The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
COP v3.0:roof-ventilation; living-environment
In Cold Roof construction the insulation is at ceiling level, and there is an air gap between the insulation and the roof surface.
With cold roof construction, the under-surface of the metal roofing will at times be quite low, so the primary tool of managing condensation is to control the concentration of water vapour in the attic space.
As some condensation is inevitable, this must be managed to ensure it is not excessive in terms of degree or duration.
In cold places such as Europe and Northern USA where heated buildings are the norm, air is 'conditioned' to control the humidity and keep the heat in. In hot countries where the emphasis is on cooling, insulation and vapour barriers are used to keep the heat and moisture out.
Buildings in hot countries are typically constructed with a warm roof. The insulation is in direct contact with the underside of the roof, and a vapour-control layer is installed on the underside of the insulation to limit moisture infiltration to the underside of the roof. These systems require careful design and engineering and are, therefore, marketed as proprietary systems. See 15.5 Insulated Panels.
Bathing and showering, cooking, heating, and clothes drying are the most obvious sources of water, but respiration, perspiration, indoor plants, and pets all produce moisture.
Areas of moisture-generating activities should be well ventilated, and preferably mechanically ventilated to outside the structure.
Some other sources of moisture are best avoided altogether, particularly unvented gas heating and kerosene heaters. Burning 1 kg of gas can release 1.6 L of moisture into the atmosphere.
Occupant behaviour is another large variable which is difficult to manage or predict.
The COP recommends opening a window when possible, and have security stays so that some ventilation can be maintained throughout the day.
During construction, timber can become wet and take some time to dry out. Activities such as plastering and painting also release water vapour.
Concrete floors are a particularly prolific source of moisture. During curing, a 100 mm thick concrete slab releases approximately 10 L of water vapour per square metre of surface area. The period over which this occurs varies, but a rule of thumb is that a concrete floor cures at the rate of 25 mm per month, therefore a concrete slab can affect internal moisture levels for a considerable period.
All new buildings, particularly those with concrete floors, must be kept well ventilated until moisture levels of construction materials have stabilised.
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 architraves and other timber trimmings. Ceiling tiles and tongue and groove ceilings generally have a greater porosity than plasterboard.
Unsealed downlights can be a major source of moisture traffic into the ceiling cavity and should be avoided where possible. Alternatively, replace them with sealed lighting units or the install roof space ventilation.
Wall cavities must be closed off at the top so that they do not transmit vapour from groundwater to the ceiling space
With low energy housing, proprietary systems can be used which limit the amount of air movement through the ceilings. These systems can be very effective if used systematically, but the COP does not recommend using impervious vapour barriers in an ad hoc manner as an alternative to roof space ventilation. If more water enters the ceiling cavity than the evaporation rate, chronic moisture problems will occur.