In the world of industrial and commercial buildings, a roofing system that can deliver high solar reflectance (the ability to reflect the visible, infrared and ultraviolet wavelengths of the sun, reducing heat transfer to the building) and high thermal emittance (the ability to radiate absorbed, or non-reflected solar energy) is a cool roof . Most cool roofs are white or other light colors.
In tropical Australia, zinc-galvanized (silvery) sheeting (usually corrugated) do not reflect heat as well as the truly "cool" color of white, especially as metallic surfaces fail to emit infrared back to the sky . European fashion trends are now using darker-colored aluminium roofing, to pursue consumer fashions.
Cool roofs enhance roof durability and reduce both building cooling loads and the urban heat island effect.
Also known as albedo, solar reflectance is expressed either as a decimal fraction or a percentage. A value of 0 indicates that the surface absorbs all solar radiation, and a value of 1 represents total reflectivity. Thermal emittance is also expressed either as a decimal fraction between 0 and 1, or a percentage. Another method of evaluating coolness is the solar reflectance index (SRI), which incorporates both solar reflectance and emittance in a single value. SRI quantifies how hot a surface would get relative to standard black and standard white surfaces. It is defined such that a standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100. The use of SRI as a combined measurment of reflectance has been disputed, since it has been shown that two different products with identical SRI numbers can yeld significantly different energy savings results depending on what geographic region they are applied in, and the climatic conditions present in this region.
Cool roofs are an effective alternative to bulk attic insulation under roofs in humid tropical and subtropical climates. Bulk insulation can be entirely replaced by roofing systems that both reflect solar radiation and provide emission to the sky. This dual function is crucial, and relies on the performance of cool roof materials in both the visible spectrum (which needs to be reflected) and far infra-red which needs to be emitted.
Cool roof can also be used as a geoengineering technique to tackle global warming based on the principle of solar radiation management, provided that the materials used not only reflect solar energy, but also emit infra-red radiation to cool the planet. This technique can give between 0.01-0.19 W/m 2 of globally-averaged negative forcing, depending on whether cities or all settlements are so treated. This is generally small when compared to the 3.7 W/m 2 of positive forcing from a doubling of CO 2 . However, in many cases it can be achieved at little or no cost by simply selecting different materials. Further, it can reduce the need for air conditioning, which causes CO 2 emissions which worsen global warming. For this reason alone it is still demonstrably worth pursuing as a geoengineering technique.
Benefits of cool roofs
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 (90 °F), reaching temperatures of 66 to 88 °C (150-190 °F). This heat increase can contribute to:
- Increased cooling energy use and higher utility bills;
- Higher peak electricity demand (the maximum energy load, in megawatts, an electric utility experiences to supply customers instantaneously, generally experienced in summer late afternoons as businesses and residences turn up their air conditioners), raised electricity production costs, and a potentially overburdened power grid;
- Reduced indoor comfort;
- Increased air pollution due to the intensification of the "heat island effect"; and
- Accelerated deterioration of roofing materials, increased roof maintenance costs, and high levels of roofing waste sent to landfills.
Any building with a dark colored roof, but particularly large buildings, will consume more energy for air conditioning than a “cooler” building – a strain on both operating costs and the electric power grid. Cool roofs offer both immediate and long-term savings in building energy costs. White reflective membranes, metal roofing with "cool roof" pigments, coated roofs and planted or green roofs can:
- Reduce building heat-gain, as a white or reflective roof typically increases only 5–14 °C (10–25 °F) above ambient temperature during the day.
- Create 15–30% savings on summertime air conditioning expenditures.
- Enhance the life expectancy of both the roof membrane and the building’s cooling equipment.
- Improve thermal efficiency of the roof insulation; this is because as temperature increases, the thermal conductivity of the roof’s insulation also increases.
- Reduce the demand for electric power by as much as 10 percent on hot days.
- Reduce resulting air pollution and greenhouse gas emissions.
- Provide energy savings, even in northern climates on sunny (not necessarily “hot”) days.
Note that today's "cool roof" pigments allow metal roofing products to be EnergyStar rated in dark colors, even black! They aren't as reflective as whites or light colors, but can still save energy over paints used just a few years ago.
Energy calculators
Calculating cost savings resulting from the use of cool roofs can be done using several tools developed by federal agencies.
U.S. Department of Energy (DOE) Cool Roof Calculator
This tool developed by DOE's Oak Ridge National Laboratory estimates cooling and heating savings for low slope roof applications with non-black surfaces.
ENERGY STAR Roofing Comparison Calculator
This tool developed by the U.S. EPA calculates the net savings accruing from installing an ENERGY STAR labeled roof product on an air conditioned building. In addition to cooling savings, the program considers any resulting differences in heating costs.
Cool roofs in cool climates
No matter where cool roofs are installed, they cut down on the urban heat island effect, and usually lower a building’s carbon footprint. Even in climates where there are more heating days than cooling days, white reflective roofs may be a worthwhile investment for many reasons. The cooling benefits of a highly reflective roof surface far outweigh the potential winter month heating benefits of a less reflective, or black, roof surface. Energy calculators generally show a yearly net savings. This is true because the sun is lower to the horizon in winter and not hitting the roof as directly or as intensely as it would in summer, it shines fewer hours and there are more cloudy days, and snow cover reflects the sun’s energy. Another reason: because cool roofs cut peak use during the summer when rates are the highest, they can help reduce the demand charge that a building pays all year on the basis of its greatest energy use.
Additionally, higher R values for insulating materials required by many sustainable design guidelines negate the incidental heat gain during winter months resulting from the use of dark roof materials. Snow on roofs also provides insulation; for a medium density of snow the resistance per 25 mm is about 0.110 (m 2 -°C)/W, 300 mm of snow cover can provide an equivalent of 50 mm of good insulating material. Cool roofs contribute to the retention of snow on roofs in moderate snow fall areas; thereby improving insulation factor of the assembly to a greater extent than the benefit of solar gain during winter months. There can be a 26 °C differential in membrane temperature between areas having 300 mm of snow cover compared to areas having no snow.
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.
There are some studies that have shown that reflective roofs are not always best in cool climates. Benchmark Inc. did a study in five different cities and used the energy star calculator and the DOE calculator to find the annual savings. Because the DOE calculator includes differences in heating losses, there were significant differences between the savings in all of the cities. However, in Chicago, the annual savings became slightly negative in one of the models because of heating costs. The following graph shows the results:
Calculations performed using the DOE Energy Star Calculator show that high-reflectivity, medium-emissivity roof coatings, such as aluminum roof coatings can yield greater savings in
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