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.
The first line of defence against excessive roof space moisture is to restrict water egress by good design and rational occupant behaviour. The second is by ventilating the ceiling cavity.
Barriers to the natural airflow in the ceiling cavity must be avoided. These barriers include non-perforated profiled filler strips at the eaves and apex, and insulation pushed up hard against the underside of the roofing material.
Underlay should terminate at the ridge purlin, or have strips or slots cut in it where it traverses the apex; this alone can double air changes in the ceiling cavity.
The COP recommends specific ventilation design in high-risk situations. Because of the small the air cavity in skillion and flat roofs, saturation points can be quickly and regularly reached with normal amounts of moisture-laden air. For this reason, they must be ventilated.
Other high-risk situations include skillion or curved roofs in which the ceiling line follows the roof cladding, so the air volume is significantly reduced, or curved or tight capped roofs where there is little or no ventilation at the apex.
Additional ventilation mechanisms include:
- Louvre vents in gable ends.
- Soffit vents.
- Fascia vents.
- Proprietary ridge vents.
- Ventilated soft edge strips on transverse flashings.
- Solar powered or wind-powered vents positioned close to the apex.
Where eave vent intake and ridge vent exits are both employed, the area of the ridge vents should be less than that of the eave vents. This arrangement prevents air escaping through the ridge vent from lowering the pressure of the attic cavity, which will encourage more ingress of moist air from the dwelling area.
For venting to be effective, an intake at the lower edge and outlet at the upper edge of the roof end is optimal.
In pitches of 30° or less, cross venting alone is generally sufficient, combined with trickle ventilation at the ridge or apex.
When ventilated ridges are used, the underlay must be terminated at the ridge purlin to allow free passage of air. The COP recommends that the underlay is terminated at the ridge, or slit or slotted to allow passive ventilation of the ceiling cavity.
While a rule of thumb of 1/300 of ventilation aperture to ceiling area exists overseas, far smaller ratios have proven sufficient in NZ conditions. The main rule is to let air in at the bottom, out at the top, and provide a free passage in between.
The function of bulk insulation is to trap air, so the effect of moderate air movement is insignificant. Wet insulation, however, is ineffective. Ventilation of spaces above insulation to remove excess moisture will allow the insulation to perform to its design capacity.
When insulation fills a ceiling cavity or takes up a significant portion of the ceiling cavity space, it inhibits ventilation. The installation of a vapour barrier to limit entry of moist air into the ceiling space has been used in some older New Zealand homes, but removing damp air by ventilation is a more practical approach unless a properly engineered vapour control system is adopted.
A minimum air gap of 20 mm must be provided between bulk insulation and the roof.
Even with good ventilation, condensation may form at times on the underside of the roof and, more commonly, on the underside of the underlay. This is acceptable, providing the quantity of condensation, and the duration of it being present is not excessive.