Multifoil divides opinion more than any insulation product we work with. In the right place it punches well above its thin profile; in the wrong place it under-delivers and the customer ends up disappointed. If you've had it recommended to you and you're trying to work out whether your house is genuinely a fit, this is the straight version from our installers.
What is it, physically? A thin multi-layer reflective product, typically 20–35mm thick, made from alternating sheets of aluminium foil and woven thermal wadding or closed-cell foam. It works by reflecting radiant heat rather than slowing conductive heat the way mineral wool or PIR do, and it uses micro air gaps between layers to trap convective heat as well. On paper, impressive. On site, it's all about the air gap.
How multifoil actually works
Roughly two-thirds of the heat lost from a typical UK ceiling or roof line is radiant — infrared energy radiating outward from warm surfaces. Bulk insulation like mineral wool and PIR tackles conductive heat loss well but does little about radiant loss. Multifoil reverses that split: it's excellent at radiant reflection (typically 95%+ of incident infrared), and it handles a meaningful share of the conductive load through its internal air gaps too.
The single most important rule with multifoil: it needs a 25mm minimum air gap on at least one side (ideally both) for the reflective surfaces to do their job. Squash it tight against rafters with no air gap and you've paid £40 a square metre for a thin thermal blanket. If the installer you're speaking to doesn't mention battens and air gaps in the same sentence as the product, that's your first red flag.
Where multifoil is the right answer
- Room-in-roof systems where you can't afford to lose 200mm of headroom to traditional insulation.
- Sloping rafter spaces where PIR boards would either be impossible to install neatly or would block ventilation paths.
- Loft conversions where the building-regs U-value target needs to be hit without raising the ceiling.
- Caravans, log cabins, garden offices and outbuildings where weight and thickness both matter.
- Refurb projects on listed buildings where the existing rafter depth can't be modified.
Where multifoil disappoints — be honest with yourself
- Open lofts with full headroom. 270mm of mineral wool quilt is cheaper, simpler, and equally effective.
- Cavity walls. Foam-bead or mineral fibre is the right answer.
- Solid walls. Internal or external solid wall insulation is a completely different specification.
- Underfloor. PIR board or sheep-wool insulation is a better fit — multifoil doesn't belong under floors.
What multifoil actually costs, fitted
| Application | Area | Typical 2026 cost |
|---|---|---|
| Room-in-roof, 3-bed semi | ~30 m² | £1,800 - £2,600 |
| Loft conversion sloping rafters | ~25 m² | £1,500 - £2,200 |
| Garden room (4m × 3m) | ~24 m² | £950 - £1,400 |
| Caravan / static home | ~40 m² | £1,800 - £2,600 |
Multifoil and grant funding
Multifoil room-in-roof insulation is an eligible measure under ECO4 and ECO4 LA Flex when it's installed by a PAS2030-accredited installer (we are). Where the household qualifies, the £1,800–£2,600 cost is fully grant-funded — zero contribution from you. Multifoil fitted to non-residential outbuildings, garden rooms or caravans isn't grant-eligible and is paid privately.
Install gotchas — the things that separate a good multifoil job from a bad one
- Air gaps on both sides. The most common failure. We batten out before AND after the multifoil, every job, no exceptions.
- Vapour control. Multifoil is vapour-impermeable. The dew point can shift if the wrong VCL is specified. Survey detail matters.
- Junctions. Every staple hole, tear or untaped joint is an air leak. We tape with manufacturer-spec aluminium tape — gaffer won't do.
- Overlap. Minimum 100mm on every horizontal joint, with the upper layer over the lower (not the other way round).
- Loft hatch. Needs a proprietary multifoil-compatible insulated hatch, otherwise the thermal bridge ruins the whole U-value calculation.
For a multifoil suitability check on your loft or room-in-roof, book a survey online or ring 0800 229 4094. We'll tell you on the day whether multifoil is the right answer for your house, or whether mineral wool or PIR will do a better job. Either way, you'll know for certain. No cost. No obligation. No pressure.
Want to find out how much you could save?
Book a free 45-minute survey. We'll check every active grant, measure your home, and give you a written quote — with no obligation to proceed.
No cost. No obligation. No pressure.