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.
Changing building techniques and materials—e.g., the increasing use of insulation, impervious cladding, and aluminium joinery—have led to buildings being much more airtight.
Occupant behaviour has also changed. More families shower daily and then leave the house unoccupied and closed-up for much of the day and night. Less activity means a low level of air changes per hour. These changes can lead to internal moisture problems.
To ensure that the installation of the roof or wall cladding does not cause moisture problems within the structure, neither the roof designer and the roof or wall cladding contractor should create conditions that could jeopardise the durability of the cladding or the structure.
The primary purpose of the roof cladding is to act as a rain screen so that no water enters the building from the outside. It is, however, equally important to ensure that the building is kept dry from within. Because metal roof and wall cladding are good heat conductors and are not absorbent, condensation forms on metal cladding under conditions of high humidity or changes in temperature.
Water is present in or on the surface of most building materials. Roofing materials are designed to tolerate a certain degree of dampness, and can also withstand greater wetness for short time spans. Excessive and prolonged wetness, however, can reduce the durability of most building materials, and cause the formation of health-threatening mould.
In gas form, the kinetic energy of the molecules overcome the bonds of attraction. This is why water vapour has a strong tendency to migrate upwards into the ceiling cavities, and why it is so important to manage the atmospheric conditions in attic spaces.
Water vapour tends to condense into liquid form when the concentration rises or the temperature drops. Condensation occurs readily when the humidity is high. At a relative humidity of 95% and a temperature of 20°C, only 1°C difference in temperature is required before dewpoint is reached; at 50% relative humidity this difference is 11°C.
The ratio of the mass of water suspended in vapour form compared to the value that saturated air could contain is known as 'Relative Humidity' and expressed as a percentage. The point at which air can hold no more water is called the 'Dew Point' or saturation point and equals 100 % relative humidity.
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 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 14.5 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.