Foam and combination foam
Standard polyethylene foam, usually 3mm, with an integral damp-proof membrane on the underside. It is inexpensive, easy to lay and adequate under laminate in a bedroom. It is also the source of most complaints about floors feeling hollow or bouncy, because foam compresses under point loads and never fully recovers. We rarely specify it under real wood.
Rubber and acoustic underlays
Dense rubber or rubber-crumb underlays are heavy, thin and highly effective at reducing impact sound to the room below. They are what leaseholders normally need to meet a specified dB reduction, and they make a floating floor feel far more solid underfoot. They cost several times what foam does and are worth every penny in a first-floor flat.
- Check the lease for a required impact sound reduction figure before buying
- Ask for the manufacturer's test certificate, not a marketing claim
- Dense underlays also improve how a floating floor sounds when walked on
Fibreboard
Wood-fibre boards laid in sheets rather than rolls. They are rigid, so they bridge minor subfloor undulations better than any roll product, and they add useful thermal insulation over a cold suspended timber floor. They are not the right answer over underfloor heating, where you want heat to pass through rather than be insulated against.
Thermally rated underlay for underfloor heating
Over UFH the underlay must have the lowest practical thermal resistance — usually 0.15 m²K/W or less across the whole build-up including the floor covering. Thick insulating underlays throttle the system, run up the heating bill and often void the heating manufacturer's warranty. This is the specification detail we most often have to correct on jobs where the flooring was bought before the survey.
Damp-proof membranes
Any floor over concrete or screed needs a moisture barrier unless the adhesive itself provides one. Some underlays have a DPM bonded to the back; otherwise it is a separate 1200-gauge polythene layer taped at the joints. We take a moisture reading at survey and specify accordingly rather than assuming.
And when you need none at all
A fully bonded floor — herringbone parquet, glue-down engineered, glued LVT — has no underlay. The adhesive bed does the job, and it is why bonded floors feel the most solid. In those cases the money that would have gone on underlay goes into levelling the subfloor instead, which is a better investment.