Passive Design

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

Design of passive ventilation

A passive ventilation system should be design to achieve air flow rates that are sufficient to remove pollutants and are comfortable for occupants.

On this page:

  • Factors affecting air flow rates
  • Calculating air flow rate
  • Designing ventilation features

The optimum air flow rate will depend on temperature and humidity. The higher the temperature and humidity, the more air flow is needed to maintain comfortable temperatures.

Factors affecting air flow rates

Air flow rates depend on:

  • prevailing wind direction
  • average wind speeds (as a guide, use half the average seasonal speed, as wind speeds rarely fall below this value)
  • how the site is influenced by daily and seasonal variations in wind such as onshore/offshore winds and how these may change during the day
  • building form – whether it enhances or restricts air flow
  • surrounding landforms and planting – will they obstruct air flow
  • orientation and position of windows, doors, roof ventilators, skylights and vent shafts
  • surface pressure coefficients around the building.

Calculating air flow rate
The air flow rate through a ventilation inlet opening forced by wind can be calculated using the formula Q = Cv x A x v where:
Q     = air flow rate (m3/s)
Cv     = effectiveness of the openings (assumed to be 0.5–0.6 for perpendicular winds and 0.25–0.36 for diagonal winds)
A     = free area of inlet openings (m2)
v     = wind velocity (m/s)

Ventilating features
When designing a natural ventilation system, the long façade of the building should be facing the prevailing wind direction, with doors and opening windows providing the ventilation openings.

Ensure that openings (inlet and outlet) are:

  • not obstructed
  • the same size
  • able to control the flow
  • located in opposing pressure zones to increase the potential air flow.

Awning windows have a relatively small opening area and are therefore less effective as ventilators.

Other ventilating features include:

  • maintaining a vertical distance between two openings to create a stack effect, i.e. hot air rising and thereby enhancing air flow
  • shafts to promote air flow
  • maximising air flow by designing open plan spaces
  • maximising air flow by having openings at different levels or near the ceiling on opposite sides of the space
  • using architectural and landscape features to direct and control air flow - for example, using casement sashes on the windward façade as these can be more efficient than other types of sasheshttp://www.environmentdesignguide.net.au/media/TEC02.pdf, and including opening windows on the leeward face

In tropical parts of Australian,it is common practice to install louvre windows to provide good air flow control as well as keeping rain out. Louvres are also fairly common in warmer, more humid parts of New Zealand.