Passive Design

Designing the building and the spaces within it to benefit from natural light, ventilation and even temperatures.

Ventilation

Effective ventilation is necessary for good indoor air quality and to help with temperature control.

On this page:

  • passive ventilation
  • using passive ventilation for cooling
  • active ventilation
  • Building Code minimum requirements
  • security and other passive ventilation issues

Ventilation is needed to remove pollutants such as moisture, volatile organic compounds (VOCs) and carbon dioxiode (CO2) from the building’s internal environment. These pollutants arise from household activities such as cooking, cleaning and heating, as well as human activities such as smoking.

There is evidence that our existing homes are not effectively ventilated. The BRANZ 2015 House Condition Survey found that 11% of owner-occupied and 31% of rental houses felt damp. Assessors found signs of mould in almost half the houses visited.

The prevalence of damp in our homes was confirmed in the 2018 Census, which recorded 318,891 homes affected by damp. 44,520 were damp all the time (3% of all homes) and 274,371 were sometimes damp (18.5%). Rental homes were far more likely to be damp than homes that were owned or held in a family trust by the household.

The Census found visible mould larger than an A4 sheet of paper was always present in 4.3% (64,536) of homes and sometimes present in 12.6% (188,319).

Creating an indoor environment where there is no damp or mould requires an effective combination of ventilation and heating. Effective ventilation is particularly important as new houses become more airtight.

BRANZ recommendations

A careful assessment of BRANZ and other research findings in 2020 prompted a shift in thinking around ventilation and airtightness. 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 passive ventilation system also an option

An airtightness target of 3 ach @50 Pa provides 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.)

Passive ventilation

Passive (naturally occurring) ventilation is when air is exchanged in a building through openings in the building envelope using the stack and wind pressures. It is made up from two sources:

  • Controlled through openings such as windows and doors or purpose-built small vents (such as trickle vents on some windows).
  • Uncontrolled by infiltration through unintentional openings such as gaps around windows and doors and between building components. This is much more common in older houses than new houses.

Passive ventilation is an essential component of passive design and is a free and environmentally friendly method of ventilation that is suitable for most New Zealand locations.

In most New Zealand homes, passive ventilation will be sufficient to meet most temperature control and air quality requirements, so long as it is carefully designed and used in conjunction with localised air extraction systems such as range hoods and bathroom extractor fans to remove moisture and pollutants.

Opening windows can reduce excess indoor moisture, provided the room is heated and occupants remember to do it every day. A north-facing room in a BRANZ test house was conditioned with a relative humidity of 70% (while heated to a constant temperature) with the internal door closed. The window was opened on consecutive mornings at 8am to different widths, over a range of external temperature and humidity conditions. The most effective case was when the window was opened to 300 mm – within 10–15 minutes, the absolute airborne moisture content had reduced by 14%.

Even after an hour with no heating, the room’s air temperature did not drop to outside levels. As air has a low heat capacity, the cost to reheat the air in the room back to the recommended minimum of 18°C was low.

A room with excess moisture and limited heating may take longer for the airing-out to work. BRANZ found that a modern home suffering condensation on double glazed windows needed to have the windows opened twice a day for 2–3 weeks to reduce the internal humidity to acceptable levels. This was mainly due to moisture stored in linings or furnishings. Moisture levels could then be maintained by simply opening windows in the mornings.

Using passive ventilation for cooling

In warmer, more humid regions, passive ventilation for cooling should be included in house design. (Design of passive ventilation explains how to calculate air flow rates).

Considerations when designing a home for passive cooling:

  • Locate, orient and design a form that maximises exposure to cooling breezes.
  • Ensure there are good air flow paths through the building.
  • Specify windows that maximise air flow but minimise unwanted heat gain
  • Where possible, specify low level (i.e. near the floor) horizontal openings as these are more effective than vertical openings for ventilation purposes.

Air speeds up to 1.0 m/s can increase evaporative cooling. Air speeds above 1.0 m/s usually cause discomfort.

Passive ventilation for cooling in the summer must be countered by restricted incoming air in winter, so that minimum fresh air requirements can be maintained without causing draughts or excessive heat loss.

Active ventilation

Passive ventilation alone will not provide enough air exchange to remove moisture from wet areas such as bathrooms, kitchens and laundries. In fact, the Acceptable Solution 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. ”In those situations, an air extraction system or other form of active ventilation is needed to remove moisture.

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:

  1. 25 L/s for showers and baths, and
  2. 50 L/s for cooktops."

Other forms of mechanical ventilation are also available and may be useful if a building is very airtight, or security or other concerns make a passive solution unviable. Heat recovery systems in particular require very airtight buildings and user education to work efficiently.

For more detail, see active ventilation.

Building Code minimum requirements

Passive ventilation must meet the requirements of Building Code clause G4 Ventilation. This includes ensuring that spaces within buildings have adequate ventilation for their intended use and occupancy, having 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. (The 5% figure cannot be used to demonstrate compliance where household units and accommodation units only have one external wall with opening windows, such as some apartments, hotels and motels.).

G4/AS1 also provides means of compliance for removing moisture and contaminants from kitchens, bathrooms and laundries using active ventilation.

Security and other passive ventilation issues

Natural ventilation design solutions may create other issues such as:

  • safety and security concerns
  • the ingress of noise and dust from outside entering through openings
  • draughts
  • aesthetic impact.

These issues must be addressed as part of the total design of the building.

Draughts and heat loss

Recent New Zealand research has found that even in new homes that meet Building Code requirements, draughts can reduce temperatures to uncomfortable levels.

The Wellington City Council built an apartment complex, finished in 2015, where the 27 one-bedroom apartments were double-glazed and insulated. Yet the tenants said the units were cold in winter.

The Department of Public Health at the University of Otago in Wellington looked into the problem. It found draughts were to blame. It also found that fixing sealing strips to doors helped to eliminate draughts and make the apartments an average of 1.36°C warmer.

Healthy homes standards

The Government introduced ventilation and anti-draught requirements for rental housing in the healthy homes standards announced in February 2019.

There will have to be openable windows in the living room, dining room, kitchen and bedrooms. Rental property owners will also have to stop any unnecessary gaps or holes in walls, ceilings, windows, floors and doors that cause noticeable draughts. All unused chimneys and fireplaces will have to be blocked.

The healthy homes standards will apply to new tenancies from 1 July 2021 and to all rental homes from 1 July 2024.

 

Update: 29 October 2020