Passive Design

Ventilation is needed to bring fresh air into a home and to remove stale, moist or polluted air. It may also be needed for cooling.

Passive ventilation is the supply and removal of this air through openings in the building’s envelope using natural wind flow and temperature differences around the building.

Passive ventilation can be:

• controllable air movement through openings such as windows, doors from wind pressures and/or indoor-outdoor temperature differences
• uncontrollable air flow through unintentional openings in the building envelope (infiltration) resulting from wind and temperature generated pressure differences across the building envelope.

Passive and active ventilation

Passive ventilation is the least expensive and most environmentally friendly way to ventilate a home. It is suitable for most New Zealand locations. However, in some situations passive ventilation on its own will not be enough:

• wet areas such as bathrooms will need extract systems to remove moist air (see active ventilation)
• airtight homes may need a supply system to bring in heated, drier outside air (see active ventilation).

Recommended ventilation rates

The ventilation rate in a room can be given by the number of air changes per hour (ac/h). The size and height of rooms can vary, so to get a consistent measure the ventilation rate can also be expressed in terms of litre per second (l/s) but this may not take account of the volume of the room being ventilated.

The recommended ventilation rate for houses is given in the Acceptable Solution to NZBC Clause G4. For spaces with natural ventilation only, an occupied space requires a net openable area of windows (or other openings) that is no less than 5% of the floor area. For spaces with a mechanical ventilation system, the rate is given in NZS 4303 for supply systems and AS/NZS 1668.2 for extract systems. Recommended ventilation rates for domestic buildings are:

• living areas (living room, bedrooms, dining room) – 0.35 ac/h but not less than 7.5 l/s/person
• kitchens – 50 l/s intermittent (or 12 l/s continuous or with openable windows)
• baths and toilets – 25 l/s intermittent (or 10 l/s continuous or with openable windows)
• laundries – 20 l/s intermittent
• spa pools – 5 l/s/m² of floor area
• garages – 50 l/s/car.

Design of natural ventilation

The design and performance of passive ventilation system is difficult to quantify. Ventilation rates achieved will depend on:

• prevailing wind direction
• average wind speeds – as a guide, use half the average seasonal speed as analysis has shown that wind speed rarely falls below this value
• seasonal and daily variations in wind speed and direction – e.g. is the site influenced by onshore or offshore winds, and how does this change during the day?
• building form – does it enhance or restrict airflow?
• surrounding landforms and planting – do they obstruct air flows?
• orientations and positions of windows, doors, roof ventilators, skylights, vent shafts
• surface pressure coefficients around the building.

The air flow rate through ventilation inlet opening forced by wind can be calculated using: Q = Cv x A x v

where

Q = air flow rate (m³/s)
A = free area of inlet openings (m²)
v = wind velocity (m/s)
Cv = effectiveness of the openings (assumed to be 0.5 to 0.6 for perpendicular winds and 0.25 to 0.36 for diagonal winds)

When designing a natural ventilation system, ensure that:

• the long façade of the building and the door and window openings are oriented to the prevailing wind direction
• inlets and outlets are not obstructed by nearby objects
• inlet and outlet openings are of equal size
• ventilation openings are able to control the flow
• windows and other openings are located in opposing pressure zones to increase the potential air flow
• a vertical distance is created between opposing openings to create a stack effect (hot air rising) and thus enhance air flow; vertical ventilated shafts within a building can promote air flows
• airflow through the home is maximised by using open planning
• airflow through the occupied areas is maximised by having openings at different levels or near the ceiling on both sides of the space
• architectural and landscape features are used to enhance and redirect airflow – ventilation flow can be enhanced by specifying casement sashes on the windward face with louvre or hopper windows on the leeward face; tall awning windows actually have a small opening area so are considered less effective as ventilator; Australian practice is to recommend louvered windows in tropical areas where breezes are often accompanied by rain.
• for humid areas, air flows are maximised to promote cooling
• mechanical extract exit points are remote from fresh air supply inlets
• outside air is not drawn from an area where contaminants may be present (e.g. not having an inlet point adjacent to the garage door).

Barriers to achieving a natural ventilation design solution have been identified as:

• safety and security concerns
• air contaminants, noise and dust from outside the building entering through the openings
• presence of draughts
• aesthetic impact
• reported problems with the (automatic or manual) controls.

To prevent annoying draughts from passive ventilation a maximum air speed of around 7.5 metres per second is recommended for cooling.

Passive cooling

In warmer and more humid climate areas of New Zealand, the benefits of passive cooling will be maximised by designing homes that: 

• are located and have a form that maximises exposure to cooling breezes
• promote air flow through the building
• have windows that minimise unwanted heat gains while allowing maximum air flow
• are well insulated
• promote summer shading and utilise reflective exterior finishes to minimise heat absorption to minimise heat gain through the envelope
• in areas with small daily temperature ranges, have low thermal mass
• incorporate back-up (active) ventilation
• incorporate evaporative cooling from water features and ponds.

At 50% RH an air flow of 0.5m/sec equates to a 3 degree drop in temperature.