Multifoil insulation explained: how it works, where it wins, where it loses

Walk into any builder's merchant and you'll see multifoil stacked in rolls beside mineral wool. Ask three people behind the counter which is better and you'll get four answers. As PAS2030 installers fitting both materials every week, we get the same question from homeowners all the time — here is the honest picture of where multifoil wins, where it loses, and what to ask any installer before they start cutting rolls in your loft.

What is multifoil insulation, in plain terms?

Multifoil is a layered composite. The outermost layers are reflective metallised films — typically aluminised polyester. Sandwiched between them sit layers of polyethylene foam, non-woven fabric and, in the newer BBA-certified systems, a honeycomb air-trap matrix. The whole sandwich is 25–40mm thick, flexible enough to fold into awkward rafter bays, and bonded at the edges so it behaves as a single product.

Mineral wool slows conducted heat. Multifoil does that too, but its headline trick is reflecting radiant heat — the infrared energy that travels as waves rather than through contact. On a pitched roof with a cold outside face, roughly 60–70% of the heat loss from the room below is radiant. Reflect that directly and you attack the dominant mode of loss — which is why multifoil can hit genuine U-values in a fraction of the thickness.

Where multifoil insulation genuinely wins

The common thread across all four is simple — you physically cannot get 270mm of mineral wool into the space without eating the room. Multifoil lets you hit a 0.18–0.22 W/m²K U-value in a 30–50mm total build-up. That's what the BBA certificates formally test, and it's reproducible on site if (and only if) the air gaps are cut in exactly as specified.

Where multifoil insulation loses to other materials

• Open lofts with 300mm of joist depth to play with. Just lay mineral wool — it's cheaper per U-value, easier to lift if a pipe needs servicing, and A1 fire-rated.
• Solid wall interiors where depth isn't the constraint. Woodfibre or PIR board gives a much better moisture profile through the wall build-up.
• Floors with deep voids. Rigid mineral wool between joists beats multifoil on both cost and acoustic performance — and your footsteps won't echo.
• Any job where the installer refuses to detail the air gaps properly. Multifoil flush-fixed against plasterboard loses most of its performance, guaranteed.

How to read a multifoil BBA certificate

The BBA (British Board of Agrément) is the only independent UK body whose U-value claims on multifoil stand up to building control scrutiny. A proper certificate tells you three things that really matter — and any installer worth their salt will know them off by heart.

1. The U-value achieved — tied to the exact build-up (layer by layer, in the exact order) used in the test.
2. The air-gap dimensions — always two, one above and one below. If the certificate says 25mm and your installer leaves 10mm, you're outside the certificate and the performance claim no longer holds.
3. The vapour-control classification. The best systems are Class 1 VCLs, which means no separate VCL layer is needed — simpler install, fewer failure points.

The three ways multifoil gets installed badly

1. No air gaps. Multifoil clamped tight between plasterboard and rafter loses 50–70% of its certified performance. We use counter-battens on both sides, sized exactly to the BBA certificate.
2. Compressed at the ridge or eaves. Any crush of the foil layers collapses the air-trap matrix. Fitters paid by the metre are usually the culprits, which is why we pay our installers by the day.
3. No vapour seal at the junctions. Edges should be taped with the manufacturer's foil tape, and any cut face sealed. Skipping this lets warm, moist air reach the cold rafter face — and within two winters you're growing mould behind your beautiful new ceiling.

Does multifoil qualify for ECO4 and GBIS grants?

Yes — multifoil room-in-roof upgrades are a named measure under both ECO4 and GBIS, provided the product is BBA-certified and the installer is PAS2030. We check your eligibility on the same survey visit. Most of our multifoil work over the last year has been ECO4-funded for no customer contribution; for middle-income households with unsuitable lofts, GBIS typically covers a useful chunk of the balance.

Getting a quote you can actually compare

Every OMEGA multifoil quote names the BBA-certified product, the exact air-gap dimensions, the ventilation strategy, and the expected U-value in writing. If another installer is quoting lower, ask them for the same four data points. If they can't provide them, the quotes aren't really comparable — one is engineered, the other is a punt.

If your loft or room-in-roof feels cold and you'd like a straight, specific answer, book a free home survey online or ring 0800 229 4094 — Monday to Saturday, freephone from any UK number. We cover every postcode across Essex, Suffolk, Kent and Cambridgeshire. No cost. No obligation. No pressure.