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What underlay does
- Evens out very small irregularities in the subfloor (it is not a substitute for levelling).
- Reduces impact noise for the rooms below and walking noise in the room itself.
- Some types include a vapour barrier for concrete floors.
Types
| Type | Good for | Notes |
|---|---|---|
| PE or XPS foam, 2–3 mm | Laminate on a budget | Light, cheap; limited sound reduction |
| Wood fibreboard, 5–7 mm | Laminate on uneven timber floors | Bridges small dips; not for underfloor heating |
| Rubber or mineral-filled, 1–2 mm | Vinyl, laminate, upstairs flats | Dense, good impact sound, often heating-friendly |
| Cork, 2–4 mm | Wood and laminate | Natural, warm, good sound |
Rules of thumb
- Vinyl: only thin, firm underlay approved by the maker. Soft, thick foam lets the click joint flex and break.
- Pre-attached underlay: do not add a second layer on top.
- Underfloor heating: keep the combined thermal resistance of underlay and floor low (often quoted as below 0.15 m²K/W).
- Flats: check the building's rules on impact sound before choosing.