Short answer
Condensation is a construction, moisture, temperature and ventilation problem, not a property of foil. A vapour-resistant foil on the warm side of a ventilated build-up reduces condensation risk; the same layer on the cold side of an unventilated one traps moisture on timbers.
- Position decides everything
- Ventilation on the cold side
- Most internal foils are vapour-resistant
- Fix existing damp first
- BS 5250 for anything unusual
The physics, briefly
Warm indoor air holds moisture. As it moves outward through a roof it cools, and at the dew point the moisture condenses on the first surface it meets. Insulation moves the dew point outward — into the cold side of the build-up. A vapour-tight layer on the warm side stops the moist air getting there. A vapour-tight layer on the cold side traps it exactly where it condenses.
When foil reduces condensation
- Multifoil or a reflective VCL on the room side of the rafters, joints taped, continuous.
- A ventilated void or a breathable membrane on the cold side so any moisture that gets through can leave.
- Eaves and ridge ventilation kept open — the most commonly broken rule.
When foil causes condensation
- Foil pressed against the underside of the tiles or felt with no ventilation between: cold surface, vapour-resistant layer, moisture trapped.
- Foil on a cold loft floor over wet or unventilated wool.
- Roof void sealed by the installation — eaves blocked with quilt, ridge closed.
- Foil fitted over existing damp timber or spray foam.
- Loft room with no extract ventilation to the house, so moisture loads rise.
See how moisture and timber checks are handled after spray foam removal →
The rules a build-up must follow
- Vapour control on the warm side, continuous, sealed at penetrations.
- Ventilation or a vapour-open layer on the cold side.
- Ventilation path shown on the specification and protected during installation.
- Room ventilation to Approved Document F — extract fans and background vents — where a loft becomes habitable.
- Follow BS 5250 for moisture design in anything beyond a simple case.
Multifoil vs other layers
Because most multifoils have high vapour resistance and are certified as the VCL when taped, no separate VCL is normally needed on the warm side; the joint taping makes it the VCL. If PIR is fitted between the rafters with multifoil under, the PIR is on the cold side of the foil and the ventilated gap above the PIR does the moisture work. Vapour barrier guide.
The survey checks the timbers, ventilation and existing insulation before any foil is recommended.
Questions homeowners ask
Does foil insulation cause condensation?
Does multifoil need a vapour barrier?
My loft is damp already — can I add foil?
Where should the ventilation be?
- BSI — BS 5250:2021 Management of moisture in buildings — checked 2 September 2026
- GOV.UK — Approved Document F, Volume 1: Dwellings (ventilation) — checked 2 September 2026
- GOV.UK — Approved Document C (resistance to moisture) — checked 2 September 2026
- Energy Saving Trust — Roof and loft insulation — checked 2 September 2026