Expert Advice
Frank DiStefano

Let Your Home Breathe

The effects of condensation build-up in a home can be unsightly and expensive to fix. Typically, blistering of paint, mould on walls and ceilings, stains and a musty smell, are all signs that condensation is an issue. However, there are ways to avoid this. One important element in controlling condensation when building is using the correct type of wall sarking.

Vapour permeable sarking is the waterproof membrane that captures any water that may penetrate a building envelope but allows water vapour to pass. It directs the water away from the structure avoiding it pooling and affecting the frame, during and after construction. It also offers protection from wind when correctly lapped and taped. An air-tight wrap of the building helps reduce the loss of heated and cooled air, improving energy efficiency. The National Construction Code NCC Part 3.5.3 Wall Claddings calls for timber claddings to be installed with a vapour permeable sarking fixed behind the boards.

“An air-tight wrap of the building helps reduce the loss of heated and cooled air, improving energy efficiency.”

Some older traditional breather foil membranes, even though perforated, may not be suitable as a wall wrap if they are ‘unclassified’ as a water barrier. Cladding producers have guidelines similar to Bowens specifying a permeable vapour barrier behind their claddings. Importantly these wraps must be classified ‘High’ as a water barrier as well as have a ‘low vapour resistance’, such as CSR Bradford™ Enviroseal™ Proctorwrap™.

Condensation typically affects homes more when they are constructed from lightweight cladding, have high levels of insulation and air tightness, and have a plastic vapour barrier as the sarking. With the internal warmth from heating, some water vapour disperses through plaster walls and roof areas, this eventually collides with the external cold temperatures of the cladding and roofing resulting in condensation in the insulation layer inside the building.

“Some older traditional breather foil membranes, even though perforated, may not be suitable as a wall wrap if they are ‘unclassified’ as a water barrier.”

Trapped moisture from condensation will cause dampness, moulds and eventually building rot and decay. Fixing the moisture problem after construction is a lot more expensive than installing the correct membrane initially.

A building practice that is not used enough, often due to the additional materials and labour costs, is to fix external cladding to timber battens which are laid over the sarking fixed to the frame. This creates an important cavity between the back of the cladding and the sarking membrane. It provides a drainage path for moisture and a ventilation area allowing condensation to dry. This cavity batten system is an installation requirement for some claddings, but I would highly recommend it for all.

“Trapped moisture from condensation will cause dampness, moulds and eventually building rot and decay.”

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