Do I need a breather membrane, and where does it go?

Every modern wall and roof build-up is designed to be vapour-tight on the inside and vapour-open on the outside. The breather membrane is the outside half of that pair, and it is the reason a construction can dry rather than accumulate moisture.

What you'll need

• breather membrane
• membrane tape
• staples or clout nails
• battens (optional)

Step by step

1. Understand the two properties.A breather membrane is water resistant from the outside and vapour permeable from the inside. It is not a contradiction — liquid droplets are far larger than water vapour molecules.
2. Place it on the cold side.Outside the sheathing or the insulation, beneath the cladding, tiles or rainscreen. It never goes on the room side.
3. Lap it correctly.Horizontal laps must shed water downwards, so upper sheets overlap lower ones. Follow the manufacturer's minimum lap, typically 100 to 150mm.
4. Tape the laps where required.Airtightness is a separate benefit, and taped laps improve it markedly. Follow the product's own guidance about which laps to tape.
5. Keep a drainage and ventilation path.Battens behind the cladding create a cavity where any water reaching the membrane can drain and any moisture can be carried away. Fitting cladding tight against a membrane defeats it.
6. Do not sandwich it between impermeable layers.A vapour-open membrane trapped behind an impermeable cladding with no cavity cannot do anything. The path to the outside air is what makes it work.
7. Understand roofing membranes specifically.Modern breathable roofing underlays replaced traditional bitumen felt. Some are approved to allow a roof to be built without a ventilated void; others still require ventilation. That distinction is on the product's certificate and it matters.
8. Support it properly on a roof.Membrane should drape slightly between rafters so water runs down it to the gutter, but not so much that it touches the insulation or ponds.
9. Check UV exposure limits.Most membranes have a stated maximum period of exposure before the cladding goes on, often a few weeks. Beyond that they degrade in sunlight.
10. Detail around openings carefully.Windows and doors are where water actually gets in. The membrane must be dressed into the reveal and lapped so any water is directed back out.

Tips

• Cold side, outside the insulation, always. It is the mirror image of the vapour barrier.
• A breather membrane needs a ventilated cavity in front of it to do its job — battens are not optional.
• Check the exposure period. A membrane left uncovered for a summer can be past its useful life before the cladding goes on.

Common mistakes

• Using a breather membrane as a vapour barrier — It is deliberately vapour permeable, so it will not stop moist internal air entering the construction. The two products do opposite jobs and are not interchangeable.
• Fitting cladding directly onto the membrane — Without a drainage and ventilation cavity, water sits against the membrane and moisture cannot escape. The membrane's permeability becomes irrelevant.

If it doesn't work

Questions people ask