Short answer
Multifoil can be draped between the joists of a suspended timber floor over a ventilated void, with a small air space below the boards and the void ventilation kept clear. Thin foil underlay is a different product doing a smaller job.
- Suspended timber floors only
- Keep the void ventilated
- ≈25mm space below the boards
- Underlay ≠ insulation
Two different jobs
| Multifoil between joists | Thin foil underlay | |
|---|---|---|
| Product | 30 – 45mm certified quilt | 2 – 5mm foam or bubble foil |
| Purpose | Primary floor insulation | Small thermal and acoustic gain, moisture barrier |
| Access needed | From below, or boards lifted | None — laid on the existing floor |
| Installed cost | £20 – £45 per m² | £3 – £8 per m² material |
Multifoil between joists
- Confirm the floor is suspended timber with a ventilated void and clear air bricks.
- From below (crawl space) or with boards lifted, drape the quilt over the joists and down into each bay, leaving a 25mm still gap below the floorboards.
- Staple to the joist sides; tape all joints; seal at the perimeter.
- Netting or battens below hold the quilt and keep a still gap to the void.
- Do not block air bricks or the void ventilation.
Alternatives
Mineral wool in netting is cheaper; PIR cut between joists is higher performing per depth but slow to fit around uneven timbers. Multifoil wins on speed and continuity in awkward voids. Underfloor guide.
Thin foil under laminate
Worth having as an underlay on a concrete or timber floor: a few degrees of comfort, some sound reduction, a vapour check. It is not floor insulation in the regulatory sense and should not be sold as such.
The check asks about the floor; a survey confirms the construction and void access before quoting.
Questions homeowners ask
Can I use foil insulation under floorboards?
Is foil underlay the same as floor insulation?
Do I have to lift the floor?
- Energy Saving Trust — Roof and loft insulation — checked 2 September 2026
- SuperFOIL — product range — checked 2 September 2026
- GOV.UK — Approved Document L, Volume 1: Dwellings (current edition, as amended) — checked 2 September 2026