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Cool roofs in South Africa

by Tania Wannenburg
Cool roofs in sa

From cool roof paint and parapet walls to a new way of installing insulation, Walls & Roofs takes a look at what is happening in the local roofing industry.

 

The South African National Energy Development Institute (SANEDI) is conducting demonstrations of cool paint on low-income houses to improve thermal comfort for residents as well as building energy-efficiency. The Cool Surfaces Project is the South African involvement in the United States Department of Energy’s Global Superior Energy Programme (GSEP) Cool Roofs and Pavements Working Group.

According to Denise Lundall, project officer for energy-efficiency at SANEDI, the Cool Surfaces programme is currently focused on bringing thermal comfort in the informal and formal structures of low-income communities, including those that are off-grid, while simultaneously collecting data about the performance of these interventions.

“At the moment we are focusing on the application of cool paint,” she states. “We considered the different aspects of the building envelope and needed to narrow it down to the quickest application with which we could get the best results that could be measured and verified.”

The pilot demonstration took place in the !Kheis community in Groblershoop in the Northern Cape. “The most common building material used for houses in the area is corrugated iron, a great conductor of heat, and the walls and roofs get so hot that one can obtain serious burns just by touching them,” she says.

“We selected a house and painted the external walls as well as the roof with white cool paint manufactured by Dow, a United States chemical company. Instantly, we measured dramatic cooling of between 7°C and 10°C.

“Of course these temperatures would vary greatly in a place like Johannesburg, which is not subjected to the extreme heat of the Northern Cape desert, but studies indicate a cooling potential from 2°C to 4°C, which is still a drastic difference in terms of thermal comfort,” she adds.

SANEDI is taking this initiative to other areas too, with ten more sites lined up for further studies. In order to ensure the responsible implementation of cool roofs, SANEDI is also presently setting up a study to determine the effect of cool paint on avian wildlife in collaboration with the Endangered Wildlife Trust.

Understanding the cool concept
According to Lundall, cool surfaces are measured by their solar reflectance, which is how much light they reflect, as well as their thermal emittance or how much heat they emit.

When sunlight hits a corrugated roof surface, a large percentage, sometimes up to 80% of that heat, radiates into the building (emittance), with some of the heat being absorbed by the roof and some reflected back into the atmosphere, according to Lundall. In contrast, when cool paint is applied to the roof, only a fraction of the heat and the light is absorbed and only about 20% of it enters the building.

Colour matters, but doesn’t limit
Although white paint is used in the demonstration projects because it is the most effective in reflecting light, Lundall points out that cool paint is not limited to only white – in fact, it comes in any colour, even black.

“It is not the actual colour per se that makes paint cool, although lighter paint definitely has higher reflectivity and lower emittance. Instead, it is the molecular structure of the paint that gives it a low emittance and allows for minimal light and heat to be emitted into the building. At the same time, it has a static ion relationship with the atmosphere that doesn’t allow dust to build up,” she explains.

From a contractor’s point of view, Nico van der Merwe, director at Ceilspray, adds that they have been working and experimenting with cooling paints for the past ten years on several types of roofs. “It is in fact true that one can reduce the room temperature by between 4°C and 8°C, irrelevant of the colour of the cooling paint,” he says.

Parapet walls
Duncan Goldsmith, founder and managing director of Rigifoam, says that another big trend adding to cooler roofs that has stood the test of time in the United States, for example, is the addition of parapet walls on the buildings to create shade over the roof in the afternoons so that less heat is absorbed. Although the initial capital expenditure is high, the benefit lasts for the lifespan of the building.

Urban heating debate
The topics of cool roofs and the urban heat island (UHI) effect have been a matter of debate for many years, and many theories and studies are still contested amongst professionals.

According to Goldsmith, when it comes to cool roofs it is important to consider the reflectivity into the atmosphere that causes urban heating. In his opinion, when the heat bounces off the cool roof and hits a concrete wall next to it, the wall’s temperature rises and it starts emitting heat. “This is how urban heat islands are created and why, in cities where building owners have gone the cool roof way, the actual buildings are cooler inside, but the surrounding areas are heating up,” Goldsmith says.

Lundall disagrees, saying that glare is a minor, site-specific problem. “Cool roofs are reflective, but the paint diffuses the light instead of reflecting it in a concentrated way like a mirror or shiny surface might. Therefore the intensity of the sun rays is drastically reduced by the time it hits the neighbouring building,” she states.

“While glare might cause some additional heating on closely packed buildings of different heights, these cases are not common and wouldn’t contribute in any measurable way to the overall urban heat island.”

