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Designing homes to conserve energy and use it efficiently, from sources that cause least environmental harm.
Ventilation is required to remove moisture and airborne pollutants from a home to protect the health and comfort of people living there. Ventilation can also be important for temperature control, particularly in summer.
The ventilation rate for houses is often described as the number of air changes per hour (ach). This is how many times a volume of air equal to the house volume has entered and left the house in 1 hour. A healthy ventilation rate is 0.35–0.5 ach. In other words, one third to one half of the volume of air in a house is replaced each hour. Lower than this risks not removing contaminants properly, while higher ventilation rates may require more active space heating.
Houses built in the last few decades of last century typically had infiltration rates around 0.3 ach, with houses built before 1960 having three times this much. These older homes provide the required ventilation just through their ‘air leaky’ construction.
However, houses built after 2000 are typically much more airtight, with closer to 0.2 ach on average.
There is good evidence that New Zealand houses are not effectively ventilated and heated. The 2018 Census recorded 318,891 homes affected by damp, with visible mould larger than an A4 sheet of paper always present in 4.3% (64,536) of homes and sometimes present in 12.6% (188,319).
Building Code requirements
Building Code clause G4 Ventilation requires that spaces within buildings have adequate ventilation for their intended use and occupancy, have adequate fresh air, and have means to remove moisture, products of combustion and other airborne contaminants.
Acceptable Solution G4/AS1 provides means of demonstrating compliance, including – for most buildings – that in occupied spaces the net openable area of windows and other openings must be at least 5% of the floor area. G4/AS1 also provides means of compliance for removing moisture and contaminants from kitchens, bathrooms and laundries, and for using active ventilation. In fact, G4/AS1 includes the comment: “Within this acceptable solution, natural ventilation…on its own is not adequate to remove moisture generated from cooktops, showers and baths.”
G4/AS1 was amended in June 2019 with the addition of extractor fans as a compliant way to ventilate bathrooms and kitchens, so that owners can more easily comply with the new Healthy homes standards (see below). The new 4th edition of G4/AS1, effective 27 June 2019, says:
“1.3.3 Spaces in household units and accommodation units that contain cooktops, showers and baths must have mechanical extract fans installed to remove moisture generated by these fixtures. Mechanical extract fans (including associated ducting) must have a flow rate not less than:
- 25 L/s for showers and baths, and
- 50 L/s for cooktops."
In most New Zealand homes, passive ventilation (including opening windows) will still be sufficient for most of the year and only localised air extraction systems are needed: range hoods in the kitchen and extractor fans in bathrooms.
However, in some situations, an air supply ventilation system may be necessary where passive ventilation is not sufficient to meet New Zealand Building Code requirements. Multi-unit blocks where some units have only one external wall could be an example. Some air supply ventilation systems use solar or heat recovery technology to provide heating as well as ventilation.
A true heat recovery ventilation system supplies fresh air from outdoors that is warmed by the heat extracted from the air being removed from indoors; it does not simply recycle roof space air.
Heat pumps are sometimes confused with ventilation systems. A split-cycle heat pump is a heating and cooling system. A heat pump does not remove indoor air or bring outdoor air indoors and is therefore not a ventilation system.
Healthy homes standards
The Government introduced active ventilation requirements for rental housing in the healthy homes standards announced in February 2019.
An appropriately sized extractor fan(s) will be required in rooms with a bath or shower or indoor cooktop.
The healthy homes standards will apply to new tenancies from 1 July 2021 and to all rental homes from 1 July 2024.
Air purifiers cannot be used to meet Building Code requirements but can still be useful for people susceptible to health problems where there is an excess of pollutants in the air. The HEPA filters in many purifiers can take out particles as small as 0.3 microns (less than one-hundredth the thickness of a human hair). This means they will trap dust, pollen, pet hair and dander.
Updated: 21 September 2020