About on- third of the unwanted heat in your home comes through the roof. This is hard to control with traditional materials. Enter the cool roof, an ingenious development in roofing that can save up to twenty percent on energy costs and help the environment too. Herein, comes Cool Roof. A cool roof is designed to reflect more sunlight and absorb less heat than a standard roof. Cool roofs can be made of a highly reflective type of paint, a sheet covering, or highly reflective tiles or shingles. Standard or dark roofs can reach temperatures of 150°F or more in the summer sun. A cool roof under the same conditions could stay more than 50°F cooler and save energy and money by using less air conditioning. When these roofs are designed to reflect increased amount of solar radiation, the urban heat island effect can be reduced through the reduced need for cooling costs in the summer. Though a more reflective roof can lead to higher heating costs in the colder months, studies have shown that the increased winter heating costs are still lower than the summer cooling cost savings.
Cool roofs have surfaces that reflect sunlight and emit heat more efficiently than hot or dark roofs, keeping them cooler in the sun. In contrast, hot roofs absorb much more solar energy than cool roofs, making them hotter. Solar reflectance and thermal emittance are the two key material surface properties that determine a roof’s temperature, and they each range on a scale from 0 to 1. The larger these two values are, the cooler the roof will remain in the sun.
Advantages of cool roofs
Cool roofing system delivers higher solar reflectance (the ability to reflect the visible, infrared and ultraviolet wavelengths of the sun, reducing heat transfer to the building) and higher thermal emittance (the ability to radiate absorbed, or non-reflected solar energy) than standard designed roofing products.
Cool roofs achieve cooling energy savings in hot summers but can increase heating energy load during cold winters. Therefore, the net energy saving of cool roofs varies depending on climate.
Covering membranes with materials that reflect ultraviolet and infrared radiation will reduce damage caused by UV and heat degradation. White surfaces reflect more than half of the radiation that reaches them, while black surfaces absorb almost all. White or white coated roofing membranes, or white gravel cover would appear to be the best approach to control these problems where membranes must be left exposed to solar radiation.
A cool roof transfers less heat to the building below, so the building stays cooler and uses less energy for air conditioning. It also reduces air pollution and greenhouse gas emissions: By lowering energy use, cool roofs decrease the production of associated air pollution and greenhouse gas emissions.
Cool roofs can help many building owners save money while protecting the environment. This guidebook has been created to help you understand how cool roofs work, what kinds of cool roof options are available, and how to determine if cool roofing is appropriate for your building.
Substituting a cool roof for a non-cool roof decreases cooling-electricity use, cooling-power demand, and cooling-equipment capacity requirements, while slightly increasing heating-energy consumption. Cool roofs can also lower citywide ambient air temperature in summer, slowing ozone formation and increasing human comfort.
In many cases, cool roofs cost about the same as non-cool alternatives. The energy cost savings you can realize from a cool roof depends on many factors, including local climate; the amount of insulation in your roof; how your building is used; energy prices; and the type and efficiency of your heating and cooling systems. The subject areas of cool roofs, surface solar absorption and emittance, Urban Heat Islands (UHI), urban air pollution, and building and city level cooling energy requirements are all highly interrelated.
The parameters of a roof’s surface can have a large influence on the surface temperature of the roof. During clear sky conditions, up to approximately 1kW/m2 of solar radiation can be incident on a roof surface, and typically, between 20% and 95% of this radiation is absorbed (Suehrcke, Peterson et al. 2008). This large range can be explained by the influence of the surface parameters on heat gain.
How do cool roof works?
To understand how roofs can keep cool in the sun, you need to first understand solar energy. Solar energy is composed of ultraviolet (UV) rays, visible light, and infrared energy. Slightly less than half of sunlight is visible; a small fraction, less than 10%, arrives as invisible ultraviolet light, sunburns, and nearly half of sunlight arrives as invisible near infrared radiation. The solar energy from the sun strikes a rooftop and mostly reflects back into the sky. However, some of that solar energy or radiation is absorbed as heat in the roof. Cool roofs reflect more sunlight and absorb less heat than traditional roofs. Traditional dark-colored roofing materials strongly absorb heat from sunlight, making them warm in the sun and heating the building. On the other hand, cool roofs stay cooler in the sun and transmit less heat into the building.
The “coolness” of a roof is determined by two properties of roofing materials and their combined effects on temperature, namely, Thermal Emittance and Solar Reflectance Index (SRI).
