10.9 Minimising Ingress of Water Vapour into the Ceiling Cavity
The first line of defence for managing roof space moisture levels is maintaining low relative humidity in the dwelling areas. The second line is preventing excessive amounts of moisture entering the ceiling cavity. It is recommended that all ceilings are square stopped and all penetrations (cables, pipes, hatches, etc.) are caulked. Only use downlights that are airtight and have a gasket
A gloss painted plasterboard ceiling presents some resistance to the passage of water vapour but is not a complete barrier. Vapour will also find its way through any minor gaps in architectural details, and it is air transport through gaps that is responsible for 95% of the passage of water vapour into the ceiling space.
Ceiling tiles and tongue and groove ceilings are considerably more porous than plasterboard. Unsealed downlights can be a major source of moisture movement into the ceiling cavity and should be avoided where possible.
In some older New Zealand homes, vapour barriers have been used to limit entry of moist air into the ceiling space, but control of air movement into the cavity and removal of damp air by ventilation is a more practical approach.
Cavity systems prescribed under E2AS1 220.127.116.11, except those behind masonry veneer, are classified as drained not ventilated. That means they must be closed at the top to restrict air movement between the cavity and the roof space. Closing a cavity off at the top still allows reasonable ventilation of the cavity while preventing excessive amounts of moisture rising to the ceiling space. Alternatively, wall cavities can be vented externally.