Rather, according to her, almost all of a city’s urban heat can be accounted for by four factors:
5.    Human activity like air-conditioners, industrial processes and vehicles.
6.    A relative lack of vegetation compared to surrounding rural areas.  
7.    The predominance of dark, impermeable surfaces that absorb sunlight and convert it to heat.
8.    The blockage of air flow through cities.

“There is a growing body of research demonstrating that increased reflectivity helps cool urban areas and the planet,” Lundall adds. “For example, researchers did a 20-year study of temperatures in the Almeria region of Southern Spain, where farmers whitewash their greenhouses every year, resulting in higher surface reflectivity to regions around it. However, the results showed that Almeria experienced a cooling trend of 0.3°C while the surrounding areas heated up by 0.4°C.”

Product testing and standards
A public-private association, the South African Cool Surfaces Association (SACSA) has been established in February 2014 under the auspices of SANEDI and is administered by the Association of Architectural Aluminium Manufacturers of South Africa (AAAMSA).

According to Lundall, SACSA is in the process of setting up a materials testing laboratory in collaboration with the South African Bureau of Standards (SABS) to determine the voluntary industry standard for cool surface testing and performance, to ensure product efficacy and durability.

The importance of insulation
Lundall further states that while the objective is to create optimal thermal comfort inside a building, it is recommended that cool paint should be used in conjunction with thermal insulation.

She mentions that the Departments of Energy and Human Settlements have started specifying bulk insulation in the formal housing as part of RDP projects.

Goldsmith echoes this, saying that the International Code Council (ICC) now also prescribes that an insulation medium should be installed together with cool roofs. “In the USA, they have experienced major issues with condensation due to cool roofs causing dampness and wood rotting,” he says.

“Because you are reflecting the radiant heat and not the convection and conductive heat, the roof metal doesn’t heat up as much. The temperature on the inside of the building then drops below dew point and you get moisture on the underside of the roof. Since there is reduced radiant heat, there is nothing to flash off the moisture,” he explains.

“What is often misunderstood, is that these radiant barriers only insulate against radiant heat, which heats up objects, but doesn’t insulate convectional or conductive heat, which heats up the ambient temperature,” he says.

According to Lisa Reynolds, sustainability director at Saint-Gobain, cool roofs have little to no effect for winter conditions as they do not assist in keeping the heat inside the buildings. “Therefore, if they are used in climatic zones that focus on reducing the heating required in winter, the designers or architects would still have to specify the level of intervention as outlined by SANS 10400-XA and the TIASA guide,” she says.

Insulating on the outside
According to Goldsmith, in America and Europe insulation is installed extensively on the outside of buildings rather than on the inside, where insulation is put on top of the roof sheeting with waterproofing.

“We have to change the way we think in order to improve the way in which we insulate,” he explains. “Locally, we insulate below the heat sink, so when you have a big steel roof absorbing heat, with the insulation beneath that, the product no longer deals with the ambient temperature but with a much higher surface temperature. When putting the insulation on the outside, the performance is much better since you reduce the temperature difference.”

Goldsmith says that, in his experience, many local architects are keen to follow this approach. However, changing the whole building concept is no simple feat, especially since the industry has been doing it in a certain way for so long.

“Contractors will have to learn new skills required for insulating on the outside of buildings and while currently the roof sheeters install the insulation, it will probably become the responsibility of the waterproofing contractors to apply the insulation. This method not only gives optimum insulation, but also extends the lifespan of the roof structure from weathering elements,” he states.

Reynolds adds that the optimum placement of insulation is dependent on the climatic zone, the shading – either in design or by neighbouring buildings – and the orientation of walls and roof pitches.

Ceilspray’s Van der Merwe agrees that inverted insulation is an effective method and says that some contractors are already doing it this way, but he cautions that this method could restrict the architect’s aesthetic design options.

Join the discussion
Walls & Roofs would like to hear your views and continue this discussion around cool roofs and inverted insulation. Email your thoughts to alet@mediainafrica.co.za.

Full thanks and acknowledgement are given to SANEDI, Saint-Gobain, Rigifoam and Ceilspray for the information given to write this article.

Benefits of cool roofs:
•    Both the roof and the equipment on it last longer.
•    As less heat is allowed in, unconditioned buildings become much cooler.
•    Cooling effects will vary by city, but studies indicate a cooling potential from 2°C to 4°C.
•    Whitening 100m² of grey roofing cancels the warming effect of 10 tons of CO₂ emissions, or 0,6 tons per year for the life of the roof.
•    Globally, it cancels 500 medium-sized coal power plants worth of greenhouse gas emissions.
•    Cool surfaces can cut energy used due to air-conditioning by up to 20% on the top floors of buildings.
•    Cooler intake air means air-conditioners work less, contributing to downsizing the units.
•    The industry can participate in the 12L EE Tax Incentive launched in November 2013.
Courtesy of SANEDI

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