COP:roof-ventilation; ventilation

9.4 Ventilation 

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.

Moisture saturation points are easily reached in skillion and flat roofs, even with normal moisture levels, because of the small air cavities. Such roofs and any other high-risk designs must be ventilated to comply with the NZMRM Code of Practice.

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.

 

9.4.2 Commercial Ridge Vents 

Permanent ventilation of the roof cavity may be achieved by using accessories such as gable louvre-ventilators, purpose-made ventilators, wind or powered extractors, or by using a vented ridge.

Continuous ridge vents used in conjunction with eave inlets can provide the necessary air movement to avoid the accumulation of condensation Their efficiency depend on the wind direction, because there is positive pressure on the windward side, and negative pressure on the leeward when the wind blows 90° to the ridge.

To be thoroughly efficient, the inlet area should larger than the outlet area. Because fluctuations in air pressure can vary from positive to negative along a section of vented ridge under gusty wind conditions, the lack of a sufficient inlet area can also promote leakage of ridge vents.

The lower down the building height the inlets are, the more efficient the ventilation will be. The steeper the pitch, the greater the stack effect and the better the air flow.

In very exposed areas, high winds may cause excessive air movement
 

9.4.3 Turbine Vents 

Rotary ventilators are generally preferable to ridge ventilators for commercial roofs.

They are more weathertight, perform well in all wind conditions, have a determinable airflow capacity, can readily be damped when airflow is not required, and can be retrofitted if more ventilation is required.

 

 

Rotary ventilators should be positioned as close to the apex as possible, at spacings of up to 6 metres apart. As with all forms of ventilation, they work most efficiently when there is ample air intake capacity at a lower level.

Capacities and air change calculations vary with design and can be obtained from the supplier. Common sizes range from 150 mm to 900 mm throat size.
 

9.4.4 Mechanical Vents 

Supply-driven and exhaust-driven mechanical ventilation systems can pressurise or depressurise internal atmospheres in different areas of the building interior, and need to be well designed and maintained to avoid the risk of them affecting internal moisture.

A balanced mechanical ventilation system where both intake and exhaust is to the outside atmosphere is preferred. Mechanical vents need specific design.