Getting the Layers Straight

Before you choose a product, it pays to be clear about what sits where in the build-up. Underlay is the thin, resilient layer that goes directly beneath the boards. It deals with impact sound, warmth underfoot and the small unevenness that every subfloor has. A subfloor board — plywood, OSB or chipboard — is a different job entirely: it creates the flat, strong deck the flooring is fixed to. Confusing the two is one of the most common reasons a new floor starts creaking and drumming within a year.

Solid and engineered boards also have very different relationships with that layer. Match the underlay to the board and you get a quiet, warm, stable floor. Get it wrong and you get movement, hollow noise, or a warranty conversation you would rather avoid.

Solid Boards Need Support, Not Sponginess

Solid oak, ash or walnut at 18–20mm is heavy and wants to be restrained. In most cases that means nailing or screwing it down to joists, battens or a structural subfloor. A thick foam underlay beneath a nailed solid board is pointless at best: it gives the nail something to compress into and the board something to move over.

Where you are laying over joists, a batten system works beautifully. Treated battens at 400mm centres, mineral wool packed between them, and a vapour barrier laid across the top before the boards go down. This lifts the finished floor slightly, improves thermal performance and cuts the impact noise that travels down into the structure below. Working over an existing deck instead, a 6mm plywood overlay or building paper is the sensible choice — not foam.

Solid timber also moves with the seasons, so leave a 10–12mm expansion gap at the perimeter and cover it with skirting or beading. No underlay will save you if that gap is missing.

Engineered Boards: Where Underlay Earns Its Place

Engineered boards are dimensionally calmer and can be floated over an underlay, which is exactly why they suit renovation work — flats, upper floors, rooms where lifting the existing deck is not an option.

For a floating installation, the underlay does three jobs at once: it decouples the boards from the subfloor, it takes up minor irregularities, and it adds a little insulation. For a glued-down installation over ply, the underlay is usually reduced to a thin membrane or omitted altogether, because the adhesive needs direct contact with the deck.

Broadly, the options break down like this:

• Foam or polyethylene (2–3mm) — inexpensive, easy to handle, fine for light domestic traffic over a sound subfloor.
• Cork (2–4mm) — natural, warm underfoot, forgiving of slight unevenness, good for older homes.
• Wood fibre (5–7mm) — excellent acoustic performance, but it needs a genuinely flat deck or it will telegraph every dip.
• Rubber crumb (3–10mm) — the strongest sound control of the lot, heavy, and priced accordingly.

Matching Thickness and Material to the Job

Thicker is not automatically better. An over-thick, soft underlay under a floating engineered board can allow the joints to flex, and in the worst cases the click system will fail along a seam. Use the board manufacturer's stated tolerance as your starting point, then work to the space.

• Solid 18–20mm, nailed over battens: no compressible underlay; use a vapour barrier with insulation between battens.
• Engineered 14–15mm floated over timber: 2–3mm foam or cork is ample.
• Engineered 18–21mm floated: 3–5mm cork or wood fibre gives better comfort and quietness.
• Upper-floor flats needing acoustic separation: 5–10mm rubber or a dedicated acoustic system, often combined with a heavier subfloor deck.

Also watch the height you are adding. Underlay plus board can easily push a door out of its swing, and trimming doors or adjusting thresholds is fiddly work you would rather plan for than discover.

Sound, Warmth and Upper Floors

If you live in a flat or a conversion, impact noise is the issue that matters most. Approved Document E sets performance targets for separating floors, and a resilient underlay is part of how you meet them — but it is only part. Mass matters more than most people expect, so a heavy board over a dense subfloor with a good resilient layer underneath will always outperform a thick underlay under a thin board.

For ground floors over a cold void, thermal performance is the priority. Batten systems with insulation, or a fibre underlay with a foil backing, make a noticeable difference underfoot. In older properties where draughts come up between boards, deal with the source first; no underlay is a substitute for sorting out ventilation and insulation properly.

Underfloor Heating and Moisture

Over underfloor heating, keep the build-up thin and conductive. Aim for a combined tog rating under 1.0 — typically a 1.5–2mm foil-backed underlay, or nothing at all if the boards are being glued directly to a prepared screed. Avoid thick rubber crumb and deep fibre boards, which insulate you from the very heat you are paying for. Engineered boards handle the cycling temperatures far better than solid, and most solid timber manufacturers will not warrant their boards over underfloor heating at all.

Moisture is the other quiet killer. Concrete slabs need a damp-proof membrane and a verified moisture reading before anything goes down — generally below 75% relative humidity with a hygrometer, or under 2% with a carbide meter. Let solid boards acclimatise in the room for several days in their packaging, and keep the finished space between 45% and 65% relative humidity once it is down. Do that, and the underlay will do its job quietly for decades.