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Cool Roofs beat the heat

Cool Roofs beat the Heat – white roofing, cool roof options, solar reflectanceConcrete structures, urban smog and rampant use of air-conditioners are turning cities into hot beds. One of the most cost-effective ways of cooling down concrete structures such as homes and buildings is by having ‘cool roofs’.

Cool Roofs deflect heat. They are based on the simple concept that dark colours absorb heat while light colours deflect heat. Dark-coloured roof cause more heat to flow into the building than a light-coloured one.

A major part of the heat that enters the top storey of a building is from its roof. Most of the buildings in our country have dark roofs which can reach temperatures of about 70-80oC in summer. This heat is transmitted to other parts of the home/building through the ceiling. This leads to a higher indoor temperature in unconditioned spaces, higher electricity consumption in air-conditioned spaces, higher peak electricity demand, ‘heat island effect’ leading to air pollution, and lowering of life of the roof material.

It has been observed that light-coloured roofs can be about 30oC cooler than dark ones. This reduces the heat flowing from the roof into the buildings. For a roof to be ‘cool’, its surface should have two important properties – high solar reflectance and high thermal emittance. Solar reflectance is the percentage of solar energy that is reflected by a surface. Higher the value, higher the solar energy reflected by the surface and therefore, lower the increase in the temperature of the surface.

Thermal emittance is defined as the percentage of energy a material can radiate after it is absorbed. Higher the value, higher is the heat the material will radiate to the surroundings. Hence material with higher emittance cools down faster than a material with lower emittance.

Besides keeping the heat out, cool roofs have a longer life. As they absorb less solar energy, they get less heated up in the day and therefore the temperature difference between night and day is reduced. This reduces the process of thermal expansion and contraction and increases the life of the roof.

The concept of cool roof is not new in tropical countries and has been traditionally known for centuries. In Rajasthan for instance, it is a practice to whitewash all the exposed surfaces (including roof) of buildings. This is a cost-effective way of making the roof cool. Cool roof is most effective when there is abundant solar radiation and when the building needs cooling. In most of the regions in India there is huge amount of solar radiation and cooling is the primary requirement.

Cool Roofs not only help cool the building, they also help in reducing energy consumption. The magnitude of energy savings depends upon various factors such as building type, level of roof insulation, ventilation rate between roof and ceiling, a/c size and efficiency, and roof solar reflectance.

Cool roof coatings

Cool roof coatings are white liquids with the consistency of thick paint applied over an existing roof structure. They can be applied over a traditional roof to achieve energy savings, an extended life span, and protection from weathering and ultraviolet radiation. The new surface can last 10 to 20 years, depending on coating quality and its thickness. Most cool roof coatings can be described as elastomeric, polyurethane, or acrylic. The reflectance of cool coatings decreases by only about 20% over the product's lifespan, and most of this decrease occurs in the first year after application.

The coatings may cost between Rs 70 and Rs 100 per square foot for materials and labour. Installation cost varies, however, depending on the condition of the roof, size of the job, number of roof penetrations or obstacles, and the ease of access to the roof.

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