Media | Is the sea of dark roofs raising the heat in Australia’s new suburbs?
In this piece, Rothelowman National Design Principal, Jonothan Cowle, comments on adaptive housing design such as green [living] roof to reduce the urban heat island effect even in mid to high-density developments.
This article was originally published in www.abc.net.au/news written by Gavin McGrath
Treeless streetscapes and higher-density housing contribute to warmer ambient temperatures on hot days.(Supplied)
A sprawling population and the popularity of black or near-black roofing is creating urban heat islands across Australia’s cities and regional centres, an energy scientist says.
While darker roofing has a role in raising the energy efficiency rating of a home, the collective mass is enough to turn a 40 degree Celsius summer’s day into a neighbourhood 45C swelter.
Architectural scientist Mahsan Sadeghi from the CSIRO’s energy business unit says high-density housing is amplifying adverse climatic effects on a city-wide scale.
“The prevalence of dark construction materials has a substantial impact on the ambient temperature within these urban environments,” Dr Sadeghi says.
Some new subdivisions have a strict roofing colour palette.(Supplied)
“Dark roofs, which are often chosen for their aesthetic appeal, tend to absorb and retain heat, exacerbating the heat island effect, while high-density housing reduces green spaces and limits natural cooling.”
Dr Sadeghi says that in contrast, cool roofs can decrease the peak ambient temperature in parts of our cities from between 1.1C to 1.4C.
And in higher-density locations the effect is magnified.
“Research shows that cool roofs decrease in average the surface temperature between 5C to 7C in high-density parts of the cities,” Dr Sadeghi says.
“The reflective materials or light-coloured coatings have the ability to significantly lower ambient temperatures.”
All hail monument
Newer housing developments are typically cheek by jowl in largely treeless streetscapes.
According to one Ballarat builder the darker hues in the Colorbond steel range — monument (black), ironstone (blue-black), woodland grey (dark grey) and basalt (mid-to-dark grey) — are some of the most popular roof colours for new homes in Victoria.
In Queensland it is monument, dark grey, charcoal and woodland grey.
New homes are synonymous with darker roofs.(Supplied: Rodney Owen)
Haymes Paint colour consultant manager Erin Hearns says monument has been Australia’s favourite roofing colour for at least a decade.
“It’s definitely a trend. Monument is popular because it looks good and it goes well with other exterior colours,” Ms Hearns says.
“Some people see it everywhere and think, ‘that’s just what they have to do’.
“It can also be about the environment. Dark colours can blend in with the surrounding vegetation, and some councils insist on [black and dark grey] for that reason.”
Warm versus cool climate
Engineer and energy-efficient housing consultant, Richard Keech, says homes in Victoria earn a higher energy efficiency rating by having a darker roof.
Home energy efficiency is rated using NatHERS (Nationwide House Energy Rating Scheme), an Australian index that calculates how much heating or cooling is needed for a home to remain at a comfortable temperature.
“In a cool climate the dark roof will tend to lessen the heating energy you need. But in a hot climate, unambiguously, a light roof is better,” Mr Keech says.
“If your goal is to lessen the annual energy use, then in a heating-dominated climate, a dark roof isn’t a problem. But if your goal is to be climate safe in a warming climate, a light roof would be better.”
Mr Keech says choosing a roof colour to negate hot weather is a trade off.
“It’s a tricky one because you are balancing the year-round energy needs versus the acute impact of really hot weather,” he says.
“On the other hand, you’ve got that bigger problem on those really hot days, so you’ve got to trade off one to the other.
“You have to make a judgement of whether it’s more important to ease the problems on a really hot day and lose the benefit year-round.
“The NatHERS star rating system does favour darker roofs in a coal-heating dominated climate. In most of Victoria, we need a lot more energy for heating our homes than we do for cooling.”
Monotonous aesthetic
Building Designers Association of Victoria past president Tim Adams suggests that while the NatHERS star rating system is one reason for colour choice, there is another less practical reason.
“It’s a fashion. That’s my big take on it,” Mr Adams says.
“The colour of the roof is just one of the myriad influences on the performance of the house.”
Black and near-black tones are Victoria’s most popular colour choices for new home roofs.(ABC Ballarat: Rhiannon Stevens)
Mr Adams says some developers have a strict design palette, offering only two or three roof colour choices.
“That’s an unfortunate thing, it leads to a monotonous aesthetic. Whereas you can have a much more interesting and vibrant aesthetic if people have more flexibility,” he says.
“In Victoria, we have dramatically different climates from one end of the state to the other.”
Ms Hearns believes the fashion is changing.
“It takes quite a lot of time but there is a shift to lighter colours more recently,” she says.
“Monument may be number one, but shale grey (light grey) is our number two, and surf mist (silver-white) is increasingly popular.”
Jonothan Cowle, principal at architecture firm Rothelowman, says that while roof colour choice is a “massive consideration”, adaptive housing design is also key to reduce the urban heat island effect even in mid to high-density developments.
“You can have a green [living] roof, you can have grass on a roof, or you can create shade by breaking up roof design,” Mr Cowle says.
“A giant flat mass will absorb heat during the day and radiate the heat at night, whereas complex surfaces are much better.”
This article was originally published in www.abc.net.au/news written by Gavin McGrath
