Australia had a justifiably great reputation for passive solar houses in the 1960’s and 70’s. Most examples, including four ‘demonstration houses’ called ‘Solar One’, were freestanding single-family homes on large sites, and each demonstrated well-researched principles, which became shibboleths of motherhood-like statements.
But when those principles were later built into regulations for higher density dwellings, they were inappropriate at best, mostly counterproductive. Examples of badly applied ideas include cross ventilation, thermal mass, passive winter sun and NatHERS regulations. Each deserves a closer look, as they have become myths that mostly go unchallenged, creating corrupted design solutions and unintended consequences. We will start with that first issue: ventilation.
There are 3 quite different kinds of ventilation: fresh, cross, and ‘coolth’. Whilst cross ventilation gets the most attention, and is held, quite correctly, to be a useful way to assist human thermal comfort, it fails spectacularly in apartments, as does fresh air ventilation, but ‘coolth’ is better and more useful, but little used. So, let’s review all three.
Fresh air ventilation
This is the supply of fresh (i.e. oxygenated) air to a room to replace the stale or ‘stuffy’ air (with excess CO or CO2) or to removes odours. This was essential when gas was the principal form of lighting and heating in the 19th century, and permanently open vents were required by building regulations, which are now redundant but often not closed up. The 20th century building codes followed British codes requiring openable windows with a minimum area of 5 percent of the floor area for all habitable rooms, despite the differing climate.
They needn’t have bothered; most Australian building quality is so poor that there are air leaks galore around doors, windows and even through floors and walls. On the other hand, the recent local rise of the European ‘Passivhaus’, has standards that so tightly seal up a dwelling that the air intake can be closely controlled, and the air leaving (whether it’s warm or cool) can be heat exchanged with fresh air coming in to reduce energy loss.
The difference in approach is dramatic: typical project homes leak 12-14 air changes per hour (AC/hr), whereas Passivhaus is based on 0.6 AC/hr, which is calculated to keep people alert and happy. Is there anything else in building design that has a 20-fold performance difference?
Typically our houses use only windows and doors for fresh air (mechanical ventilation being used only in bathrooms and kitchens). Hopefully the openings are sized for 10 percent light and 5 percent ventilation, although there is nothing to show how these standards were derived beyond some thumb ruling, or there is no empirical evidence that these standards are in any way related to orientation, climate variations, wind speeds or building height.
But in a classic case of lazy and bad building standards we apply those same dumb rules to apartments, even though wind speeds at height can be double to quadruple those for houses at ground level.
Apartments are often better served with a form of Passivhaus: close the doors and windows (keeping out the noisy neighbours, road and planes) and have a controlled flow of fresh air through purpose designed vents. These could be natural wall vents, such as ‘SilenceAir’, a clever box of acoustic attenuation that can be opened or closed; or vertical ducts from the apartment to the roof, driven by buoyancy and the Bernoulli effect; or mechanical venting such as an AC unit with fan use only.
But Council approvals defer to the Building Code of Australia (BCA) and don’t allow for flexibility without a massive cost imposition for specialist consultants. On the contrary, the codes should promote air tightness at the same time penalising the lazy leaky units that abound, with no effective ventilation control.
Cross Ventilation
This ventilation method is intended for better occupant thermal comfort in summer by promoting evaporative cooling. This works by passing volumes of external air, even though its often warm, over the occupants in a room to create a cooling effect by evaporating moisture from the skin.
As the air takes up the moisture the skin becomes cooler (from the latent heat of vaporisation), giving a sense of cooling. But the air becomes more humid and has to be exhausted from the room by keeping the airflow going to remove the humidity. This airflow can be created both naturally and mechanically.
For natural air flow there needs to be two differently sized openings on opposite sides of the room to create the 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. It also works well for widely spaced detached homes, where urban or background noise is low and open windows are not an acoustic privacy issue. Think the ‘Queenslander’ on generous quarter acre blocks from Brisbane to FNQ.
In denser, urban areas the wind paths may be blocked, or redirected, by surrounding buildings, and landscape, and cross ventilation becomes less reliable. Moreover, the acoustic concerns become greater as houses get closer. Townhouse #3 doesn’t want to open the balcony doors to hear ‘Game of Thrones’ at top volume in Townhouse #1, or the impending divorce in Townhouse #5.
And cross ventilation works less well in temperate climates, where wind speeds and direction are more variable, (the wind direction on summer afternoons in Sydney can be anywhere from north-east to south) and perversely the hottest times have the stillest winds (again, Sydney away from the coast where most houses are). Providing openings to meet all those varying conditions is rarely practical.
As an aside, another form of cross ventilation is ‘mechanical evaporative cooling’, hugely successful in hot dry climates such as Perth and Adelaide and all inland towns. A fan mounted in a ‘cooler’ on the house roof draws air over water dripping through fabric, raising its low humidity and dropping its temperature, and cooling the occupants. It’s the Coolgardie safe system, and the hessian bag on the front of the Kingswood ute.
