We have talked extensively about how insulation keeps heat inside your home. But there is something else that constantly tries to escape through your walls and ceiling: water vapour.
Every time you take a hot shower, boil pasta, or simply... breathe, you add moisture to the indoor air. This warm, humid air tends to move outward, seeking the cold. If it manages to penetrate the building materials and cools suddenly, it turns to liquid water (condensation). And water inside a wall means one thing: mould, rot and destruction of the insulation.
To prevent this, we use the Vapour Barrier.
Very simply, it is a completely impermeable membrane (usually made of special nylon, polyethylene or aluminium foil). Its job is not to keep heat in. Its job is to forbid the humid air of the house from passing into the insulation.
It is always (always!) placed on the warm side of the insulation – that is, on the interior room side.
In an external thermal facade (ETICS) with EPS, we usually do not need a separate vapour barrier, because the brick wall and render slow vapour sufficiently, and the EPS "breathes" in a controlled way.
However, there are 3 cases where failing to install a vapour barrier will cause your house to rot:
This is the most dangerous case. If you insulate from the inside, the external wall (the brick) becomes freezing in winter. If the living-room moisture passes through the plasterboard and mineral wool, it will hit the frozen brick, turn to water, and within a year the wool will become a soaked sponge full of black mould. The vapour barrier goes right behind the plasterboard to stop it!
Wood is an organic material. If the warm, humid air from the house rises to the roof, passes through the insulation and condenses on the timber beams, the rot fungus will destroy the roof's structural integrity.
Bathrooms, swimming pools, restaurant kitchens. Here the vapour production is so enormous that a strong vapour barrier under the cladding is mandatory.
We decide to internally insulate a bedroom because we could not erect scaffolding outside. We build a frame, install mineral wool and close with plasterboard. Outside it is 0°C, inside we have 22°C and 60% humidity.
The worker screws the plasterboard directly onto the frame. The water vapour passes easily through the plasterboard. It reaches the external wall, which is freezing (at 5°C). There it condenses. The mineral wool soaks up the water. After 2 winters, the room smells strongly of mould and the plasterboard shows black stains at the bottom.
Before screwing the plasterboard, the worker spreads a nylon membrane (vapour barrier) over the entire wall, carefully sealing the joints with special tape. The humid air hits the nylon and stops there. It never passes into the insulation. The mineral wool stays dry, warm and efficient for decades. The room is healthy and safe.
The Final Conclusion: Insulation keeps you warm, but the vapour barrier keeps the insulation alive. When doing interior renovations, ceiling linings or building lofts, the words "vapour barrier" should be the first thing you write in the contract with your contractor. It is an ultra-cheap membrane that prevents ultra-expensive damage!
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