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 ceiling vents. Most systems have a single fan that forces air through ducting to multiple ceiling vents. Ductless systems consist of one or more self-contained ceiling vents that have their own built-in fans.
How well these systems ventilate the whole house depends on:
- performance of fans
- distribution of the ceiling vents around the house
- building air tightness.
In draughty houses, ventilation systems struggle to force air into each room of the house. The system can also 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.
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.?
We strongly recommend that you ask your supplier to install an extra duct that enables the ventilation system to source your supply air directly from outside, not from the roof cavity.
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 benefits are small
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 in summer to avoid overheating your house.
Temperature and humidity sensors
Beacon Pathway research found that systems that aren’t controlled by humidity sensors can increase moisture levels in your house. Look for a ventilation system that uses both temperature and humidity sensors to control the air supply.
Optional electrical heating add-ons
Some suppliers offer an electric heating unit to pre-heat supply air when it’s cold outside. These are known as electric in-line duct heaters. Most of these heaters don’t provide enough heat to meet a home's heating needs and they also lose heat through the ductwork, so it’s better to use a dedicated heating appliance in the room(s) you want to heat.
Ask your supplier to provide independent test performance reports for the system they’re proposing. You should also get a ‘no questions asked' guarantee of performance, which includes removing the system if it doesn't work and repairing all modifications to your home.