Positive pressure ventilation systems

Positive pressure/roof cavity ventilation systems are the most common type available in New Zealand. They force filtered air from your roof space into the house through single or multiple ceiling vents.

Performance

How well these systems ventilate the whole house depends on:

  • performance of fans
  • distribution of the ceiling vents
  • building air tightness.

In draughty houses, ventilation systems struggle to force air into each room of the house. The system can short-circuit if the roof cavity is not properly sealed from the house interior (for example if you have older recessed downlights in your ceiling). Indoor air can migrate into the roof cavity and be pumped back into the house over and over again.

Air supply

Roof spaces are often polluted with dust, mould, insects and vermin droppings. To keep the air supply clean, positive pressure/roof cavity ventilation systems are usually fitted with filters. The quality of the air entering the house depends on the filter type and how often it’s cleaned. As filters are hidden away in the roof space, filter cleaning is often overlooked over time. 

As suppliers are yet to prove that home ventilation system filters effectively reduce contaminants, we strongly recommend sourcing your air supply from outside, not the roof cavity.

Standards

The New Zealand Building Code requires homes to ventilate using outdoor air to maintain air purity. Ventilation systems that draw air from the roof space and not directly from outside do not comply with ventilation standard NZS4303:1990 "Ventilation for acceptable indoor air quality". They cannot be used to comply with the Building Code Acceptable Solution for ventilation.

Heating and temperature control

University of Otago research shows that moving roof space air into your home doesn’t provide adequate heat to keep your house warm in winter. It also found that this could often push internal temperatures away from the desired level, rather than toward it.

In summer, roof cavities quickly become excessively hot. Systems that pump roof space air into your house without a summer bypass should be turned off for extended periods to avoid overheating your house.

Beacon Pathway research also found that systems that aren’t controlled by humidity sensors can increase moisture levels in your house. Look for a ventilation system that controls both the temperature and humidity of your air supply.

Testing ventilation systems – Beacon Pathway website