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Cool Roofs: A step towards sustainable architecture

The heating and cooling demands of a building structure are affected by the thermal characteristics of its envelope. For flat normal roofs, the solar irradiation contributes considerably to the total heat gain of the building and its solar reflectivity and thermal emissivity are very relevant in determining the cooling and heating requirements of the building. Dr. Mohammad Arif Kamal, Professor & Director, Al Amir College of Architecture, Thiruppathur, Tamil Nadu reviews the role of cool roofs in providing thermal comfort and its significance in energy conservation.

A cool roof is a roofing system which is able to reflect solar heat and keep roof surfaces cool under the sun. This is due to the properties of the reflective and emissive materials used which reflect solar radiation back into the atmosphere. As the roof stays cooler, it reduces the amount of heat transferred to the building below, keeping a cooler and more constant temperature in the interior. The high solar reflectance (ability to reflect sunlight) and high thermal emittance (ability to radiate heat) of cool materials helps roofs to absorb less heat and stay up to 28-33°C cooler than conventional materials during peak summer weather. These roofs can generate air-conditioning savings and peak demand reductions of 10-30%.

Materials

Cool roofs can be made of a highly reflective type of paint, a sheet covering, or highly reflective tiles or shingles. By installing a cool roof, you can lower the temperature of your roof and save energy and money by using less air-conditioning. Cool roofs make spaces like garages or covered patios more comfortable. Nowadays, numerous roofing materials are rated with relatively high reflectance and emittance values, including materials for low and steep sloped roofs (white, colored, photo-catalytic, etc.). This provides more choices for designers to employ aesthetic solutions, either for commercial and industrial buildings or residences, in both new construction and existing buildings. Many materials are available such as coatings (white-tan, light gray), single-ply membrane (white), granule surface asphalt cap sheets and painted metal (white, light, cool colored).

Cool roofs are defined by the Cool Roof Rating Council as a product with solar reflectivity of at-least 0.70 and infrared emissivity of at-least 0.75. It reflects most of the solar irradiation and also highly emits thermal radiation in the infrared part of the spectrum. The cool roofs have been proven to reduce the building cooling energy use by 10% to 50%. They also show a decrease in summertime air temperature of the building surroundings of 1-2° K, reducing the Urban Heat Island Effect.

Types of Cool Roofs

There are many types of roof systems available, but the surface exposed to the sun is the one that determines if a roof is cool or not. A new or existing roof cool can be installed by selecting the appropriate surface.

Cool roof coatings are white or special reflective pigments that reflect sunlight. They are very thick paints that can protect the roof surface from ultra-violet (UV) light and chemical damage, and some offer water protection and restorative features. Products are available for most roof types.

Single-ply membranes are pre-fabricated sheets rolled onto the roof and attached with mechanical fasteners, adhered with chemical adhesives, or held in place with ballast (gravel, stones, or pavers). Reformulate or black membranes should be coated to make them reflective.

Built-up roofs consist of a base sheet, fabric reinforcement layers, and (usually) a dark protective surface layer. The surface layer can be made different ways, and each has cool options:

• Substitute reflective marble chips or gray slag for dark gravel in a flood coat of asphalt

• Use reflective mineral granules or a factory-applied coating rather than a dark coating on a mineral surfaced sheet

• Apply a cool coating directly on top of a dark asphaltic emulsion coating.

Modified bitumen sheet membranes have one or more layers of plastic or rubber material with reinforcing fabrics, and are surfaced with mineral granules or a smooth finish. These can also be used to surface a built-up roof—known as a “hybrid” roof. A pre-coat with a cool roof coating should be done, which is available from the market.

Spray polyurethane foam roofs are constructed by mixing two liquid chemicals together that react and expand to form one solid piece that adheres to the roof. Foams are highly susceptible to mechanical, moisture, and UV damage, and rely on a protective coating. The protective coatings are usually already reflective and offer cool roof performance.

Shingle roofs consist of overlapping panels made from a variety of materials such as fiberglass asphalt, wood, polymers or metals. The cool asphalt shingles which use specially coated granules that provide better solar reflectance. (Coating existing asphalt shingles to make them cool however, is not normally recommended or approved by shingle manufacturers.) Other roof shingles can be coated at the factory or in the field to make them more reflective.

Tile roofs can be made of clay, slate, or concrete. Tiles can be glazed to provide waterproofing or coated to provide customized colors and surface properties. Some are naturally reflective enough to achieve cool roof standards, and surface treatments can transform tiles with low solar reflectance into cool roof tiles.

SRI of Cool Roofs

• The Solar Reflectance Index (SRI) is a measure defined by the Lawrence Berkeley National Laboratory as the roof’s ability to reject solar heat, as shown by a small temperature rise

• SRI is an alternative method for considering the radiative properties of roofing materials. SRI is defined by ASTM Standard E1980-01 and is a calculation that uses solar reflectance and thermal emittance.

• EPA summarizes SRI as “the relative steady-state surface temperature with respect to the standard white (SRI=100) and standard black (SRI=0) under the standard solar and ambient conditions.”

• SRI is often used as an alternative for products that have a low thermal emittance but a very high solar reflectance—the theory is that the higher solar reflectance will outweigh the impact of a low thermal emittance.

• The light colored, reflective roofs have an SRI of 50% or more. A fine example of higher SRI is the use of broken china mosaic and light colored tiles as roof finish, which reflects heat off the surface because of high solar reflectivity, and infrared emittance, that prevents heat gain.

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