There are three, quite different forms of ‘ventilation’, often confused by designers, so let’s first sort out those types.

Ventilation for fresh air

This is the supply of fresh air (with oxygen) through a window in a room to replace the stale or ‘stuffy’ air (which has excess CO₂). The NCC/Building Code of Australia (BCA) Vols 1 + 2 requires an opening with a minimum area of 5% of the floor area for all habitable rooms, almost always a window to the outside.

Where an operable window is not possible (e.g., for noise reasons), mechanical ventilation supplies fresh air. In earlier times, using gas lighting and heating, people were not trusted to open windows (in cold weather), so it was mandatory to install wall ventilators in rooms, which are now redundant.

Nowhere in codes for fresh air is there any reference to ‘air flow paths’, or ideas such as two windows giving ‘cross ventilation’, although this would obviously help.

Ventilation for adiabatic or evaporative cooling

In this ventilation method, external air (often warm) passes over occupants in a room, evaporating moisture from the skin to create a cooling effect. As the air absorbs moisture, the skin cools (latent heat of vaporisation), and the air becomes more humid. This creates a cooling sensation on the skin and in the air, but the air must be exhausted from the room to maintain airflow and remove humidity.

This airflow can be created both naturally (cross ventilation) and mechanically (ceiling or wall fans).

For natural airflow, you need two differently sized openings on opposite sides of the room to create a pressure differential. One opening, preferably the larger one, needs to be facing a breeze and the other as far away as possible to create the ‘breeze path’.

This form of cross ventilation works well in the humid tropics, where wind flows are more predictable, detached homes are more common, and urban background noise is low, so open windows don't compromise acoustic privacy.

It works less well in temperate climates, where two issues arise: wind directions are far more variable, so providing openings to meet those unpredictable conditions is often impractical; and in denser urban areas, surrounding buildings may block or redirect wind paths, and acoustic concerns are greater, particularly in apartments.

Where natural cross-ventilation is not possible or desirable, mechanical means such as ceiling or standing fans, or fan-driven ventilation, are more effective and reliable and don’t require large openings that can infringe aural privacy.

Whilst some codes, such as the calculations in NatHERS or Basix in NSW, encourage the use of ceiling fans as an energy-saving device as a supplement to air conditioning (AC), not necessarily as a replacement for it.

But others, notably the NSW Apartment Design Guide (ADG), invoke the need for ‘natural cross ventilation’ in at least 60% of apartments. Ridiculous contortions in external forms result. A few well-placed ceiling fans can achieve a better level of thermal comfort, with better acoustic amenity for occupants (without hearing the neighbours through the nearby windows), and at a lower capital cost.

Ventilation for ‘coolth’

Even better than natural or mechanical ventilation is designing for ‘coolth’. Essentially, this uses the night’s cooler external air to replace warmer internal air in summer, thus cooling the space and the occupants. It is the opposite of passive design for ‘warmth’.

Coolth ventilation is dependent on three factors: having access to cool external air, having ventilation across the space, and having ‘thermal mass’ inside the building to store the ‘coolth’.

The first factor occurs in temperate climates such as coastal cities where the diurnal range of temperature (from day to night) is considerable (>15°) and occurs regularly in summer. For instance, a day of 35°C+ heat is usually followed by a night of temperatures of less than 18°C, and the difference in temperature allows the heat in a dwelling, which may be > 30°C, to be cooled to, or below, 18°C.

For the second factor to work, the cooler air must be drawn in from late evening to early morning (preferably midnight to 5am). This is often frustrated by the paucity and unpredictability of breezes at this time, and so some form of mechanical assistance is usually required to cross ventilate.

The third factor relies on the ‘thermal mass’ (heavyweight materials) of the building holding the ‘coolth’ in the same way that the mass holds warmth from the winter sun. Unlike ‘passive heating’, which relies on heat rising from the floor, ‘passive cooling’ works best when the thermal mass is in walls, and particularly ceilings.

The heat of the day can rise up to be stored in the overhead mass before being drawn out of the dwelling by high-level ventilation (natural or mechanical) that will not disturb the occupants (blowing over furnishings, paper, plants, etc.).

Our perception of thermal comfort is derived more from radiant sources than from convective (see P.O. Fanger on Thermal Comfort, 1974). Put another way, surrounding surface temperature is as important as, if not more important than, air temperature. We need ‘building conditioning', not ‘air conditioning’.

Given these three factors, the best way to ensure ‘ventilation for coolth’ is to use a small fan on a thermostat/timer to pull air from high-level openings, such as fanlights, from the exterior on one side to the other side of the dwelling. The fan can be fitted with an acoustically lined duct to limit fan noise inside and out.

This form of ‘cross ventilation’ delivers far greater energy savings than other methods because it reduces the space's internal temperature through the ‘coolth’ stored in the thermal mass. The dwelling or apartment will stay cooler longer before occupants feel the need to use air conditioning (AC), if at all.

The energy saved is many times greater than the energy required to run the mechanical fan, given the low volumes of air moved over several hours. Reducing AC use is critical to sustainability, because AC is highly energy-consuming, leading to overloads and brown/blackouts.

Sadly, NatHERS notoriously underrates thermal mass (warmth and coolth), an ongoing issue for the last twenty years, so designers and industry have had little incentive to pursue this form of energy saving. Nevertheless, designing for ‘coolth’ is most desirable and will always outperform ‘cross ventilation’ by itself.

In summary: natural and cross ventilation are not interchangeable; it’s time the ADG gave up the confusion and its demand for something without scientific value or benefit. That fact alone makes me cross that such stupidity has lasted so long.

Cross-Sections are Friday musings from Tone Wheeler, formerly of Tone on Tuesday. Amateurs draw plans; professionals draw sections – never at cross purposes.

Tone Wheeler

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