Australia's climate is characterised by high ambient temperatures and long hours of sunshine over most of the continent for much of the year. High rates of skin cancer relative to those in most developed world countries would attest to this fact. Indeed, the average maximum shade temperature in January exceeds 30°C over almost all of the continent1. Yet shade temperatures are a poor indicator of how hot surfaces exposed to sunlight might react, as the temperature of any material is influenced, among other things, by its mass, specific heat and reflectivity. In terms of reflectivity, colour is an important factor. For example, with a maximum air temperature of 40°C, Martin2 quotes a flat white roof as reaching 66°C, while a black roof reaches 81°C. This is also well illustrated in Fig. 1 which shows how over the daily range of temperature increases, dark colours increase relative to light colours.
The impact of paint colour on temperature has three important ramifications.
FIGURE 1: Impact of colour on daily temperature of insulated wall surfaces exposed outdoors. This measure is possible where heat is not absorbed but impacts on the insulated surface.
1. White and pastel coloured paints and other light organic coatings will deteriorate slower than darker coloured ones of the same type outdoors, as the reactions which hasten their demise tend to occur faster as the temperature increases.
In a similar vein, for two surfaces of the same colour, the one with greater mass or higher specific heat will stay cooler. For example, a thin metal wall will get hotter and therefore degrade more quickly than a much thicker material with a high heat sink, such as brick and concrete. The effect will be more apparent if the same colour is applied to different adjacent substrates. In these situations, there is therefore an advantage in painting the metalwork a different colour from the concrete or brick.
FIGURE 2: Painted galvanized shed and concrete wall showing marked colour differences using the same paint, due to more rapid chalking because of the higher ambient temperature reached on metal.
2. The hotter the surface, the greater the rate of heat transfers through the building fabric. This means that darker coloured buildings are likely to heat up more quickly than lighter coloured ones. This is particularly true for metal roofs and walls which are comparatively thin and good conductors of heat. For example, for an off-white Colorbond roof exposed to solar radiation at 850 W/m2*, zero wind speed and 30°C, the manufacturer claims only about 1/8th of the heat is radiated downward, while for an identical tawny brown one about 1/4 of the heat is transmitted.
FIGURE 3: Chalking is more obvious on darker and brighter coloured paints.
3. As paints weather, the binder at the exposed surface breaks down to form chalky powder. However, because chalked paint is a light colour, any chalking of a darker or brighter paint is more obvious than with white or pastel colours (Fig. 3).
Therefore, for maximized service life, particularly on thin metal sheeting, the use of bright and dark colours is best limited to areas protected from solar radiation or at least for orientations where solar radiation is less severe, e.g. south and east. While this is not always practicable, the principle of minimising the use of bright and dark colours in direct sunlight, particularly in north and west facing surfaces outdoors, is always worth considering.
One advantage of darker surfaces is that staining and soiling tends to be much less apparent. Thus light painted doors and other frequently trafficked surfaces are less serviceable than dark painted ones in similar situations.
The colour temperature effects described here apply to after-fabrication galvanized steel, to much the same degree as if applied to steel direct.
The same greater severity on thin steel sections will prevail where the galvanizing thickness has no measurable temperature reduction influence.
These factors mean less in more temperate zones but have interesting design implications in tropical and arid high temperature areas where climate is an important factor of the structural and architectural design.
For example, if a hollow section is open ended and vertical, a chimney effect is created and the hollow serves to cool the surface. If it is horizontal or enclosed it will heat up, as the specific heat of air is very low and the heat from the steel will not be absorbed by the hollow. This is one reason why horizontal paint surfaces will degrade faster than vertical ones.