Short answer
Foil-backed insulation is not one product. It usually means foil-faced PIR board, a multifoil quilt, a thin foil-faced foam roll or a reflective membrane — and the four insulate, handle moisture and get certified in different ways.
- PIR: the core insulates, the foil is a facing
- Multifoil: core plus certified air spaces
- Foam rolls and membranes are not primary insulation
- Use the certificate for the exact product, not the category
The four products people mean
| Product | What the foil does | Where the insulation comes from | Thickness | Typical use | Vapour behaviour |
|---|---|---|---|---|---|
| Foil-faced PIR board | Facing: protects the core, low-emissivity face towards a cavity, helps airtightness when taped | Rigid polyisocyanurate core (low lambda) | 25 – 150mm | Between/over rafters, floors, walls, flat roofs | Facings are vapour-resistant; joints must be taped |
| Multifoil quilt | Multiple reflective layers each facing an air space | Wadding/foam core plus reflective air-space contribution | 30 – 100mm | Under/between rafters, stud walls, floors | Usually vapour-resistant; some breathable versions |
| Foil-faced foam roll | Single reflective face | Thin foam core | 3 – 10mm | Garages, floors under laminate, behind radiators | Vapour-resistant |
| Reflective membrane | Low-emissivity face on a functional membrane | None of its own beyond an adjacent air space | <1mm | VCL on the warm side or breather layer on the cold side | Either — by design |
Why they are not interchangeable
- A foil-faced PIR board's performance is in its core; the foil is a facing. It reaches a target in a fixed depth and does not depend on maintaining air spaces.
- A multifoil's certified performance combines its core with defined air spaces; take the spaces away and the system figure no longer applies.
- Foil-faced foam rolls and membranes are accessories or components, not primary insulation for a heated room.
Which one for which job
- Fixed-depth target, single certificate, flat roof or floor: PIR.
- Limited headroom, awkward geometry, fast fit: multifoil, often with a PIR layer.
- Outbuilding comfort or a radiant barrier behind radiators: foam roll. Foil wrap guide.
- Moisture control layer in a designed build-up: membrane. Vapour barrier guide.
Which product does your roof need?
The check asks about depth, existing insulation and the space's use, then shows the likely route.
Questions homeowners ask
Is foil-backed insulation the same as multifoil?
No. Foil-backed usually means a foil-faced PIR board; multifoil is a flexible quilt of many reflective layers.
Is foil-faced PIR better than multifoil?
Different jobs. PIR reaches a target in a fixed depth with one certificate; multifoil wins on headroom and awkward geometry, usually with a PIR layer to hit a target.
Does the foil on PIR board need an air gap?
The board's core performs regardless. A low-emissivity face towards an unventilated cavity adds resistance in some calculations; the certificate says whether it is counted.
Sources and when they were last checked
- BBA — Agrément certificates search — checked 2 September 2026
- LABC — Registered details and product assessment — 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