I will declare, my Melbourne roof is coated by an advanced heat reflecting, insulating coating. It has helped to make my home a bit cooler in summer while having an unmeasurably small impact on heating energy use in my Melbourne home. It has also extended the life of my rusting 50 year-old steel roofing by more than a decade, and is still going.
I think people are having the wrong debate. The focus of policy and regulation should be on the performance of the roof system, not the colour, and it should factor in both individual and societal benefits. All options have pros and cons. A performance requirement could drive innovation and take advantage of subtle design and installation details. If it is effectively enforced.
A heat reflective roof has been repeatedly shown to reduce urban heat island effect.
Some propose that it can also reflect solar energy back to space instead of reradiating infrared heat that is trapped by greenhouse gases in the atmosphere, adding to global heating. These are local and global impacts not necessarily recognised or valued by individual home designers, builders and occupants.
A heat reflective roof has been repeatedly shown to reduce urban heat island effect.
Also, we should be designing buildings for the next 50 years or more, yet most building modelling relies on historical weather data. It can be interesting to look at the changes in energy values associated with a given building NatHERS rating in a given climate zone since more recent weather data files were introduced.
At an individual level, “cool” surfaces help keep a building cooler and can make a big difference to summer radiant comfort and cooling bills. Heat reflective roofs and walls can help to limit overnight temperatures.
I remember cooking overnight in my bedroom with a west-facing uninsulated double brick wall – and it wasn’t even a very dark colour. Then freezing in winter.
A cooler roof may improve the efficiency of PV (solar photo voltaic) and airconditioners mounted on roofs, as both are more efficient at lower temperatures. I haven’t looked into this in detail, but it seems to make sense.
Some heat reflective coatings claim additional insulating properties due to features such as hollow microspheres and infrared pigments.
Some can be colour-tinted while maintaining reasonable performance. (See this 2010 paper: A Review of Heat-reflective Paints John Pockett and Martin Belusko, Sustainable Energy Centre University of South Australia A Review of Heat-reflective Paints for a useful overview).
But reflective surfaces, including glazing, can cause glare. They may also increase winter heating requirements though, as discussed below, how the colour of a roof (and walls) impacts on the thermal performance of a building can vary widely.
A dark roof is effectively an unglazed solar collector, with much of its area not facing north. In most of Australia, summer sun’s impact on the temperature and energy collection efficiency of a dark roof is much greater than in winter, so it not a very effective solar collector when you need it. And it’s very effective when you don’t want it.
Summer sun is the equivalent of pointing a single bar radiator at each square meter of sun-exposed roof at the hottest time of day. It is a lot of energy. So a surface that reflects instead of absorbing heat does make a difference then.
Installing a reflective “radiant barrier” under a dark roof reduces the radiant heat load on the ceiling insulation and, if the roof is very well ventilated, the cavity temperature will be reduced.
How well do our building energy rating schemes address these multiple issues?
The NatHERS rating uses historical temperature data, not what our new buildings will face over their lives. Relatively high thermostat settings in the regulated mode of the software and other variables seem to under emphasise the impact of summer heat.
Will there be someone home to pull down the light-coloured blinds when the solar radiation on a window exceeds a couple of hundred watts per square metre? Will the radiant heat from nearby paving be factored-in? Will occupants avoid using air-conditioners below the specified temperatures?
A dark roof drives higher local heat island effects and increases the trapping of heat by greenhouse gases
Will all of the real-world shading from adjustable shading, trees and other vegetation, and changes in the microclimate over the next 50 years be considered?
To be fair, NatHERS is pretty good, but not everyone seems to understand its limitations, the underlying assumptions specified by regulators and settings selected by some assessors.
In some climates, installing reflective foil and sufficient bulk insulation under a well-ventilated pitched roof may be more important for a household than a light-coloured roof, though it drives higher summer heat gain.
But a dark roof drives higher local heat island effects and increases the trapping of heat by greenhouse gases. Adjustable shading that limits heat input from surrounding areas while allowing winter sun to enter is very important.
