Which side of the insulation does the vapour barrier go?
The warm side rule, why getting it wrong soaks the insulation, and how a vapour barrier differs from a breather membrane.
- Difficulty
- intermediate
- Time
- 20 min
- Read
- 4 min
Short answer
The vapour control layer always goes on the warm side — directly behind the plasterboard and in front of the insulation. Vapour travels from warm to cold, so stopping it before it reaches the cold zone is the whole point. A breather membrane, which is a different product, goes on the cold side.
Warm air holds far more moisture than cold air. When indoor air migrates outwards through a wall or a roof, it cools as it goes, and at some point it can no longer hold its moisture. Where that happens decides whether your insulation stays dry.
What you'll need
- vapour control layer or foil-backed board
- vapour barrier tape
- breather membrane for the cold side (optional)
Step by step
- Establish which side is warm.The heated interior. In a wall that is the room side; in a warm roof it is below the insulation; in a floor over an unheated space it is the top.
- Put the vapour control layer on that warm side.Between the plasterboard and the insulation. Anything that stops vapour entering the construction from the room does the job.
- Understand why the cold side is wrong.A barrier on the cold side lets vapour into the insulation and then traps it there. It condenses at the barrier and soaks the insulation from the inside, which is both a performance and a rot problem.
- Make the layer continuous.Its value depends on completeness. Tape every lap, seal round every service penetration, and continue it into the junctions with floors and walls.
- Seal around downlights, sockets and pipes.These are the routes vapour actually takes. A well-laid membrane with unsealed penetrations performs far worse than its area suggests.
- Use the cold side for a breather membrane instead.A breather membrane sheds liquid water from outside while remaining open to vapour, so anything that does get into the construction can dry outwards.
- Do not put impermeable layers on both sides.That traps moisture with nowhere to go. Vapour-tight inside, vapour-open outside, is the principle.
- Consider foil-backed plasterboard as an alternative.It combines the board and the vapour control layer, which reduces the number of ways to get it wrong — but the joints and edges still need attention.
- Treat older breathable buildings differently.Solid-wall houses built with lime mortar and lime plaster are designed to let moisture move through and dry out. Sealing them with an impermeable layer can cause damp rather than prevent it, so get advice for a pre-1919 building.
- Get the design checked for anything substantial.Loft conversions, warm roofs and internal wall insulation all involve condensation risk calculations. This is one of the places where getting it wrong is expensive and invisible for years.
Tips
- Warm side, always. If you remember only the phrase, you will get it right in every situation.
- The tape is as important as the membrane. An untaped lap is a hole.
- In an old solid-walled house, adding an impermeable layer can trap moisture in the wall — take advice rather than following modern-build detailing.
Common mistakes
- Putting the vapour barrier on the cold side of the insulation — Vapour passes through the insulation, reaches the cold impermeable layer and condenses there, soaking the insulation from within. The insulation loses performance and the timber behind it starts to rot.
- Treating a breather membrane as a vapour barrier — They do opposite jobs. A breather membrane is deliberately vapour-open so moisture can dry outwards; it will not stop vapour entering the construction.
If it doesn't work
Damp insulation and black staining on the cold side
Cause: Interstitial condensation, often from a missing or wrongly positioned vapour barrier — Fix: Open up, dry out, replace wet insulation and fit a continuous taped vapour control layer on the warm side.
Mould appearing on a newly insulated internal wall
Cause: Vapour reaching a cold surface, or a cold bridge at the edges — Fix: Check the continuity of the vapour layer and the insulation at reveals and junctions, where cold bridges most commonly occur.
Condensation dripping in a converted loft
Cause: Warm moist air reaching a cold roof surface — Fix: Seal the vapour layer, seal all penetrations, and check the ventilation path above the insulation is not blocked.
Damp in an old solid wall after internal insulation
Cause: Breathable construction sealed with impermeable materials — Fix: Take specialist advice. Vapour-open insulation systems exist specifically for older buildings.
Questions people ask
Is polythene good enough?
Heavy-gauge polythene, properly lapped and taped, is a common and effective vapour control layer. Purpose-made membranes are tougher and have printed lap guides, which makes a continuous job easier.
Do I need one under a suspended timber floor?
The principle is the same — the barrier on the warm side, above the insulation. Underfloor ventilation on the cold side must not be blocked.
What about behind a bath or shower wall?
Wet areas need tanking on the surface as well as vapour control in the build-up. They are separate problems and both need addressing.