Does roof colour really make a difference?
This aerial photo shows Jenny’s House second from the left, another of our designs (Higgins House) on the far left and then three non-Light House homes to the right. The Light House homes have smaller footprints and much more green space around them (including on the verge). At Light House we take the integration of building and landscape design very seriously.
This is a question that Jenny gets asked a lot. It comes up frequently in the My Efficient Electric Home Facebook group, and came up during Jenny’s recent Sustainable House Day webinar and again during a Guardian article about proposed planning changes in NSW just last week (which triggered some social media discussion and another ABC radio interview for Jenny). So it is time to clear up some misconceptions.
The answer depends a little on where you live.
In a well-designed, and properly insulated home (where there are very few ceiling penetrations or insulation gaps and little opportunity for air leakage) in Canberra, roof colour makes very little difference - in winter or summer. We do light, medium and high solar absorptance roofs. Jenny’s own roof is Colorbond ‘Deep Ocean’ which is classed as dark with a high solar absorptance of 0.75.
In Canberra’s cool temperate climate (climate zone 7 of the NCC, climate zone 24 in NatHERS) which, despite climate change, is still heavily dominated by heating requirements, it is actually ever so slightly beneficial at the house level to have a darker roof. Another thing that people don't think about is that lighter roofs actually get colder in the depths of winter making condensation underneath them more likely.
At the 2020 Bushfire Conference, Dr Mark Dewsbury from the University of Tasmania, showed footage of it raining condensation inside a roof cavity. This particular roof cavity had low solar absorptance (was light in colour) and was always cold, making it a great environment for condensation. You can watch Dr Dewsbury’s presentation here.
Jenny has used her own home extensively as a science experiment, to demonstrate how a house designed for its’ climate and site, and built with standard construction techniques and materials, can survive through the weather extremes in Canberra. Her graphs of internal, external and roof cavity temperatures during a heat wave are impressive and show how little impact a dark coloured roof has on the temperature gauge where it matters - INSIDE her well insulated and airtight home. Here is a link to that post: My Efficient Electric Home (MEEH) | INSULATION and AIRTIGHTNESS and NO ROOF VENTILATION - UPDATE POST HEATWAVE | Facebook
When it comes to the community level ‘heat island’ argument, people get very distracted from the MAJOR problem, which is simply that houses are so big and have so little green space around them. Landscaping around smaller footprint homes will have a much greater impact on suburban temperatures than a change of roof colour.
Jenny worries that people think changing roof colour is going to have a dramatic impact in our suburbs. Only smaller homes and more gardens will do that. People in McMansions, with little landscaping and greenery, should not feel virtuous for having a light coloured roof. They also should not expect their home to feel way more comfortable… if it does, then we strongly suspect the ceiling is not properly insulated or draught sealed.
Of course, in warmer climates (which don't get winter extremes like Canberra), where energy for cooling is the major issue, then a lighter coloured roof will be more worthwhile. Speak to an energy assessor in your area to see what is most suitable in your climate.
To be very clear, we are NOT against light roofs (as mentioned earlier we do all shades). We simply want to encourage people to think more holistically about keeping our homes and suburbs cooler.
On that note we strongly encourage you to check out the suburban micro-forest movement that is gaining momentum in Canberra.