Reflective roofs reduce energy demand in buildings by reflecting more sunlight and absorbing less heat than conventional roofs. In this article, we take a look at cool coated steel roofs as well as the use of a cool paint coating.
The term “cool roofs” is commonly used to refer to reflective, roofs, whether the surface is made of coated metal sheeting, reflective tiles or coated with reflective paint. The point is that cool roofs reflect more sunlight and absorb less heat than conventional roofs to make buildings more energy efficient.
Steel is a popular choice for a roof covering material since it has a high solar reflectivity and a low thermal mass, and when it comes pre-coated with a cool roof coating, it goes a long way in improving the energy efficiency of a building. Although white is generally the colour that reflects the most light, with the solar reflectance technology available today, cool metal roofs come in a wide range of colours.
To determine a surface’s effectiveness in reflecting solar heat, the Solar Reflectance Index (SRI) is used by green building tools. The SRI indicates how hot a surface would get, relative to standard black and white surfaces. It is indicated using a value between 0 and 100, which takes into account solar reflectance and thermal emittance, plus a medium wind coefficient.
It is however possible for the SRI of a product to score more than 100 on the SRI scale as certain white thermoplastics and cool white coatings may be more reflective than standard white roofs. Similarly, very hot roofs can have negative values.
Even when coated, a black steel roof will still have a lower SRI than a white coated steel roof because of the dark colour’s lower reflectivity. With light-coloured coated steel roofs boasting an SRI of about 80, they may generally perform far better than conventional roofing materials such as concrete or clay tiles, of which the SRIs are both lower than 40.
Cool coatings a low-cost option
At the other end of the scale, not all projects can afford the all-in-one, pre-coated roof sheeting. A second option though, according to Denise Lundall, project officer for energy efficiency, cool surfaces and communications at the South African National Energy Development Institute (SANEDI), is the application of a cool coating.
Cool roof coatings contain special reflective pigments that reflect sunlight and protect the roof from ultraviolet (UV) rays and damage. “This is a fantastic passive solution for those people who can’t afford expensive building materials, and HVAC systems,” she says.
The Cool Surfaces Project
South Africa joined the United States Department of Energy’s lead Global Superior Energy Programme (GSEP) Cool Roofs and Pavements Working Group in 2013. Since then, SANEDI has been focused on bringing thermal comfort into the informal and formal structures of low-income communities and reducing the heat island effect over human settlements, while simultaneously collecting data about the performance of these interventions.
Through its Cool Surfaces Project, SANEDI has done several demonstration projects in the Northern Cape and Gauteng, with more to follow over the rest of the country. In the !Kheis municipality in Groblershoop in the Northern Cape, dramatic cooling of between 7°C and 10°C was measured, but in a more moderate climate zone, the cooling potential is closer to between 2°C and 4°C.
“Currently, we are in the process of rolling out the 500 Roof Heat Island Mitigation Project in Sternham in the Northern Cape, a demonstration project that aims to prove that the ambient temperatures of the atmosphere can be significantly and positively impacted by the simple application of a cool coating,” says Lundall.
“Most of the buildings we coat are either made with corrugated zinc or have tile roofs. The coating is applied to the roofs, and sometimes the walls, of these structures.”
The numbers behind cool surfaces
When sunrays (comprising heat and light) strike surfaces on the earth, up to 40% heats the general atmosphere, 50% heats the city air, 5% is reflected towards space and 5% of the heat transfers into buildings.
A surface covered with a cool coating reflects up to 80% of light and heat towards space. Only 10% heats the atmosphere, 8% heats the city air and 2% heats the building.
Traditional dark roofs can reach temperatures of 66°C or more in the summer, while a cool roof under the same conditions could remain more than 28°C cooler. This trend is repeatedly noted in multiple studies.
New: The scattering effect
Lundall points out that the problem with reflecting heat into the atmosphere is that it gets trapped by greenhouse gasses and increases the temperature of the planet. But thanks to recent advancements, the paint does not only reflect sunlight, but now also breaks up and scatters sunrays so as not to cumulatively increase the atmospheric temperature that is contributing to the heat island effect and climate change.
The paint formula was developed by Millennium Roofing Solutions, a California-based cool coatings manufacturer, who is training local companies in the mixing and application of this paint, following the successful demonstration project within the !Kheis municipality in Groblershoop in the Northern Cape.
According to Lundall, there are currently four primary accredited companies in South Africa who are manufacturing this paint.
Working with the government
SANEDI is in talks with the Department of Defence to coat old asbestos roofs as a short-term solution until these roofs can be removed and replaced. “Coating these roofs seals in the harmful particles and prevents them from being released into the atmosphere,” Lundall explains.
In addition, ways to adapt the cool paint technology for alternative applications, such as coating metal-armoured vehicles and portable tents, are also being explored.
“We are further trying to develop a stronger relationship with the Department of Housing to get the decision-makers on board and educate them about the benefits of the cool paint, since they can only make decisions based on what they know,” she states.
Looking forward, the Cool Surfaces Project is supporting the development of quality coatings that could make a sustainable impact.
The introduction of waste coal ash as a reflective filler to the cool coating composition and as an additional binder to strengthen the product is being researched. The project seeks to absorb the waste ash from the environment and turn it into a useful commodity.
In addition, the use of waste coal ash is being tested to manufacture reflective paving bricks for rural areas. There are currently in excess of 60% of rural roads that are unpaved. Instead of heat-inducing blacktop tar roads, the cheaper and cooler surface option would be the proposed reflective bricks.
“While the immediate concern was to improve the thermal comfort of humans’ occupation in buildings, the larger aim is to reduce the impact of humanity on the heating of the planet’s atmosphere, which is inducing undesirable climate change,” says Lundall.
“It is the project’s intention to roll out a scaled-up project (like in Sternham) in each of the seven climatic zones to measure the effect on climate change. Ideally, we would like to do a substantial project in each of the nine provinces, concluding two a year.”
Full thanks and acknowledgement are given to bluescope.co.za, www.safalsteel.co.za, www.chromadek.com, www.facilitiesnet.com and SANEDI for the information used to write this article.
Caption: A cool paint coating was used on the roof of this building in the !Keis municipality in the Northern Cape as part of SANEDI’s Cool Surfaces Project.
Courtesy of SANEDI.