In hot tropical climates, low thermal mass materials such as steel with light-coloured roofs and walls can be used to reduce energy demand for internal cooling. 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. Such thermal comfort is maintained by the dirt-resistance property in Clean COLORBOND TM steel. These properties are enhanced by way of BlueScope’s world leading THERMATECHTM coating technology.
With the addition of THERMATECHTM solar reflectance technology, darker coloured Clean COLORBONDTM steel products absorb less energy, creating the advantages of a cooler roof and a cooler building, similar to lighter shaded colours.
Due to demand from architects, 5 new colours have been added and there are now a total of eight colours in the Clean COLORBONDTM range that have Solar Reflectance Index (SRI) values of 80 or higher. They are:
What is the SRI?
The Solar Reflectance Index (SRI) is widely used by green building rating tools to mitigate the Urban Heat Island (UHI) effect. SRI is a value that incorporates both solar reflectance and thermal emittance in a single value to represent a material’s temperature in the sun. SRI quantifies how hot a surface would get relative to standard black and standard white surfaces. The table below indicates the colours with SRI values of 80+.
Solar properties of roofing materials
Definition of solar reflectance
Solar reflectance is the proportion of the total solar radiation that is reflected back from a surface to the atmosphere. Any solar radiation that is not reflected is absorbed into the building envelope, requiring further energy to cool it, or partially convected into the atmosphere and thus increasing the ambient air temperature in the surrounding environment.
Definition of thermal emittance
Thermal emittance is the proportion of absorbed solar radiation that is radiated back to the atmosphere. Most unpainted metallic coated steels have low thermal emittance, while pre-painted steels have high thermal emittance values of around 0,85. A roofing material with a higher thermal emittance will re-emit absorbed thermal energy more quickly than a material with a low emittance and keep a roof temperature lower.
Steel is 100% recyclable and is globally one of the most recycled materials by volume. The amazing metallurgical properties of steel allow it to be recycled continually with no degradation in performance, and from one product to another. Steel is produced using two different processes: basic oxygen furnaces (BOF) and electric arc furnaces (EAF). Both processes consume recycled scrap steel to produce new steel. When steel completes its useful life, it can be easily recycled and not landfilled.
The new colour range provides architects with a whole new scope of design for buildings. It adheres to the old adage “form follows function” but provides beautiful alternatives to white roofs.
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