When selecting materials the building designer will often have to determine the reflectivity of their product of choice. There are a number of issues with which they have to contend. Firstly, all materials reflect sunlight to a degree, however some colours reflect light more than others. Materials and/or colours with a high degree of reflectivity will give better thermal performance than those with lower reflectivity. Therefore colour choice will affect heating and ventilating costs.
It is difficult to quantify the reflectivity of a material in isolation The following considerations will affect the degree of reflectivity or colour selection:
- When sunlight scatters and reflects colour from adjoining surfaces, the colour of the building in question will be compromised, in that it is tinted by the reflected colour. Therefore the observable colour in practice is not the colour necessarily selected.
- The orientation of a building will determine the amount of light reflected, for example if a roof is viewed from the north, it is unlikely that any glare will be experienced.
- The pitch of the roof and the topography of the ground will affect the angle at which the light will strike a building and hence how it is reflected.
- Seasonal variations such as the height of the sun in the sky will affect the amount of glare, with summer conditions obviously being the worst.
- As pre-painted steel roofs age, they naturally weather and the effect of glare is reduced proportionately.
Giving consideration to the above points, it is necessary to assess each building on its merits rather than applying a ‘rule of thumb’ judgement based on numerical criteria such as a Tristimulus value.
The correlation between thermal absorption and reflectivity whilst not being exactly inversely proportional, generally does follow this pattern. Therefore Table DD ES CC 001 can be of assistance in assessing the relative degree of reflectivity of various colours. In general terms, the colours that have a solar absorption of >0.70 will have a low degree of reflectivity in comparison to the colours having a value of <0.35.
The Building Code of Australia (BCA) and the New South Wales Building and Sustainability Index (BASIX) both have a classification system which groups colours into light, medium and dark to assist designers in determining their colour classification. Table DD ES CC 001 classifies the Bluescope Steel COLORBOND® range to the BCA and BASIX systems, as well as the Solar Absorption ratio.
For further information please refer to the resources on bluescope.com