Thermal Emittance describes how efficiently a surface cools itself by emitting thermal radiation. Thermal emittance is measured on a scale of 0 to 1, where a value of 1 indicates a perfectly efficient emitter. Bare, shiny metal surfaces, like aluminum foil, have low thermal emittance, which helps them stay warm. A bare metal surface that reflects as much sunlight as a white surface will stay warmer in the sun because it emits less thermal radiation.
Solar Reflectance Index (SRI) is another metric for comparing the “coolness” of roof surfaces. It is calculated from solar reflectance and thermal emittance values. The higher the SRI, the cooler the roof will be in the sun.
Both of these properties are measured on a scale of 0 to 1 (0% -100%) – the higher values, the cooler the roof. Together, these properties help roofs to absorb less heat and stay up to 50–60°F (28–33°C) cooler than conventional rooftops during peak summer weather.
A roof can qualify as cool in one of two ways. The first way is by meeting or exceeding both the minimum solar reflectance and thermal emittance values. The alternative way is to meet or exceed the minimum SRI requirement. This allows some roofs that have a low thermal emittance and a high solar reflectance (or vice versa) to still qualify as a cool roof.
Different types of cool roofs
Different roofing systems present different surface options. By selecting the right surface, you can usually make your new or existing roof cool. Here are some common roof systems. Cool Roof Coatings contain white or special reflective pigments that reflect sunlight. Coatings are like 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 as well. Here are some common cool roof systems.
Low Sloped Roofs
Single-ply Membranes – Single-ply Membranes are prefabricated sheets that are rolled onto the roof and attached with mechanical fasteners, adhered with chemical adhesives, or held in place with ballast (gravel, stones, or pavers). Some kinds of membranes, like TPO and PVC, are typically white and reflect sunlight well. Others, like EPDM, are typically black, and must be formulated differently or coated to make them reflective.
Built-Up Roofs – Built-Up Roofs consist of a base sheet, fabric reinforcement layers, and a protective surface layer that is traditionally dark. The surface layer can be made in a few different ways, and each has cool options. One way involves embedding mineral aggregate (gravel) in a flood coat of asphalt. By substituting reflective marble chips or gray slag for dark gravel you can make the roof cool.
Modified Bitumen Sheet Membranes – Modified Bitumen Sheet Membranes are composed of one or more layers of plastic or rubber material with reinforcing fabrics, and are surfaced with mineral granules or with a smooth finish.
Spray Polyurethane Foam roofs – 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. Since foams are highly susceptible to mechanical, moisture, and UV damage, they rely on a protective coating.
Steep Sloped Roofs
Shingled Roofs – Shingled Roofs consist of overlapping panels made from any of numerous materials. Fiberglass asphalt shingles, commonly used on homes, are coated with granules for protection. Cool asphalt shingles use specially coated granules that provide better solar reflectance. While it is possible to coat existing asphalt shingles to make them cool, this is not normally recommended or approved by shingle manufacturers.
Tile Roofs – Tile Roofs can be made of clay, slate, or concrete. Clay and slate tiles come from the ground, so their colors differ depending on the earth’s composition. Some varieties will naturally be reflective enough to achieve cool roof standards. Tiles can also be glazed to provide waterproofing or coated to provide customized colors and surface properties.
Low and Steep Sloped Roofs
Metal Roofs – Metal Roofs are available with natural metallic finishes, oven-baked paint finishes, or granular coated surfaces. Usually, unpainted metals are good solar reflectors but poor thermal emitters so they rarely satisfy low slope cool roof requirements, though some may still have a high enough SRI to count as a cool roof. Paint applied at the factory or in the field can increase a metal roof’s solar reflectance and thermal emittance, allowing it to achieve cool roof status. Alternatively, cool reflective coatings can be applied as with low sloped metal roofs.
Cool roofs have many benefits, from lowering indoor temperatures and improving air quality to reducing cooling costs and greenhouse gas emissions. Especially the summer season approaching, it is a great choice for your building structure. Painting the roof white, or implementing any type of cool roof helps you to conserve energy, reduce their carbon footprint and provide a more comfortable workplace for employees and clients alike. For Cool Roof rating, The Cool Roof Rating Council was created in 1998 to develop accurate and credible methods for evaluating and labeling the solar reflectance and thermal emittance (radiative properties) of roofing products and to disseminate the information to all interested parties. India is yet to catch up…what should be standard for cool roofing rating in India? We wonder.