Nov 25, 2014: Think light for a cooler roof

In our last newsletter we talked about the how Passive Solar Design helps to keep the Greeny Flat cool in summer and how we can help this process by operating the windows and blinds correctly.

Another factor in the design of the Greeny Flat that has a major impact on summer cooling is the choice of colour and material for the roof.

Most people understand that darker colours absorb more heat than lighter colours. So, in order to keep a building cool in the summer, it is generally best to use a lighter colour for the roof.

The thermal mass of the roofing material also makes a difference. Basically, the heavier the material the more heat it will absorb from the sun shining on it and the longer it will hold that heat after the sun goes down. A light weight roof material on the other hand will release heat quickly once the sun is gone.

So ‘think light’ when choosing a roofing material to help you stay cool in the summer. Light weight and light colour.

It seems to be the fashion at the moment in Australia to build brick veneer McMansions with very dark tile roofs. It makes me want to laugh and cry at the same time when I see massive subdivisions of huge new homes with dark tile roofs baking in the Aussie summer sun. You know that all of those new homes have air conditioning systems which have to work much, much harder to keep them cool because they have chosen just about the worst possible roofing material.

And not only do those dark tile roofs heat the interior of the homes, they also heat up the entire neighbourhood. This is a well documented phenomenon known as the ‘heat island effect’. Basically, a lot of dark roofs and asphalt paving can cause a whole urban area to be much hotter than its surrounding rural area. What this means is that those air conditioning systems not only have to work harder but they operate less efficiently when the surrounding air temperature is higher which all adds up to a whole lot more fossil fuels being burned in order to overcome the effects of a bad choice of roofing materials. In Australia I believe it should be illegal to use a dark tile roof on a building.

To quote from the Wikipedia article on Urban Heat Islands, ‘Mitigation of the urban heat island effect can be accomplished through the use of green roofs and the use of lighter-colored surfaces in urban areas, which reflect more sunlight and absorb less heat.’ This is why, for the Greeny Flat we chose to use a light coloured (Shale Grey) Colourbond steel roof. Without going into too much technical detail, this colour has a ‘solar absorptance’ (SA) rating of 0.43 which compares very favourably (lower number is better) with galvanised steel which has an SA of between 0.65 and 0.80 (depending on how weathered it is), or a dark grey tile roof which has an SA of 0.96.

The Wikipedia article also states that, Despite concerns raised about its possible contribution to global warming, comparisons between urban and rural areas show that the urban heat island effects have little influence on global mean temperature trends.’  This suggests that the Heat Island Effect is not contributing to global warming. I disagree because, what the article doesn’t consider is the amount of fossil fuels required to be burned in order to keep the inside of buildings cool in urban areas. Buildings with dark roofs have higher air conditioning loads and urban areas with higher temperatures make air conditioning systems less efficient. Plus, all those air conditioning systems actually expel heat to the outside of those buildings which heats up the surrounding area even more. This all adds up to a huge amount of fossil fuels burned globally to keep buildings cool which certainly contributes greatly to global Co2 levels and climate change.

So, for the benefit of the occupants of a building, the surrounding neighbourhood, and the greater environment, think light colour and light weight for a cooler roof.

And, of course, don’t forget that the roof also needs to be insulated well to stop the heat that is absorbed by the roof from making its way into the interior of the building.


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