If you thought last week's column demolishing the idea of cross ventilation was confounding, then stop reading now, because this column is about the failure of a centrepiece of green design: passive solar heating. Peak gold-plated hyper-heresy.
Free winter heating from the sun is a fundamental tenet of solar houses, a vitally strong design movement in Australia since the 1960’s. There were plenty of books on “Solar Houses” about 50 years ago. But it's a classic example of fine in theory; failure in practice.
The idea is simple: take a well-insulated house with air tightness, position the glazing approximately north to pick as much winter sun as possible, store the warmth in thermal mass, then cover the windows at night to prevent heat loss. The warmth radiated into the house is trapped by the greenhouse effect, and the house remains warm throughout the night.
Simple really, but we can pick apart that theory with a failure at every step.
Firstly, our standard houses are not well-insulated. The worst by world standards actually. The controversial ‘pink-bat’ program of 2009 was intended to address the shocking statistic that one third of Australian homes had no roof insulation. And this is the most fundamental requirement to make an ‘Esky’-like house. Wall and floor installation are not mandatory in Australia, despite it being required in almost every temperate city location in the Western world. As discussed last week, our flimsy ‘veneereal’ construction has air change rates 20 times higher than they need to be.
The first rule of a passive solar house is to ensure that the envelope is extremely well insulated, otherwise every step on the path after that will be in vain, and passive solar will never work. Poor standards mean converting an existing house is usually frustrated, and a passive solar house requires starting from scratch, with a more expensive specification.
The second step is to orient the living and bedroom glazing to 15 degrees either side of North. Given the indiscriminate shaping of house sites in subdivisions this is not always possible, further exacerbated by street location, block size and slope, and trees planted to provide an attractive landscape that will eventually block the sun.
Third step, the glazing in North-oriented windows should be double or triple glazed with a low E coating, which allows short-wave solar radiation to penetrate, but prevents longer wave energy being re-radiated, so it is trapped inside, enhancing the greenhouse.
All that glazing has to be in a well-insulated frame. Standard Aussie lightweight aluminium frames won't cut it, being ‘super-conductors’. Timber frames (sometimes with a vinyl protective layer), or a split aluminium frame with a thermal break (between outside and inside frames) are better at reducing heat loss and crucially, condensation. All of which is of a higher quality, and thus far more expensive. Given that ‘cheapness at all costs’ underpins Australian housing these windows are rarely installed in anything other than a highly customized house.
That glass needs to be shaded: and given our stronger, longer summers an overhang of 450 to 600 is totally inadequate. More like a metre, which requires purposeful design that again adds costs. Maybe a per-go-la in Sydney or a per’gla in Melbourne. Deciduous trees are of great use here – they lose their leaves to allow winter sun in but regrow foliage to keep sun out in summer; but they are sometimes banned by greenie nationalistic councils in favour of indigenous species.
Fourthly, thermal mass: heavy weight materials like brick and concrete that store energy. Three characteristics are critical: it has to be ‘isolated’ by insulation (the ‘R’ or resistance level), it has to have weight (the ‘T’ value) and absorb heat quickly (the ‘Y’ value). Mass will store heat only in the top few centimetres of the area of the room where the sun falls, but only slowly. It takes ages for it to infiltrate into the body of the thermal mass, so the room will severely, and uncomfortably, overheat unless it has more mass on all surfaces: floor, walls, and even the ceiling. Which again rarely happens without conscious thought and an agreeable client (with deeper pockets).
Another issue that thwarts absorption of heat deep into the thermal mass are floor coverings and furniture. And a client’s willingness to have them fade as they prevent solar radiation from reaching the floor (and mass). Unlike the myth of daylight saving, it really does fade the curtains and the cruel the furniture, not always an acceptable trade off.
Finally, if the energy is to be stored for later use, then the windows need to be protected from heat loss by conduction and convection (double glazed etc) and radiation (curtains and blinds). Curtains or blinds should open for six to seven hours (9.30 am to 3.30 pm) and closed for the balance. But who does that? No longer a dutiful wife at home in some parody of 50’s domestic bliss, rather the blinds could now be automated with a slave motor hooked into the internet of things.
All in all, passive solar heating is a great idea, but is also difficult, its costs are high, rewards small, and operation is only possible with some exerted level of green fanaticism and disposable furniture. Unless the sunshine coming in is greater than the heat loss, and it is effectively stored, all that design energy is for nought.
Alternatively, we can now put photovoltaics on the roof, at close to 20 percent efficiency, which can run an air conditioning system at 400 percent efficiency, making free heating (and cooling) on demand without all the operational downsides of passive solar. So, passive solar (opposed to a little north sun) will remain niche, well away from the mainstream.
As Samuel Johnson once said (on other matters): “It is not done well; but you are surprised to find it done at all”. On the other hand, perhaps I should cut my curmudgeonly comments and follow George Bernard Shaw’s advice: “People who say it cannot be done, should not interrupt those who are doing it”.
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].