Some design codes require specific roof and wall colours for energy efficiency. This bulletin explores this issue and suggests possible design standards.
Light roofs for energy efficiency?
In 2021 the New South Wales Government proposed to ban dark roofs on houses. The intent was to reduce the amount of energy used to cool houses and to reduce the urban heat island effect. It was part of a plan to reach net zero by 2050. The Victorian Government is likewise proposing cool roofs for commercial and residential development. Western Australia’s residential design codes suggest dark roofs should be avoided because of their heating effect. They also warn of glare from highly reflective roofs. In future, statutory controls on roof tones and reflectivity are likely all around Australia.
The eight climate zones as used by the Building Code of Australia.
How can an estate’s design code mandate or encourage lot owners to achieve energy efficient roofs and walls? The first step is to determine the climate zone of the estate. The Building Code of Australia can assist here. It divides Australia into eight climate zones.
Over the course of a year, in most climate zones in Australia, a house will require more cooling than heating. In those zones, roofs and walls with light tones are best, as they reflect more of sunlight. However, in the southern parts of the mainland and Tasmania, a house needs more heating over a year than cooling. In those zones, roofs and walls with dark tones are best, as they absorb more sunlight.
A comparison of energy and temperature between a black and a white roof. (Cool roof toolkit)
When we know the estate’s climate zone, we can determine the appropriate reflectivity of roofs and walls. For that, we want to know a material’s solar reflective index (SRI) value. A material’s SRI value is measured twice, once when the material is new and again after 3 years, so it’s had time to weather. To explain the SRI, we’ll need to first explain solar absorbance, solar reflectance and thermal emittance.
Solar absorbance (SA) is the proportion of solar radiation that is absorbed by a material. The measure is expressed as a value between 0 and 1. The higher the value the more solar radiation is absorbed by the material. This increases the temperature under the roof or inside the wall. That’s good when one is trying to heat a house, bad when trying to cool it. Typically, the darker the material, the higher the solar absorbance value.
Colorbond colours, showing their solar absorbance and solar reflective index values.
Solar reflectance (SR) is the proportion of solar radiation that is reflected by a material. This measure is also expressed as a value between 0 and 1. 0 is pure black and 1 is pure white. Solar reflectance is effectively the inverse of solar absorbance.
Thermal emittance (TE) is the relative ability of a material to radiate heat. This is also expressed as a value between 0 and 1.
The solar reflective index (SRI) is calculated using a material’s SR and TE values. SRI is expressed as a value between 0, pure black, and 100, pure white. A pure black surface will reflect almost no sunlight, while a pure white one will reflect almost all of it. The SRI indicates the ability of a material to return solar energy to the atmosphere.
In the warmer climate zones, a roof or wall material with a high SRI value will provide better thermal comfort for a house over the course of a year than a material with a lower value. It will also add less to the urban heat island effect. Conversely, in the cooler climate zones, a roof or wall material with a low SRI value will help to heat the house.
Research shows the tone of a roof makes a big difference to how hot the roof gets. New white roofs reflect about 70 to 80% of sunlight, compared to no more than 20% for dark roofs. New white roofs are typically 28˚C to 36˚C cooler than dark coloured roofs and aged white roofs are typically 20˚C to 28˚C cooler.” (Cool Roofs, City of Bayswater Research Paper, February 2022, page 8)
The profile of the roof cladding also makes a difference. Research comparing two cream tiles, with similar solar absorption values, found a house with a roof using a high-profile roof tile needed 11 % less energy to cool it than a house using a flat tile. (‘The properties of roof tiles.’ Australian Roof Tile Association, page 7)
Design standards
Design standards can be used to mandate certain levels of thermal comfort and to ameliorate the urban heat island effect. Standards can specify solar absorption values, solar reflective index values, or particular tones or colours.
“The solar absorptance rating of the house roof must be less than 0.5.” (Menangle Park, NSW)
“Roof colour and shade structures must be of a light tone to meet the current Sustainability measures with Solar Reflective Index (SRI) values: (a) For roof pitched < 15 degrees, a three year SRI > 64 … (b) For a roof pitched > 15 degrees, a three year SRI > 34…” (Kala, Pakenham East, Victoria)
Rather than mandating specific energy efficiency techniques, an estate developer may mandate an overall energy rating.
