Most of the roofs in the world (including over 90% of the roofs in the United States) are dark-colored. In the heat of the full sun, the surface of a black roof can increase in temperature as much as 50 °C (126 °F), reaching temperatures of 70 to 90 °C (158 to 194 °F). This heat increase can cause negative effects on cooling energy use and environments.
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 5–14 °C (10–25 °F) above ambient temperature during the day
Savings of up to 15% the annual air-conditioning energy use of a single-story building[6]
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.[7]
Research and practical experience with the degradation of roofing membranes over a number of years have shown that heat from the sun is one of the most potent factors that affects durability. High temperatures and large variations, seasonally or daily, at the roofing level are detrimental to the longevity of roof membranes. Reducing the extremes of temperature change will reduce the incidence of damage to membrane systems. Covering membranes with materials that reflect ultraviolet and infrared radiation will reduce damage caused by u/v 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.[8]
If all urban, flat roofs in warm climates were whitened, the resulting 10% increase in global reflectivity would offset the warming effect of 24 Gigatonnes of greenhouse gas emissions, or equivalent to taking 300 million cars off the road for 20 years. This is based on the fact that a 1,000-square-foot (93 m2) white roof will offset 10 tons of carbon dioxide over its 20 year lifetime.[9] In a real-world 2008 case study [10] of large-scale cooling from increased reflectivity, it was found that the Province of Almeria, Southern Spain, has cooled 1.6ºC over a period of 20 years compared to surrounding regions, as a result of polythene-covered greenhouses being installed over a vast area that was previously open desert. In the summer the farmers whitewash these roofs to cool their plants down.
When sunlight falls on a white roof much of it is reflected and passes back through the atmosphere into space. But when sunlight falls on a dark roof most of it is absorbed and converted into much longer wavelengths that we call "heat" which cannot pass back through the atmosphere because they are absorbed by the greenhouse gases. The atmosphere is transparent to sunlight but opaque to heat, which is why white roofs help cool the planet and dark roofs warm the planet. [11]
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 5–14 °C (10–25 °F) above ambient temperature during the day
Savings of up to 15% the annual air-conditioning energy use of a single-story building[6]
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.[7]
Research and practical experience with the degradation of roofing membranes over a number of years have shown that heat from the sun is one of the most potent factors that affects durability. High temperatures and large variations, seasonally or daily, at the roofing level are detrimental to the longevity of roof membranes. Reducing the extremes of temperature change will reduce the incidence of damage to membrane systems. Covering membranes with materials that reflect ultraviolet and infrared radiation will reduce damage caused by u/v 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.[8]
If all urban, flat roofs in warm climates were whitened, the resulting 10% increase in global reflectivity would offset the warming effect of 24 Gigatonnes of greenhouse gas emissions, or equivalent to taking 300 million cars off the road for 20 years. This is based on the fact that a 1,000-square-foot (93 m2) white roof will offset 10 tons of carbon dioxide over its 20 year lifetime.[9] In a real-world 2008 case study [10] of large-scale cooling from increased reflectivity, it was found that the Province of Almeria, Southern Spain, has cooled 1.6ºC over a period of 20 years compared to surrounding regions, as a result of polythene-covered greenhouses being installed over a vast area that was previously open desert. In the summer the farmers whitewash these roofs to cool their plants down.
When sunlight falls on a white roof much of it is reflected and passes back through the atmosphere into space. But when sunlight falls on a dark roof most of it is absorbed and converted into much longer wavelengths that we call "heat" which cannot pass back through the atmosphere because they are absorbed by the greenhouse gases. The atmosphere is transparent to sunlight but opaque to heat, which is why white roofs help cool the planet and dark roofs warm the planet. [11]
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