Last week's column on passive solar heating elicited a number of critical comments, mostly pointing out that passive solar homes work really well. I wholeheartedly agree and said so. That wasn’t my issue, the question I was trying to answer was “if passive solar homes are a such great idea why are so few built?”

By my count less than 1000 of the 203,000 dwellings built in 2018 were 10-star passive solar design. And most of these are concentrated in the two coldest climates of Canberra and Tasmania, where my critics come from. So, I am having another go at exploring the issue.

My focus is making passive design universal; the default idea is to improve the standard of all vernacular project homes, rather than having to engage an architect to overcome the resistance to passive ideas, to design to the particularities that should be universal, and of course to wrangle the costs.

Why passive solar homes? Because traditionally thermal comfort is the largest single user of energy in a home and passive design can make you thermally comfort for the lowest amount of energy. And if that energy is carbon based then it = CO2  and is a key driver of climate change. Ultimately we want all homes to be climate adapted or zero carbon homes, and passive design can help get us there.

Understanding thermal comfort is key to unlocking why passive design has had so little impact in Australia. Thermal comfort is quite tricky; it is highly variable and standards for thermal comfort are hard to define.  

The seminal standard reference text, Thermal Comfort, by P. O. Fanger (1970) analysed the many issues including: location, season, cultural expectation, physical perception of heat, relationship between temperature and humidity, layers of clothing (which he described on a scale of 1-4 ‘CLO’), and so on.

There are two key lessons from this book. Firstly, thermal comfort is individual. No two people can agree on the ideal combination of temperature, humidity and airflow. Secondly, and crucially, irrespective of this personal preference, by and large we respond more to radiant temperature than to convective or conductive. That is, we respond more to the surface temperature around us more than the air temperature.

The first observation was well known: the wide variety of thermal comfort settings makes it difficult to set ideal exact standards, so mechanical engineers express it in a range called the PMV or Predicted Mean Vote. For instance, in a room with 10 people, on average one person will feel it is too cold and one person will feel it is too hot. Some prefer a dry atmosphere, with relative humidity (RH) at 40 percent to 45 percent, others say they are just as comfortable with the RH at 60 percent. So, we are always dealing with averages.

The second part, recognising the importance of the radiant temperature, was radical heresy in the middle of the air-conditioned century. We knew that we had made buildings for two thousand years to keep us comfortable by ‘passive means’, but the arrival of air conditioning (AC) seemingly removed that memory.

Fanger was reminding us of the way those buildings worked, from the Alhambra in hot dry Spain, to earth covered houses in frozen Norway: they stored radiant energy, as warmth or coolth, to keep us comfortable. By contrast heating and cooling the air was a quick fix that led to stuffiness and static friction in winter, and higher humidity and energy waste in summer.

He gave us scientific confirmation that we need building conditioning, not air conditioning.

Passive solar design plays a key role in building conditioning; essentially it is a way of gathering, storing and releasing radiant heat in the building fabric rather than the air. We need to have the most efficient house possible in which to ‘trade the sun’s warmth’ or the ‘summer breezes’ to drive thermal comfort. This is the equation from last week:

Passive design = Insulation + Orientation + Openings + Thermal Mass + Operation

Put another way: ideal passive design for thermal comfort starts with isolating the interior from the exterior to avoid weather extremes; then orient the key glazing to north; then design those openings for heat loss (glazing) and heat gain (shading); then add warmth or coolth into thermal mass so it can have the maximum effect and lasts as long as possible, and then find ways to get that warmth from the sun and coolth from night time breezes.

Passive solar design is well understood in architectural physics and there is no disputing that can work well in practice. But it is hardly ever adopted in our standard homes. There are three issues here: the standard of house construction, the difficulties of subdivisions, and problems of regulations.

Project homes are all wrong for passive design. Poorly insulated, thermal mass of brick veneer on the outside not the inside, poor quality glazing, indiscriminately oriented, never properly shaded, and no consideration to operation for ‘free energy’. Consumers say they want ‘green’, but their preference in purchases is to adopt air conditioning to meet the PMV variations.

And they may also have a poor view of passive design’s internal problems: the impact of the sun on furniture and the interior of the house. Whilst it is good for humans, it is not so good for possessions and building fabric. This is one of many other considerations in housing that come before thermal comfort; passive design doesn't translate into sales.

Our subdivisions are not conducive to site layouts that orient the private areas of the house to North. Slopes, topography and landscape can prevent solar access to the house, particularly with trees beyond the control of the owner or other features such as adjacent buildings. These can all be overcome, but it needs design commitment, which is so lacking in vernacular houses.

Finally, thermal comfort in housing is regulated in much of Australia through NatHERS, the National Housing Rating Scheme, a computer program that is both lauded and vilified. It’s failure to ensure standards are maintained in Australian housing will be the subject of next week's column.

Suffice it to say that passive design for thermal comfort is the greatest idea that is rarely used in Australia. We continue to have the world's worst climate adapted houses.

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].