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.
This is a much better and balanced article on the subject than the other article written by SEBASTIAN PFAUTSCH, WESTERN SYDNEY UNIVERSITY AND RICCARDO PAOLINI, UNSW SYDNEY.
However, it still refers to ‘heat reflective coatings [that] claim additional insulating properties due to features such as hollow microspheres’. These products have been proven to be well and truly bogus. A study entitled ‘Review of Heat-reflective Paints’ by John Pockett and Martin Belusko from the Sustainable Energy Centre at the University of South Australia in 2010 found that:
Much of the advertising for paints with hollow microspheres incorrectly state or imply that the paints function as an insulating layer (Hy-Tech, viewed 20/10/2010) with the hollow microspheres acting as ‘thermos bottle’-like cells (Hy-Tech, viewed 21/10/2010) with empty interstitial volumes. A similar idea is found in cartoons for INSULADD (INSULADD, viewed 20/10/2010). Paint layers containing more than 40 percent total volume of pigments and microspheres would have insufficient cohesion to be a viable layer against abrasion and would not provide corrosion protection. The thermal conductivity through a thin membrane made of microspheres, pigment particles and paint medium will be almost identical to that of a similar thickness of a layer without the microspheres. Even high quality thermal insulation batts which are almost entirely made of void require 50 to 150 mm thickness to achieve good thermal insulation. The heat reflecting paint layers have only a small proportion of voids and are usually well under 1 mm in thickness. It is not possible for a properly formulated heat reflective paint containing hollow microspheres and forming a cohesive, integral layer of less than 1 mm thick to act as an effective thermal insulator. Despite advertising material to the contrary, thermal conduction plays an insignificant role compared with the importance of Total Solar Reflectance and emissivity in minimising temperature build-up. . . .
Advertising of heat-reflective paints often refers to insulation properties; however, restricting heat flow through paint films is not a significant part of the benefits of heat reflective paints. The predominant factor reducing solar heat load on buildings etc. is the Total Solar Reflectance of the exterior surface and not thermal conductivity, but emissivity does play a small role. There is a lack of technical accuracy in much advertising material.The most cost effective solution for non-critical applications is using two coats, for infra-red opacity, of a conventional gloss white acrylic paint over an appropriate primer to reflect heat and reduce the heat load from sunlight. Gloss paint is recommended to minimise dust and dirt collection. It is preferable to spray the gloss paint wet to attain the smoothest surface possible that will not quickly capture dirt and dust.If the application is critical, such as coatings over insulation around piping carrying refrigerated fluids, or in extremely hot climates, then optimal solar heat reflection can be attained with a heat-reflective version of a pure white paint.If architectural or aesthetic reasons eliminate the use of a pure white paint because of the glare, then as light as possible coloured paint should be used. The paint should be heat-reflective to ensure that as much infra-red as possible is reflected at the surface and reduce the heat load on buildings and pipework”
I agree with much of what Daniel has said. No matter how high the insulation value of a couple of millimetres of insulation is, it is not big. But when it is compared to the almost non-existent insulation properties of unpainted steel decking or dark ceramic tiles with little or no insulation or the excellent heat absorption properties of dark steel colours, it can be significant! It’s all relative, which is why I flagged the significance of a reflective ‘radiant’ barrier under a dark roof. It’s also why I did not expect a big improvement in performance when I had my well insulated roof with a foil barrier roof coated – though I did note a recognisable summer benefit. For me, delaying the need to replace my roof was a big factor.
At the same time, even with reflective foil, if the ventilation rate in the roof cavity is not high enough, the temperature and radiant heat driving the heat flow through the bulk ceiling insulation may still be high, as I pointed out in my article.
In Melbourne I have a white/reflective coating on my roof and I think it delivers net benefits, despite my reflective foil and good insulation. And the NatHERS rating.
A lot of the advanced roofing solutions are being advocated in situations where buildings have little or no insulation and unpainted or dark roofs and no reflective foil. Under these circumstances a combination of a white/light colour roof and even a small amount of insulation makes a big difference. And the societal benefits of reduced local heat island effects and global heating are probably more important than the impacts on individual homes and buildings, but they are not properly evaluated.