What is vapour barrier foil?
Vapour barrier foil (vapour barrier or vapour retarder) is a foil 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 vapour barrier foil 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 toward the outside, it cools down. At the point where the air reaches the dew point, the moisture condenses — right in the middle of your insulation or on the timber.
This causes:
- Wet insulation (loses its insulating properties)
- Mould on timber and plasterboard
- Wood rot in the structure
The vapour barrier foil prevents this by blocking the water vapour at the interior side.
Where is vapour barrier foil used?
| Application | Position |
|---|---|
| Timber frame wall | Behind the plasterboard, in front of the insulation |
| Insulated roof | Under the finish, 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
- On the warm side — Always on the interior side of the insulation
- Apply continuously — The foil must form a continuous barrier without openings
- Overlap — Minimum 10-15 cm per strip
- Tape all seams — Seal all joints and connections with vapour-tight tape
- Connections — At frames, sockets and pipe penetrations: carefully seal with collars and tape
Common mistakes
- Forgetting the foil — No vapour barrier = moisture in the structure. In timber frame construction, this guarantees problems.
- Holes in the foil — Every hole (screw, pipe, socket) is a leak. Seal everything.
- Wrong side — The vapour barrier belongs on the warm (interior) side. On the cold side, a breathable membrane goes.
- Foil 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 | Virtually zero | Limited |
| Material | PE foil, aluminium | Special membranes |
| Application | Standard in timber frame | Renovation, specific situations |
A vapour retarder is a smarter variant: it slows down water vapour in winter (when moisture wants to escape outward) but allows moisture to dry back inward in summer. This reduces the risk of trapped moisture.
Related terms
- Breathable membrane
- Timber frame construction
- Insulation value (R-value)
- Mineral wool
- Knee wall