ENERGY STAR qualified roof coating products reduce the amount of air conditioning needed in buildings, and can reduce energy bills by up to 50%. The Cool Roof Program defines certain standards for reflectivity, and maintenance of reflectivity, in roof coating systems. These light colored roof coatings reflect the suns heat and UV rays and often lower the temperature of the roof by up to 100 degrees.
ENERGY STAR (DOE) qualified roof coating products reduce the amount of air conditioning needed in buildings, and can reduce energy bills by up to 50%. The Cool Roof Program defines certain standards for solar reflectance, 3-year aged maintenance of reflectivity, and emmisivity in roof coating systems. Reflectivity is the percentage of the sun's heat a roof keeps off a building, and emissivity is the percentage of heat a roof lets out of a building.
Lighter colored (white) roof coatings reflect the suns heat and UV rays and often lower the temperature of the roof by up to 100° degrees F.
The most common cool roof coatings used over sprayed polyurethane foam (SPF) and other roof deck surfaces are light/white colored. The most common types are:
During warmer summer months, temperatures can reach 170° degrees F on a low-slope black asphalt roof, and they can drop to -20° degrees F during the winter in northern and higher elevated regions. Much of the heat generated by absorbing the sun's ultraviolet (UV) radiation transfers to the building interior, and can have significant adverse effects on energy consumption.
Urban Heat Zones / Islands
Installing Cool Roofs in cities can have significant positive impact on the environment as well. Urban Heat Zones are caused in part by a significant number of large dark colored roofs in urban areas that can reach extremely high temperatures. This macro heat effect from hot buildings and pavement causes smog, increased energy consumption and discomfort to building occupants . A cool roof can help save the environment and save money. Many states offer utility discounts and rebates to building owners who install cool roofs. Dark colored materials absorb more heat from the sun. Good example of this can be noticed from wearing dark colored or black clothes, on a hot sunny day. Tests have shown that black surfaces in the sun can become up to 70°F (40°C) hotter than the most reflective white surfaces. If those dark surfaces are roofs, some of the heat collected by the roof is transferred inside the building.
Staying comfortable in under a dark shingle roof often means more air conditioning and higher utility bills. These roofs also heat the air around them, contributing to the heat island effect. Cool roofs can reduce the heat island effect and save energy.
In a study funded by the U.S. EPA, the Heat Island Group, http://eetd.lbl.gov/, carried out a detailed analysis of energy-saving potentials of light-colored roofs in 11 U.S. metropolitan areas. About ten residential and commercial building prototypes in each area were simulated. Both the savings in cooling and penalties in heating were considered. Estimated saving potentials of about $175 million per year for the 11 cities. Extrapolated national energy savings were about $750 million per year.
The Heat Island Group has also monitored buildings in Sacramento with lightly colored, more reflective roofs. They found that these buildings used up to 40% less energy for cooling than buildings with darker roofs. The Florida Solar Energy Center performed a similar study, also showing up to 40% cooling energy savings.
Urban Heat Islands Consume Energy and Contribute to Increased Pollution and Environmental Damage
Urban heat islands are dark areas like parking lots, highways, and commercial roofs. Because cities have higher concentrations of buildings, roads and pavement, most urban heat islands are largest and most costly in these areas. Higher temperatures in urban heat islands directly translate to increased energy use, mostly due to a greater demand for air conditioning. As increased air conditioner use takes place, power plants burn more fossil fuels. This in turn increases both the pollution level and energy costs.
For example, on warm afternoons in Los Angeles the demand for electric power rises nearly 2% for every degree Fahrenheit the daily maximum temperature rises. In total, it is estimated that about 1-1.5 gigawatts of power are used to compensate the impact of the LA heat island. This increased power costs the Los Angeles ratepayers about $100,000 per hour, about $100 million per year.
An additional consequence is that the probability of smog also increases by 3-5% for every degree °F rise in daily maximum temperature above 70°F in our cities.
The impact of these pollution levels is also seen in smog. The formation of smog is highly sensitive to temperatures; the higher the temperature, the higher the formation and, hence, the concentration of smog. In Los Angeles for example, at temperatures below 70°F, the concentration of smog (measured as ozone) is below the national standard. At temperatures of about 95°F all days are smoggy. Cooling the city by about 5°F would have a dramatic impact on smog concentration.
Los Angeles Urban Heat Island Study
Cities all over the world have been warming up in the summer over the years. Los Angeles, is a striking example of how a city was transformed into an urban heat island.
In the 1930s, Los Angeles was an area covered with irrigated orchards. The high temperature in the summer of 1934 was 97°F. Then, as pavement, commercial buildings, and homes replaced trees, Los Angeles warmed steadily, reaching 105°F and higher in the 1990s.
Currently, about 40% of the area in the LA basin is covered by buildings and roads which could realistically be made 30% more reflective during their next resurfacing. If this were done, summer temperatures in LA at 3 p.m. on an average sunny day could become 5 to 9°F (or 3 to 5°C) lower. LA would then consume 1/2 to 1 GW less in peak power, energy worth at least $100,000 per hour. Most areas would also have improved air quality, and the population-weighted average predicts an ozone reduction of 10 to 20% overall.
Because the rate of smog formation depends on temperature, this same model was used to estimate the effect on the region's smog, taking into consideration wind patterns, moisture, and other factors specific to the area. The results showed an overall reduction in smog by about 10%, the equivalent of removing three to five million cars from the roads.