A vapour barrier (vapour control layer or vapour retarder) is a membrane applied on the interior (warm side) of an insulated structure to prevent warm, moist indoor air from penetrating into the insulation and structure. It blocks water vapour and thus prevents condensation within the wall or roof.
Why is a vapour barrier needed?
Warm air holds more moisture than cold air. In a heated house, the indoor air is warm and relatively humid. As this air moves through the wall or roof towards the outside, it cools down. At the point where the air reaches the dew point, the moisture condenses — right inside your insulation or on the timber.
This causes:
- Wet insulation (loses insulating performance)
- Mould on timber and plasterboard
- Timber rot in the structure
The vapour barrier prevents this by blocking water vapour on the inside.
Where is a vapour barrier used?
| Application | Position |
|---|---|
| Timber frame wall | Behind the plasterboard, in front of the insulation |
| Insulated roof | Under the finishing, above the insulation (on the warm side) |
| Knee wall | On the room side of the knee wall |
| Insulated floor | Above the insulation (on the warm side) |
Installation
1. On the warm side — Always on the interior side of the insulation
2. Apply continuously — The membrane must form a continuous barrier, without openings
3. Overlap — At least 10-15 cm per strip
4. Tape — Seal all joints and connections with vapour-tight tape
5. Connections — At frames, sockets, pipe penetrations: seal carefully with collars and
Common mistakes
- Forgetting the membrane — No vapour barrier = moisture in the structure. In timber frame construction, this guarantees problems.
- Holes in the membrane — Every hole (screw, pipe, socket) is a leak. Seal everything.
- Wrong side — Vapour barrier belongs on the warm (interior) side. On the cold side, you use a breathable membrane.
- Membrane not continuous — A vapour barrier with holes is useless. It must be a hermetically sealed barrier.
Vapour barrier vs. vapour retarder
| Vapour barrier | Vapour retarder | |
|---|---|---|
| Sd value | > 100 m | 2-100 m |
| Permeability | Practically zero | Limited |
| Material | PE film, aluminium | Special membranes |
| Application | Standard in timber frame | Renovation, specific situations |
A vapour retarder is a smarter variant: it slows water vapour in winter (when moisture wants to escape outward) but allows moisture to dry back inward during summer. This reduces the risk of trapped moisture.
Related terms
- Breathable membrane
- Timber frame construction
- Insulation value (R-value)
- Mineral wool
- Knee wall
Discover the complete DIY building plans package at fredsdiyplans.com