Energy-efficiency and dirt resistance of roofing materials

by Darren
Energy efficiency and dirt resistance

Green building rating tools encourage the use of building materials that mitigate the intensity and impact of the Urban Heat Island (UHI) effect.


Solar reflective index considerations

Green building rating tools encourage the use of building materials with high Solar Reflectance Index (SRI) values to mitigate the intensity and impact of the Urban Heat Island (UHI) effect.

The SRI is a numerical expression of a constructed surface’s ability to reflect solar heat, based on a roof’s solar reflectance, thermal emittance and medium wind coefficient.

Sunlight that is not reflected by a building’s roof, for example, is converted into heat that dissipates through convection, conduction or radiation. This heat is either absorbed into the building envelope or partially convected into the atmosphere, thereby increasing the ambient air temperature in the environment surrounding the building, termed the UHI effect.

Energy-efficiency of roofing

There are numerous ways to improve energy-efficiency, such as:
• Using light-coloured roofing materials to reduce solar heat gain.
• Utilising low thermal mass materials that respond to temperature changes comparatively quickly as opposed to high thermal mass materials that take longer to heat or cool.
• Insulating walls, ceilings, roofs and suspended floors.
• Favourably orientating the building and providing exterior shading to control solar heating and wind flow.
• ThermatechTM solar reflectance technology.

Low thermal mass materials such as steel with light-coloured roofs and walls can be used to reduce energy demand for internal cooling, especially in warm tropical climates. For example, a light-coloured Clean COLORBONDTM steel roof not only reduces the amount of solar radiation absorbed, but it is also very effective at re-radiating heat.

With the addition of ThermatechTM solar reflectance technology, darker-coloured Clean COLORBONDTM steel products also absorb less energy, creating the advantages of a cooler roof and a cooler building, similar to lighter-shaded colours.

BlueScope’s world-leading ThermatechTM coating is a solar reflectance technology that is incorporated into Clean COLORBONDTM steel to lower surface temperature by absorbing less heat from the sun. It optimises the thermal performance of every colour in the standard Clean COLORBONDTM steel palette, without changing their appearance.

Dirt resistance also assists in cooling

The Clean COLORBONDTM steel further boasts a dirt-resistance property that maintains the thermal comfort, due to ThermatechTM Technology.

The comparison below, between a Clean COLORBONDTM and a conventional steel roof, both uninsulated, illustrates the positive effect of dirt resistance technology. After two years’ exposure, assuming an irradiance of 1000W/m², exterior temperature of 36°C and a high thermal emittance value of 0.85, typical of pre-painted steels, the Clean COLORBONDTM steel roof’s temperature is almost 10° cooler than the conventional steel roof and it radiates more than 40W/m² less heat down into the building.


Energy efficiency and dirt resistance of roofing materials

Steel is 100% recyclable to boot and is globally one of the most recycled materials by volume. In fact, the remarkable metallurgical properties of steel allow it to be recycled continually with no degradation in performance, and from one product to another.

For more information on Bluescope’s new range of architectural colours with an SRI of 80 and above, contact them today.

Tel: 021 442 5420
Fax: 021 448 9132
Website: www.bluescopesteel.co.za


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