Underlay is invisible, so it is the first thing people cut from a quote. It is also what determines how a floor feels, how quiet it is, and — in the case of carpet — how long it lasts. Here is what each type does and how to specify it properly.
What underlay actually does
It absorbs impact. Every footstep compresses the floor covering, and without a resilient layer beneath, that force goes straight into crushing carpet pile or stressing plank joints. A good underlay takes that energy instead, which is why carpet on quality underlay stays looking new far longer.
It insulates. Underlay adds thermal resistance measured in tog, which in a poorly insulated older property makes a real difference to how a room feels underfoot and to heating bills.
It reduces noise, both within the room and to the space below. And under floating hard floors it evens out minor subfloor imperfection, letting the click joints seat properly rather than flexing over a high spot.
Carpet underlay
PU foam (recycled polyurethane) is the standard modern choice: light, resilient, warm and good value. Thickness for domestic use runs 8–11mm, with 11mm being the comfortable, premium-feeling option for bedrooms and living rooms.
Crumb rubber is denser and heavier, and it is the right choice for stairs and hallways because it resists crushing under concentrated load. It costs more and it is harder work to fit, but on a staircase it lasts noticeably longer.
Sponge rubber and felt are older technologies. Felt is still specified under wool carpets and in period properties where a firmer feel is wanted; it is less bouncy and more supportive.
The upgrade from a builder-grade 7mm foam to an 11mm PU underlay typically costs £4–£7 per square metre — about £60 on a bedroom — and it makes a mid-range carpet feel and wear like a considerably more expensive one. It is the best value decision in the whole quote.
- Bedrooms and living rooms: 10–11mm PU foam
- Stairs and hallways: 8–10mm crumb rubber for crush resistance
- Underfloor heating: low-tog UFH-rated underlay only, total build-up under 2.5 tog
- Wool carpets in period homes: felt for a firmer, traditional feel
Underlay for laminate and click flooring
Floating hard floors need a thin, firm underlay — typically 2–3mm. Thicker is emphatically not better here: too much cushioning lets the planks flex, which stresses the click joints and eventually causes them to separate or creak.
On ground-floor concrete, the underlay must include a damp-proof membrane, or a separate 1000-gauge polythene DPM must be laid first with the joints taped and turned up at the perimeter. Skipping this is the most common cause of a laminate floor cupping and lifting a year after installation.
For upstairs rooms and flats, an acoustic-rated underlay is worth the extra. The difference between a basic 2mm foam and a 3mm fibreboard or rubber acoustic underlay is very audible from the room below, and in a leasehold flat it may be a requirement.
Underlay for LVT
Glue-down LVT does not use underlay at all. It is bonded directly to a prepared subfloor, and adding a compressible layer beneath would defeat the point — the floor would deflect and the adhesive bond would fail.
Rigid click SPC and WPC often come with an attached pad, in which case no additional underlay should be used. Where they do not, use a thin high-density LVT-specific underlay, typically 1–1.5mm. Standard laminate foam is too soft and will cause joint failure.
If acoustic performance is needed under a click LVT floor, use a purpose-made acoustic LVT underlay rather than a thicker general-purpose one. These are engineered to be dense enough to support the joints while still absorbing impact noise.
Reading tog and dB figures
Tog measures thermal resistance. Higher is warmer — good for a bedroom over an unheated space, bad over underfloor heating. Over UFH, the combined tog of underlay and floor covering should stay under 2.5, and lower gives faster response.
dB figures on underlay packaging refer to impact sound reduction (ΔLw). 15–18 dB is basic, 19–23 dB is a genuine acoustic product, and above 24 dB is a high-performance system. These figures address footfall noise, not voices or television.
Density, quoted in kg/m³, is the most useful indicator of quality that manufacturers publish and the one buyers ignore. A dense 8mm underlay will outperform and outlast a soft 11mm one, particularly in a hallway.
Cost and false economies
Carpet underlay ranges from about £4 per square metre for basic foam to £12 for premium crumb rubber, fitted. Laminate underlay runs £3–£9 depending on whether it includes a DPM and acoustic rating. On a whole house that is a few hundred pounds at most.
Reusing existing underlay is almost never worth it. It has compressed unevenly along the old traffic lanes, and putting a new carpet over it means the new carpet immediately inherits the old wear pattern. The saving is small and the cost is years of carpet life.
The one thing genuinely worth economising on is thickness where it does not help — do not buy a 5mm underlay for a click floor because it feels more substantial in the shop. It will cause problems, not solve them.
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Frequently asked questions
What thickness underlay do I need for carpet?
10–11mm PU foam for bedrooms and living rooms, and 8–10mm crumb rubber for stairs and hallways where crush resistance matters more than softness.
Do I need underlay for LVT?
Glue-down LVT needs no underlay — it bonds directly to a prepared subfloor. Click SPC either comes with an attached pad or needs a thin 1–1.5mm high-density LVT underlay, never soft laminate foam.
Can I reuse my old underlay?
It is rarely worth it. Old underlay has compressed along existing traffic lanes, so a new carpet inherits the old wear pattern immediately. The saving is small compared with the loss of carpet life.
What underlay works with underfloor heating?
A low-tog underlay specifically rated for underfloor heating, keeping the combined tog of underlay plus floor covering below 2.5. Standard thick carpet underlay will insulate the heat away from the room.