Vapour Barrier / Vapor Barrier
A sheet material installed within walls, floors, or roofs to prevent moisture from passing through the building structure, reducing the risk of damp and mould.
A vapour barrier (spelled vapor barrier in the US) is a layer of moisture-resistant material — typically a polyethylene sheet — installed within the construction of walls, floors, or roofs. Its purpose is to prevent warm, moist air from inside the home from passing through the building fabric and condensing on cold surfaces, which would lead to damp, mould, and structural damage over time.
How a vapour barrier works
Warm air holds more moisture than cold air. In winter, warm indoor air tries to escape outward through walls and ceilings. When that moist air hits a cold surface within the wall or roof structure, the moisture condenses — just like water droplets forming on a cold glass. A vapour barrier blocks this moisture migration, keeping the building structure dry.
The vapour barrier is typically positioned on the warm side of the insulation — between the insulation and the interior plasterboard.
Where vapour barriers are used
- External walls — especially timber-framed or insulated walls
- Roofs and loft spaces — behind plasterboard on insulated ceilings
- Ground floors — a damp-proof membrane (DPM) beneath a concrete slab or screed serves a similar function, preventing ground moisture from rising into the home
- Bathrooms and kitchens — areas with high moisture levels where the risk of condensation within the structure is greater
Why vapour barriers matter in a renovation
If your renovation involves insulating walls, converting a loft, or building an extension, vapour barriers are part of the construction that your contractor should include:
- Loft conversions — the new insulated roof structure needs a vapour barrier on the room side
- External wall insulation — whether internal or external, getting the vapour barrier in the right position is critical for preventing interstitial condensation (condensation forming within the wall itself)
- Building regulations may require vapour barriers in specific constructions
Practical tip
Do not confuse a vapour barrier with a breathable membrane. A breathable membrane (used on the cold, outer side of the insulation) lets moisture escape outward while blocking wind and rain. A vapour barrier (on the warm, inner side) stops moisture entering the structure. Both work together but do opposite jobs. When reviewing your scope of work, check that both are specified where appropriate.