When undertaking an assessment, assessors are required to nominate the colour of the external walls and roof elements in the software tool.
The colour of a wall or roof is represented by its solar absorptance value, a decimal number that is an indicator of the colour’s ability to absorb radiation from the sun.
The following extract is from the Short Course in Building Thermal Performance Assessment (Residential) training notes for assessors by Dr Holger Willrath of Solar Logic.
“Absorptance, (α), is a measure of the ability of a surface to absorb radiation. It is the ratio of the thermal radiation absorbed by a surface to that absorbed by a perfectly absorbing surface”, and “Solar absorptance, refers to the ability of a surface to absorb all the radiation wavelengths of the sun's emission spectrum”.
The software tools allow assessors to enter the actual solar absorptance values for wall and roof elements.
Assessors should research the solar absorptance values of the colours specified e.g. colorbond colours, and enter the relevant value into the assessment. Manufacturers of many products and finishes publish this information on their websites.
BERS Pro includes an additional feature that calculates the solar absorptance value for assessors when the red/green/blue (RGB) values are known for a particular colour e.g. paint colour of a wall.
But what impact does the colour of a wall or roof have on a star rating?
The answer to this question is dependent on a range of variables but the impact of colour is primarily determined by a building’s location and the level of insulation in the walls and under the roof sheeting.
Assessors often enter the solar absorptance values of walls and roofs as Light (0.30), Medium (0.5) or Dark (0.85).
In some locations and for some construction systems, this choice can impact on the achieved rating.
Construction systems such as cavity brick or single skin concrete block walls, or tile roofs without sarking are essentially uninsulated and the solar absorptance value of these elements will affect the rating achieved for the building.
In Melbourne, a cooler climate, changing the wall colour from Light to Dark can reduce the heating loads by around 5% that results in a better rating.
In Darwin, a hot tropical climate, selecting Light will significantly reduce the cooling loads and give a better rating.
Add insulation to either of these systems and the benefit from choosing the correct colour is greatly reduced.
Assessors are often faced with a building that just falls short of achieving the desired rating. Recommending a colour with a solar absorptance value appropriate to the building’s location and construction system may provide the improvement in the thermal performance of the building to reach the target rating.
Michael Plunkett is Principal of SmartRate based in Cairns and a practising ABSA accredited assessor.
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