If you've got rafters to insulate — whether it's a room already in the roof or a loft you're thinking of converting — the headroom problem is the first thing you'll bump into. Traditional mineral wool needs around 200mm of depth to hit today's Building Regulations. In most UK rafters, 200mm is simply not there. Multifoil is the installer's answer to that constraint: the same U-value in roughly a third of the depth. It's become our fastest-growing category — we fit more multifoil between rafters in 2026 than we fitted total multifoil in 2022. Here's why it works, and what it costs.
The physics — why thickness matters less with multifoil
Conductive insulation like mineral wool or PIR works by trapping still air. The thicker the layer, the better it performs — but you need actual thickness, and that eats rafter depth. Multifoil adds a second mechanism: the shiny outer face reflects radiant heat (the infra-red that actually carries most of the heat out of a roof) back into the building. You get the conductive performance of the multi-layer core plus the radiant reflection — so a 45mm multifoil build-up delivers a similar U-value to roughly 200mm of mineral wool stuffed between rafters.
What BBA-certified multifoil actually delivers
BBA (British Board of Agrément) testing strips out the marketing and measures real-world performance. The three products we fit most often — Actis Hybris, SuperFOIL SF60 and TLX Silver — all have BBA certificates lodged at registered depths. Hybris at 45mm installed, combined with 25mm PIR below the rafter, achieves a tested U-value of 0.18 W/m²K in a typical UK rafter configuration. That hits current Building Regulations Part L comfortably, and clears the tighter ECO4 / GBIS thresholds for rafter insulation funding.
| Build-up | Total insulation depth | Achieved U-value | Rafter headroom kept |
|---|---|---|---|
| 200mm mineral wool between + 50mm below | 250mm | 0.18 W/m²K | None — eats below-rafter space too |
| 150mm PIR between + 50mm below | 200mm | 0.18 W/m²K | Loses 50mm into the room |
| 45mm multifoil stapled + 25mm PIR below | 70mm | 0.18 W/m²K | Keeps ~80mm of headroom |
| 45mm multifoil + 50mm PIR + batten | 95mm | 0.14 W/m²K | Keeps ~55mm of headroom |
Cost — and the ECO4 / GBIS angle
Supply-and-fit on a typical 3-bed room-in-roof with multifoil is around £3,100 in 2026 self-funded. The exact number depends on rafter area, whether we need to lift battens, and the top-up PIR spec. Under ECO4, multifoil between rafters is a fundable measure as long as the installer holds PAS2030 for it and the property meets the core eligibility (EPC D–G, qualifying benefits or LA Flex, council-tax band A–D for GBIS).
Where multifoil does NOT win
Multifoil has a deserved fan club but it is not a silver bullet. Three applications where we still recommend mineral wool or PIR instead:
- Flat cold-deck lofts with generous headroom — 270mm of mineral wool rolled across the joists is cheaper, fully grant-funded on ECO4, and fine. Multifoil adds cost without adding value here.
- Acoustically sensitive spaces — home studios, hospitality rooms. Mineral wool and PIR have much better absorption; multifoil is reflective for sound too.
- Thermal mass installations — thick exterior walls in old stone-built properties, where we want slow thermal response. Multifoil kills the mass effect.
How OMEGA installs it
- Survey — we measure rafter depth, check for existing felt/breather membrane condition, identify any ventilation issues.
- Spec — pick the multifoil product (Hybris, SF60 or TLX) matched to depth and Building Regs target. Calculate PIR top-up.
- Prep — lift plasterboard if this is a retrofit. Install breather membrane if none exists. Check 50mm ventilation gap above the multifoil.
- Fit — staple multifoil tight between rafters with reflective face down, tape all laps with foil tape, install PIR below rafters.
- Service — plasterboard back, skim, decorate. Register the install with BBA for warranty cover.
The whole job on a typical 3-bed room-in-roof is 2 days on site, 1 day decorating to finish. Included in price: scaffolding where needed, full make-good, PAS2030 paperwork, 25-year BBA product cover, 5-year OMEGA installation guarantee.
Multifoil in pitched rafters FAQs
- Does multifoil actually meet Building Regulations on its own?
- Yes, when specified with the correct BBA-certified build-up. Actis Hybris 45mm + 25mm PIR below the rafter achieves a Part L U-value of 0.18 W/m²K in a standard UK rafter spec. The BBA certificate is the document Building Control asks for.
- Why is there so much argument about multifoil R-values online?
- Older multifoils in the 2000s were sold with hot-box test figures that did not reflect real-world in-rafter performance. BBA certification since 2014 uses hot-plate + in-situ tests that match how the product actually performs. Current BBA-certified multifoils (Actis Hybris, SuperFOIL SF60, TLX Silver) have independently verified numbers.
- Can I DIY multifoil?
- Technically yes for a small loft top-up, but not if you want it to be warranty-backed or fundable. Staple spacing, lap sealing, ventilation gap, and PIR interface detail are where DIY jobs typically fail the BBA criteria — and therefore the 25-year cover.
- Is multifoil eligible for ECO4 or GBIS?
- Yes, under both — multifoil between rafters and in rooms-in-roof is an approved measure on the ECO4 and GBIS lists. The installer must hold PAS2030 for that specific measure, and the property must meet the core eligibility thresholds.
- Do I need a vapour barrier with multifoil?
- The foil face acts as the vapour control layer when installed continuously with taped laps. You do not need an additional VCL behind the plasterboard — in fact doubling up can trap moisture. Correct install is the whole story.
A straight answer for your roof
Book a free home energy survey online or ring 0800 229 4094. One of our surveyors will measure your rafter depth, recommend the right spec, tell you whether ECO4 or GBIS funds it, and quote a fixed price inside 24 hours. No cost. No obligation. No pressure.
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