A vapour barrier (dampspærre) is an airtight membrane — typically polyethylene plastic — installed on the warm side of insulation in walls, ceilings and roofs. It prevents water vapour from warm indoor air from penetrating the building structure and condensing into liquid water.
Why does it matter to you?
Moisture inside a building structure is one of the most common and costly causes of building damage. When warm, humid indoor air meets a cold surface deep inside a wall or roof, it condenses into water. The result: rot, mould and degraded insulation — damage that can cost tens of thousands of pounds to repair.
A correctly installed vapour barrier is your primary defence. It’s especially critical:
- When retrofitting insulation to existing roofs and walls
- In bathrooms and kitchens with high humidity
- In basements and crawl spaces
How it works
Water vapour always moves from warm to cold — from indoors to outdoors. The vapour barrier is placed on the warm (interior) side of the insulation, stopping the vapour before it reaches the cold structural elements.
The vapour barrier must:
- Be installed continuously without gaps or holes
- Be taped at all joints
- Be sealed tightly around pipe penetrations, electrical boxes and other openings
A single incorrectly sealed electrical box can compromise the entire vapour barrier.
Frequently asked questions
Is a vapour barrier the same as a wind barrier?
No. A vapour barrier goes on the inside and stops water vapour. A wind barrier goes on the outside and stops cold air and rain — but is vapour-open so the structure can dry to the exterior.
Do all buildings need a vapour barrier?
All insulated structures in Denmark should have a vapour barrier. Many older houses don’t have one, which can cause moisture problems when retrofitting insulation.
What does it cost to repair moisture damage from a missing vapour barrier?
Moisture damage to structure can cost £15,000–75,000+ to repair depending on extent. Far more expensive than doing it right from the start.