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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).
A careful assessment of BRANZ and other research findings in 2020 prompted a shift in thinking around airtightness and ventilation. BRANZ now recommends that all types of new residential building:
- should be built to an airtightness target of 3 ach @50 Pa. This is an achievable target requiring minimal additional cost
- should have mechanical ventilation as the default option, with a carefully designed natural ventilation system also an option
An airtightness target of 3 ach @50 Pa would provide a very low average infiltration rate. (The 50 Pascals pressure is chosen for testing because it is far higher than the normal pressure differences naturally generated by wind and temperature, so the influence on the measurement of these background pressures is reduced.)
Current 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 includes extractor fans as a compliant way to ventilate bathrooms and kitchens. The Acceptable Solution 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."
Healthy homes standards
There are specific requirements for active ventilation in rental housing in the healthy homes standards. An appropriately sized extractor fan(s) is required in rooms with a bath or shower or indoor cooktop. In the bathroom, extract fans installed after 1 July 2019 must have a minimum diameter (including ducting) of 120 mm or an exhaust capacity of at least 25 litres per second. In the kitchen, rangehoods or extract fans installed after 1 July 2019 must have a minimum diameter (including ducting) of 150 mm or an exhaust capacity of at least 50 litres per second. Extracted air must be ventilated to the outdoors.
There is a Ventilation Tool for guidance on the Tenancy Services website.
The healthy homes standards came into force for privately-owned rental properties and boarding houses on 1 July 2021. Rental home owners must ensure that their properties comply with the standards within 90 days of any tenancy that starts or is renewed after 1 July 2021. The rules apply to boarding houses immediately. The standards apply 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: 06 October 2021