The science behind cool roofs

by Tania Wannenburg

Safal Steel explains the science behind new-generation roofing materials to reduce the heat load in buildings.

As mean temperatures continue to rise alongside rocketing energy (read cooling) costs in South Africa, savvy architects, builders and property owners are looking at new-generation roofing materials to aid them in reducing the heat load in buildings.

Steel is an obvious choice, with its high solar reflectivity and low thermal mass (tendency to absorb and retain heat energy). Innovative aluminium-zinc (AZ) coating technology such as Safal Steel’s has ramped up the durability and versatility of steel sheeting, while a pre-painted option provides roofing durability, ease of maintenance and good looks.

But how does colour-coated steel perform thermally compared to a conventional steel roof or other building materials?

Understanding the Solar Reflective Index (SRI)
To calculate a constructed surface’s ability to reflect solar heat, the SRI takes into account a roof’s solar reflectance (the proportion of total solar radiation that is reflected back into the atmosphere), plus its thermal emittance (the proportion of absorbed solar radiation that is radiated back into the atmosphere) as well as a medium wind coefficient to generate a value of between zero and 100.

A black shingle roof will hence have a far lower SRI than a white-coated steel roof because of the former’s far lower reflectivity (owing to its dark colour) and higher thermal mass (owing to its greater density and higher insulation properties).

Interestingly, unpainted metallic-coated steels have a lower thermal emittance than painted ones, whatever the colour. In general, colour-coated steel also performs far better than traditional roofing materials such as concrete or clay tiles (which have dismal SRIs of 17 and 36 respectively).

With darker colours being less heat-reflective, it may be logical to think that a dark tint on colour-coated roof sheeting will cancel out the material’s superior thermal performance. However, leading producers of colour-coated steel are turning to science to produce thermally efficient paints that mitigate the low reflectivity of dark colours.

Thermally efficient paint
Heat-reflective pigments in darker paint colours raise the thermal performance of colour-coated steel and improve durability in terms of gloss, colour stability and film integrity. Safal Steel has recently introduced such a paint system in the two darkest colours, “Slate” and “Thunderstorm”, in its COLORPLUS® range.

Safal’s COLORPLUS® range is produced by continuously hot-dipping rolled steel in a 55% aluminium, 43,5% zinc and 1,5% silicon alloy, then skin-passing it for the colour-coating line, where it is coated with a two-layer modified polyester paint system and baked. Manufactured under licence to BIEC lnternational lnc., the worldwide licensor and acknowledged leader in AZ technologies, Safal’s COLORPLUS® range boasts properties that make it long-lasting and aid its heat reflectivity, such as anti-chalking, fade-resistant pigments and particle resistance.

Advantages of thermally-efficient paint systems include:
•    A cooler building.
•    A cooler city with less of the discomfort and environmental implications associated with urban heat islands (UHIs).
•    Reduced costs, both in terms of air-conditioning equipment needed and operational costs.
•    Reduction of environmental impact in terms of heat generation and energy requirements for cooling.

The superior reflectivity and durability of Safal’s colour coating system, including the paint technology on its darker colours, coupled with competitive pricing, top-notch quality, ultraviolet (UV) resistance and corrosion resistance, hardness and flexibility, make COLORPLUS® an ideal product for local South African conditions.

Safal Steel
Tel: 031 782 5500
Website: www.safalsteel.co.za

You may also like