Short answer
Foil products are not all vapour-equivalent. Many internally fitted multifoils are vapour-resistant and, taped, are certified as the vapour control layer; some reflective membranes are deliberately vapour-open. Position and declared vapour resistance decide the role.
- Insulation, VCL and breather membrane are three jobs
- Taped multifoil usually acts as VCL
- Breathable reflective products exist
- Never two impermeable layers with insulation between
The three layers
| Layer | Job | Vapour behaviour | Position | Examples |
|---|---|---|---|---|
| Reflective / multifoil insulation | Reduce heat loss (radiant + core resistance) | Usually vapour-resistant (check declared value) | Warm side of the rafters, with air spaces per certificate | Multifoil quilts, bubble foils |
| Breathable reflective multifoil | Insulation plus vapour-open weather layer | Vapour-open | Over the rafters, under the counter-battens | e.g. TLX Silver-type products |
| Vapour control layer (VCL) | Stop moist indoor air entering the build-up | Vapour-tight or vapour-resistant | Warm side of the insulation | Polythene, reflective VCL, taped foil face |
| Breather membrane | Keep weather out, let vapour escape | Vapour-open | Cold side, under tiles or cladding | Roofing underlays, wall breather membranes |
When foil is the vapour barrier
Reflective insulation products are not all vapour-equivalent. Many internally installed foil-faced and multifoil systems have significant vapour resistance and may form part of a vapour-control strategy when installed and sealed in accordance with their specification. Other reflective membranes are specifically designed to be vapour-open. Always use the declared properties and certified build-up for the exact product.
A vapour-resistant multifoil with taped joints, continuous across the roof and sealed at penetrations, performs the VCL role where its certificate says so. Adding a separate polythene VCL on its warm side is unnecessary; adding one on its cold side would trap moisture between two impermeable layers. Reflective VCLs — thin foil-faced membranes — are used where the primary insulation is PIR or wool and a low-emissivity service void is wanted.
When it is not
- A single bubble foil in a shed is not a designed VCL; joints are rarely sealed.
- Foil on the cold side of wool or PIR is an accidental vapour barrier in the wrong place.
- A foil-faced PIR board has a foil face but its joints must be taped to act as a VCL.
Order of layers, room to sky
- Plasterboard
- Still air gap / service void (battens)
- Multifoil (taped: acts as VCL) — or VCL + PIR/wool
- Still air gap (multifoil) or PIR between rafters
- Ventilated void or breathable membrane
- Roof covering
The survey sets out the build-up in order, with the membranes named, before a quote.
Questions homeowners ask
Is foil insulation a vapour barrier?
Do I need a vapour barrier with multifoil?
Which side does the vapour barrier go?
- BSI — BS 5250:2021 Management of moisture in buildings — checked 2 September 2026
- GOV.UK — Approved Document C (resistance to moisture) — checked 2 September 2026
- GOV.UK — Approved Document L, Volume 1: Dwellings (current edition, as amended) — checked 2 September 2026