays are aimed at protecting the floor from daily wear and tear such as falling objects and walking, as well as shielding the floor from rising residual building moisture or damp.”

Source: Underlays — MMFA (Multilayer Modular Flooring Association)

Carpet and underlay: the 2.5 tog ceiling

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.

maximum total tog of carpet and underlay	2.5 tog

“most carpets are suitable for use with wwUFH as long as the total tog of all materials, including any underlays, does not exceed 2.5 tog.”

Source: BEAMA Underfloor Heating — Choosing your floor covering to maximise your underfloor heating efficiency (April 2020)

Who publishes this guide

Homes and Floors Ltd, registered in England and Wales, company number 11762665, incorporated in January 2019. Endeavour House, Floor 3, Coopers End Road, Stansted CM24 1SJ.

Where a guide quotes a standard or a regulation, the source is named and linked. Product figures come from the specifications on the product pages. If something here is wrong, tell us at info@homesandfloors.com and we will check it.

First published 20 October 2026.

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