Some years ago when I was visiting Australia, from time to time I would say not-so-nice things about people who had dark-coloured roofing material in this hot climate. Now that I am living here I am using many times more power heating my house in winter than cooling it in summer, therefore I was starting to think that dark-coloured roofs were the best – until I read an article about how we should paint our roofs white. So can you please enlighten me and others: Is it because I have a stone-walled house here in South Australia?
- Brian Polkinghorne, via email.
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A white roof (or darker one, for that matter) is not an energy bill silver bullet. There’s interplay between the design and materials of the house itself and its suitability to the local climate that results in your home’s heating and cooling needs. Although our country has a reputation for sunshine and surf, the Building Commission of Australia defines eight different climate zones for building considerations.
Unlike places like Northern Europe or Canada, very few of our homes have snow piling up at our doors in winter. We’ve been able to get away with lighter construction, so our homes aren’t typically as well insulated and airtight. But many regions still get cold winters and/or cold nights. Your experience of higher winter bills is not uncommon or unexpected in southern SA.
To make homes more energy efficient, architects and builders incorporate multiple measures, including passive solar design (using free heat from the sun), insulation, thermal mass (the ability of some materials to store and re-release heat) and the placement and efficiency of windows. For a summary of design considerations by climate zone, visit the Your Home website (www.yourhome.gov.au) and click on ‘Design for Climate’.
Stone has a relatively high thermal mass, but is a poor insulator, so your stone walls may let heat escape. I can’t be more specific without seeing your home, but I’d recommend having a qualified home energy auditor assess your house to provide some site-specific advice. Roof colour is only one consideration, and there may be other measures that would make a greater difference.
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New heat-reflective paint technology allows consumers to improve the energy efficiency of their buildings without resorting to white roofs. White coloured roofs are often not permitted by many councils due to glare and aesthetic reasons; and often are not allowed near flight paths. Check out our website for consumer information on this subject and to obtain an assessment from an Accredited GreenPainter with qualifications in sustainability.
I'm a thermal performance assessor and I've found that roof colour actually makes pretty much no difference to the heating or cooling load of a house when the roof is insulated properly, which is really the only way to keep your house comfortable while using as little mechanical temperature control as possible.
By properly insulated roof I mean at least R5 on the ceiling and preferably with a foil covered bubble wrap under the roof itself. Also, preferably don't use recessed downlights, but if you have to, make sure they're encased in little boxes that allow the insulation to be pushed right up against them.
I'm talking mostly about ratings done in Melbourne though, which requires less heating than Adelaide. But I did a rating in the Pilbara and found that insulation was the most important factor in keeping a house cool.
On a larger scale though, light coloured and light weight (metal) roofs may help to mitigate the urban heat island effect. Metal roofs can also be totally recycled at the end of their life, unlike heat trapping tiles.