subject: How Cool Roofs Can Save The Environment [print this page] A cool roof is an innovation in construction that ensures that terraces stay relatively cool when compared to the ambient temperature. This is possible because they reflect the insulation (incoming solar radiation) thereby reducing the heat absorbed by the roof and thereby the building and improving occupancy comfort.
Most of the terraces in the world (including over 90% of the terraces in the United States and India) are dark-colored. When the sun is at its highest, the surface of a dark colored roof can increase in temperature by as much as 50 C (126 F), reaching temperatures of 70 to 90 C (158 to 194 F). This is essentially because of the simple fact that dark colors and black absorb the solar radiation compared to mild colors and white. This heat increase percolates down the roof and increases room temperature. This means that the air condition system needs to work harder to compensate for it. This leads to increased power consumption and that means a larger carbon footprint.
Cool roofs, on the other hand, offer both immediate and long-term benefits including:
Reduced building heat-gain, as a white or reflective roof typically increases only 514 C (1025 F) above ambient temperature during the day
Savings of up to 15% the annual air-conditioning energy use of a single-story building
Extended service life of roofs
Improved energy efficiency of roofs, especially when there isn't adequate insulation provided in the roof envelope
Improved thermal comfort in buildings that do not have air conditioning
Reduced air pollution and greenhouse gas emissions, as well as a significant offsetting of the warming impact of greenhouse gas emissions.
Cool canopies are used in commercial and industrial buildings and fall into three basic categories - roofs made from cool roofing materials, roofs made of materials that have are coated with solar reflectors and finally green planted roofs.
Inherently cool roofs
White vinyl canopies, which are made up of thermoplastic white vinyl, for example, can reflect 80% or more of the sun rays and emit 70% of the solar radiation that the building absorbs. An asphalt canopy only reflects between 6 and 20% of solar radiation, resulting in obvious disadvantages as discussed before.
Coated crowns
Roofs can be made up reflective by applying a solar reflective coating to its surface. The reflectivity and emissivity ratings for over 1000 different reflective roof products can be found in the CRRC (cool roof rating council) website.
Green terraces
Green terraces provide a thermal mass layer, which helps in reducing the flow of heat into a building. The solar reflectance of green roofs varies depending on the plant types. Green roofs reflect less sunlight and absorb more solar heat than white roofs. The absorbed heat in the green roof is trapped by the greenhouse effect and then cooled by evaporate-transpiration.
Given the number of advantages, it is critical that cool roofs should not be a luxury for an exclusive few. Further investment in technology and research will ensure that better and cost effective solutions are developed. But one thing is certain- cool roofs are there to stay.
