Using Rooflighting for Cooling
The Urban Heat Island Effect (UHI) is a phenomenon whereby the concentration of structures and waste heat from human activity (most notably air conditioners and internal combustion engines) results in a slightly warmer envelope of air over urbanised areas when compared to surrounding rural areas. It has been suggested that UHI has significantly influenced temperature records over the 20th century with rapid growth of urban environments.
Cape Town and Johannesburg, among the fastest expanding cities in the world, have been a particular area of concern. It is clear that urban areas are struggling to control the environmental impacts of growth as many cities are becoming “heat islands”.
Not only do UHIs tax the local buildings’ air-conditioning (AC) units, they have a significant impact on pollution. Cities with UHIs tend to exhibit higher ozone levels. This is not only a serious health threat, but as international standards for pollution control become stricter and mandates are imposed, cities stand to lose funding and support for international events because of noncompliance. Ross McKitrick, a professor at the University of Guelph, and Patrick Michaels, an environmental studies professor at the University of Virginia published a paper claiming that half the global warming trend over the last 20 years has been caused by urban heat islands.
The three main causes of urban heat islands are:
- the reduction of urban vegetation, such as grasses, trees, shrubs (56% contribution to urban heating);
- increased dark roads and parking surfaces (6% contribution);
- and the concentration of buildings with dark roof surfaces (38%).
Increasing urban vegetation is certainly desirable. They reduce the environmental impact and improve the aesthetic appearance of the city. Unfortunately, planting trees or vegetation may not prove to be a long-term solution as the demand for more space in urban areas will always be a deciding factor. It is also not economically viable or ecologically viable to replace roads and parking lots with cooler surfaces. Roofing, however, is a fairly simple and cost-effective way to reduce the effects of urban heat islands.
“Cool roofing” is a process encouraged to lower the energy consumption of buildings and reduce urban heating. A cool roof is one that reflects the sun’s heat and emits absorbed radiation back into the atmosphere. The roof literally stays cooler and reduces the amount of heat transferred to the building below, keeping the building a cooler and more constant temperature – much like wearing a white t-shirt on a hot day.
However, a cool roof needn’t be white. There are many “cool color” products which use darkercolored pigments that are highly reflective in the near infrared (non-visible) portion of the solar spectrum. With “cool color” technologies there are roofs that come in a wide variety of colors and still maintain a high solar reflectance. Cool roofing is characterized by levels of reflectance and emittance high enough to help reduce the amount of solar energy retained by a building and potentially transmitted into the structure. Reflectance, or albedo, is the percentage of solar energy reflected by a surface; the higher the percentage, the more energy reflected from the surface. Emittance deals with the energy actually absorbed, and is defined as the percentage of energy a material can radiate away. Materials with low emittance tend to heat up more easily because they collect and trap heat. To stay cool, then, a surface should ideally have both high reflectance and high emittance. The roofing product must also maintain a reflectance of at least 50% after three years of weathering to be defined as a “cool” roof.
One such example is the recently launched MODEK Heat Stop, a polycarbonate profiled extrusion, manufactured in South Africa, which has a co-extruded layer of UV stabilised surface protection on the weather side. Microscopic reflective particles are incorporated in the core material which create the silver colour and the reflective qualities of the product.
MODEK Heat Stop eliminates 60% of the radiated heat of the sun and reduces the cooling load of the building. This means that the occupants are more comfortable, leading to a reduction in the use of air conditioning…leading to 10-30% energy savings.
Cool roofs directly reduce green house gas emissions by conserving electricity for air conditioning therefore emitting less CO2 from power plants. Cool roofs also cool the world independently of avoided carbon emissions, simply by reflecting the sun’s energy as light back to the atmosphere, thereby mitigating global warming. A Lawrence Berkeley National Laboratory study found that world-wide reflective roofing will produce a global cooling effect equivalent to offsetting 24 gigatons of CO2 over the lifetime of the roofs. This equates to R4200 billion in savings from CO2 emissions reduction.
Cities can be warmer than surrounding areas due to dark materials, including roofs, which absorb the sun’s light energy as heat during the day and release it at night as heat. This phenomenon removes the opportunity for air to cool down at night and results in higher temperatures being maintained longer. By immediately reflecting solar radiation back into the atmosphere and reemitting some portion of it as infrared light, cool roofs result in cooler air temperatures for the surrounding urban environment during hot summer months.
Cool roofing, by mitigating the effect of urban heat islands and ambient air temperature, can also improve air quality as it reduces the creation of smog. Smog is created by photochemical reactions of air pollutants, which increase at higher temperatures. Therefore, by reducing the air temperature, cool roofs decrease the rate of smog formation.
There is also a benefit to building owners. Not only is a material such as polycarbonate 200 times more impact resistant than glass, it is also significantly cheaper and lighter to transport. The real benefit, however, is seen in operating costs. International tests on reflective roof membranes found that a typical 4645-m2 roof can realize cooling energy savings of up to R69 650 per year as the temperature inside the building would be 5-7°C cooler than with a dark roof.