Q: Is it worth pursuing the very best passive performance in a building or is it more practical to do what's easy, then add a big PV system to cover all the remaining energy demand?
A: Passive design is worth pursuing before all else for several reasons.
First, it makes nicer buildings to live or work in — to be naturally comfortable is hard to put a price on, but thousands of home owners know it well, and staff at companies like Lend Lease who work in naturally comfortable buildings like their Sydney headquarters, know it too.
Second, it makes economic sense — running costs are lower with little or no energy input, while the initial investment may be no more than a non-passive building.
Thirdly, the embodied energy cost is lower — using a large PV system to heat and cool a poor building will require not only large sums of money, but the production of many PV panels which are not actually reducing our overall energy consumption (and emissions), just compensating for a bad building.
Doing what's easy in passive design (say 5 star), compared to going all the way (to say 8 star), may require a little or a lot more embodied energy and expenditure, depending upon the site and climate zone, etc. But until the cost of PV comes down dramatically, passive design is still the most cost-effective way of reducing energy consumption for space heating and cooling.
Q: In considering insulated cavity brick construction, the role of the cavity insulation is to disconnect the internal thermal mass from external influence. But does injected foam insulation work better than foil, given that the foam has an R value but does not let the cavity breathe, whereas the foil only works with radiant heat but allows the cavity to breathe freely?
A: This is a vexatious question at present, and the ferocity of the arguments on both sides needs to be hosed down with some clear definitive research in a variety of climate zones. And I suspect that this is key to the issue — the way different climate zones work, in relation to their temperature and humidity variations.
There are two answers, in my view.
The short answer: bulk insulation in cavities should fill the cavity to prevent humidity from entering in the first place (but this must also prevent moisture tracking through the wall). If foil is used, the cavity should be ventilated according to the BCA. We have used both techniques with great success. What should never happen any more is for cavities to remain uninsulated.
The long answer: some of the really high performance injected foam systems coming out of North America have been developed in response to high humidity and extremes of temperatures (both high and low), whereas we typically don't see anything like the extreme sub-zero range that the US mid-west enjoys for five months every year.
We, however, enjoy high levels of radiant heat with highly variable humidity. Compare Wollongong, Colangatta or Geelong with Penrith, Ipswich or Bendigo, then compare with Alice Springs and then Cairns, and finally Darwin — you get a picture of variations in temperature and humidity, with one common problem for all — radiant heat.
We know what the dew point of water vapour is at different temperatures — this is simple physics. What is needed is some research on built examples to see how it manifests itself in different construction systems and climate zones. This has been done in small parts, and most manufacturers have a library of horror photos that demonstrate the folly of not using their product. While these may be based on sound science, the breadth of building types and construction methods and the range of climatic conditions justifies serious local research.
Dick Clarke is principal of Envirotecture, which provides design and consulting services. He is an accredited building designer with 30 years experience, focusing exclusively on ecologically sustainable and culturally appropriate buildings. Clarke is director of sustainability and past president of the Building Designers Association of NSW.
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