But back to natural cross ventilation for evaporative cooling. The problems of designing openings for variable wind speeds and directions, and acoustic privacy, are grossly magnified in apartments. Orientations are constrained, flow paths through the units are not easy and balconies next to each other intrude on aural privacy.
Nevertheless, the NSW Apartment Design Guide (ADG) requires 60 percent of all units in a building to have ‘cross ventilation’ (even if it is poorly defined). Why 60% has never been explained – if it’s that important why not 80%? Or 100%. Again, it’s arbitrary rule making that makes no sense.
So, we should forget natural cross ventilation in apartments, and adopt a simple mechanical means: a ceiling fan. They have an extremely low energy demand, are controllable, far more reliable and effective. The sensible solution is to make ceiling fans mandatory in every habitable room in inner urban areas of our cities. And they work when there is no wind.
But again, the codes defeat good design: in NSW the Basix code for dwelling sustainability is intended to supplant all other energy, water and thermal requirements, and rewards ceiling fans; but the ADG recognises only ‘cross ventilation’ (which is a form of thermal comfort) and does not allow a credit for ceiling fans. This creates an illogical contradiction that the courts have been unwilling to address.
The unnecessary and unyielding demands in the ADG have led designers to develop some extraordinary contortions of form, which will be seen from the future as formulaic cookie-cutter ‘twenty-teens flats’. A $200 ceiling fan works better. And it can be in 100% of habitable rooms and balconies in 100% of apartments.
Coolth Ventilation
This form of ventilation cools the building, and thereby the occupants, rather than the occupants directly. It works in summer by using cooler outside night-time air to replace the warmer internal air that builds up during the day, thus cooling the space, and thereby the occupants.
It is dependent on 3 factors: access to cool external air at night; cross ventilating that cool air through the dwelling; and ‘thermal mass’ inside to store the ‘coolth’ (the opposite of warmth).
Firstly, cool external air at night in summer is a characteristic of temperate coastal climates where the diurnal range of temperature (from day to night) is considerable (>20o) and occurs regularly in summer. For instance, a Sydney day of 35o+ is usually followed by a night-time temperature of less than 18o. The difference allows the 30o+ temperature built up in the dwelling, to be cooled to 18o.
Secondly, the cooler air must be drawn in from late evening to early morning, a time often frustrated by the paucity and unpredictability of breezes, so some form of mechanical assistance is usually required to cross ventilate.
Thirdly ‘thermal mass’ (heavyweight materials) is required internally, holding ‘coolth’ in the same way it holds warmth from the winter sun. ‘Passive heating’ relies on heat rising from the floor; by contrast passive cooling’ works best when the thermal mass is in the ceiling. 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 cross ventilation that will not disturb the occupants (by blowing over furnishings, paper, plants etc).
This is the best form of cooling: our perception of thermal comfort is derived more from radiant sources, than from convective; the temperature of the surrounding surfaces is more important than the air temperature (see Thermal Comfort, P.O. Fanger, 1974). What we need is ‘building conditioning’ not ‘air conditioning’.
Most modern houses are not suitable for ‘coolth’; they are poorly insulated and lightweight internally (suffering from brick ‘veneereal’ disease), with no ceiling mass. By contrast apartments are naturally better suited to this form of cooling: structurally they require heavier mass: concrete floors that can have exposed ceilings below, with brick or concrete walls (and columns). The large area of common walls in units reduces heat loss and gain – rendering the biggest issue one of cooling not heating.
The optimum way to ensure ‘ventilation for coolth’ in apartments is by using a small fan pulling air from high level windows, such as fanlights, or vents on one side to the exterior on the other side of the dwelling. Where no opposite exterior wall is available the air can be drawn through an acoustic lined duct through corridors to the exterior.
This form of ventilation can deliver significant energy savings because it reduces the internal temperature of the space, through the ‘coolth’ stored in the thermal mass. The apartment will be cooler for longer before the occupants feel the need to use AC, if at all. The energy saved is many times greater than the energy for the mechanical fan, which is quite small given the low volumes of air to be moved over several hours.
Reducing AC use is critical to sustainable systems because it is the single most power-hungry appliance in a house (with high costs to occupants). Designing for ‘coolth’ is the most desirable form of ventilation for energy efficiency and sustainability. Yet it is not promoted or required by any building or apartment or sustainability code.
Rather, our design and building codes rely on transferring outdated and unworkable solutions from house codes to apartments, with no recognition of how different apartments are as a typology. Apartments are not a ‘stack of houses’. They are a completely different animal, which cannot be fed on an old diet.
Tone Wheeler is principal architect at Environa Studio, Adjunct Professor at UNSW and is President of the Australian Architecture Association. The views expressed here are solely those of the author and are not held or endorsed by A+D, the AAA or UNSW. Comments can be addressed to [email protected].
