Cool Roofs Are Ready to Save Energy, Cool Urban Heat Islands, and Help Slow Global Warming
Cool roofs are ready to save energy, cool urban heat islands, andhelp slow global warming.Cool roofing is the fastest growing sectorof the building industry, as building ownersand facility managers realize the immediateand long-term benefits of roofs that staycool in the sun. Studies exploring theenergy efficiency, cost-effectiveness, andsustainability of cool roofs show that inwarm or hot climates, substituting a cool rooffor a conventional roof can: • Reduce by up to 15% the annual air- A white roof saves energy by strongly reflecting sunlight conditioning energy use of a single- (high solar reflectance) and efficiently emitting thermal radiation (high thermal emittance). story building • Cool interior spaces in buildings that How Cool Roofs Work do not have air conditioning, making Traditional dark-colored roofing materials occupants more comfortable strongly absorb sunlight, making them • Reduce carbon emissions by lowering warm in the sun and heating the building. the need for fossil-fuel generated White or special “cool color” roofs absorb electricity to run air conditioners less sunlight, staying cooler in the sun and • Potentially slow global warming by transmitting less heat into the building. This cooling the atmosphere reduces the need for cooling energy if theIf you are installing a new roof or reroofing building is air conditioned, or lowers thean existing building, a systems approach to inside air temperature if the building is notproviding an energy efficient roof should be cooled.taken with a cool roof considered as one of The “coolness” of a roof is determined bythe options available. two properties and their combined effects on temperature: • Solar reflectance — the fraction of sunlight that is reflected • Thermal emittance — the efficiency with which a surface cools itself by emitting thermal radiation Both properties are measured on a scale of 0 to 1 — the higher the values, the cooler the roof.
Cool Roofs Save Energy and Money in • It is rarely economical to replace aMany Settings mechanically sound roof just to increaseCool choices now exist for most traditional its solar reflectance.roofing materials. Cool roofing should be • Cool versions of some roofing materials,considered whenever construction or an including thermoplastic membranes,energy retrofit is being planned. coated metal products, and clay tiles are available at little or no additional cost. • For low-slope roofs (pitch ≤ 2:12), cool Cool asphalt shingles currently sell for thermoplastic membranes, elastomeric up to $0.50/ft2 more than conventional coatings, and metal products are asphalt shingles. Cool versions of available. some other roofing materials, such • For steep roofs, cool asphalt shingle, as modified bitumen, may require a clay tile, concrete tile, and metal reflective surface coating that adds products are available. Urban Heat Islands • Lower the local air and ground (Adapted from EPA Urban Heat Island temperatures Basics Compendium) • Improve human comfort and health in uncooled buildings and outdoors http://www.epa.gov/heatisland/resources/ • Reduce the need for electricity to pdf/BasicsCompendium.pdf cool buildings Air temperatures in many urban and • Decrease emissions of nitrogen suburban areas are higher than those in oxides (NOx), sulfur dioxide (SO2), their rural surroundings; this regional carbon dioxide (CO2), and/or elevation in air temperature is called mercury (Hg) from electrical power an “urban heat island.” Many factors plants contribute to urban heat islands. In rural areas, trees and other vegetation provide shade and also reduce air temperatures by releasing water to the surrounding air. Vegetative surfaces are also moderately solar reflective. In contrast, urban areas are characterized by dry and impervious surfaces that strongly absorb light, such as conventional roofs, sidewalks, roads, and parking lots. As these urban surfaces are warmed by the sun, they heat both the interiors of buildings and the outside air. Philadelphia, PA reduced temperatures in row houses by installing cool roofs, which improves comfort Choosing solar-reflective roofing cools for occupants and may help reduce deaths from heat the buildings and the outside air. Cool waves. Baltimore, MD took similar steps following roofs in an urban area can therefore: the success in Philadelphia.
about $1/ft2 to the installed cost. • DOE’s Building Energy Software Tools • Installing a white roof on a commercial Directory provides information on 370 building yields annual energy savings building software tools for evaluating worth up to $0.20/ft2 and installing a energy efficiency, renewable energy, cool color roof on a home yields annual and sustainability in buildings. energy savings worth up to $0.05/ft2. http://apps1.eere.energy.gov/buildings/ Savings vary, and are greatest in hot tools_directory and warm climates. Use the DOE or • The DOE and the U.S. Environmental EPA calculator referred to later in this Protection Agency (EPA) each offer document to estimate your savings. online calculators that estimate the • A cool roof may increase the need for cooling energy savings and heating heating energy in winter by reflecting energy penalties attained by installing a sunlight that would otherwise warm cool roof. the building. However, winter penalties DOE calculator: can be smaller than summer savings http://www.ornl.gov/sci/roofs+walls/ even in mixed climates (locations facts/CoolCalcEnergy.htm with comparable heating and cooling EPA calculator: requirements). http://roofcalc.com • The EPA lists thousands of ENERGYHow to Select and Buy Cool Roofing STAR approved cool roofing materials.Materials http://www.energystar.gov/index.The energy and cost savings that can be cfm?c=roof_prods.pr_roof_productsachieved by using cool roofing technologies • The Cool Roof Rating Council (CRRC)depend on many factors, such as climate and maintains a program for measuringbuilding characteristics. and reporting the solar reflectances andThree aspects of cool roofing technologies thermal emittances of roofing materials.also affect their cost-effectiveness: Their Rated Products Directory lists • The aged solar reflectance and thermal over 1000 materials. emittance of the roof (since many roofs http://coolroofs.org become less reflective over time, energy savings should be based on long-term For more information, contact: values of solar reflectance and thermal Marc LaFrance, DOE Technology emittance). Development Manager for Cool Roofs • The incremental initial cost of the cool 202-586-9142, email@example.com roof (if any). Andre Desjarlais, Oak Ridge National • The incremental cost of keeping a cool Laboratory Building Envelopes Group Leader roof clean and reflective (if any). 865-574-0022, firstname.lastname@example.org Ronnen Levinson, Lawrence BerkeleySeveral tools are available to help consumers National Laboratory Heat Island Group,and professionals choose appropriate cool 510-486-7494, email@example.com materials and estimate their benefits.
The Emergence of Cool Roofs Twenty years after DOE initiated its workCool roofing is not a new concept. In the on cool roofs, these products dominate themid-1980s, researchers at DOE national commercial roofing marketplace in warmlaboratories in Tennessee and California and hot climates, partly because the Statewere measuring the energy-saving benefits of California changed its energy code toof “solar radiation control coatings” on prescribe cool roofing for most commercialtest roofs. Although energy savings were buildings with low-sloped roofs.confirmed in these early studies, they were Todaynot sufficient to lead roofing designers and Today both white and “cool color” productsinstallers to switch away from traditional are available for low-slope and steep-slopedark-colored roofing systems. roofs.A parallel effort was launched to determine • Cool products are generally economicalthe influence of light-colored roofing on low-slope roofs for commercial orproducts on summer urban heat islands. industrial buildings.This research demonstrated that solar • Cool options are available for mostreflective roofs, solar reflective pavements, traditional roofing materials.and vegetation could lower urban air • White roofs are coolest, but cool colorstemperatures, saving additional energy and are a popular alternative for roofs thatimproving air quality. can be seen by neighbors.Rolling blackouts during the summer of • The U.S. EPA lists about 3,0002001 motivated California’s public utilities ENERGY STAR® compliant cool-to expand the scope of their peak load roofing materials.management programs. They found that cool • Federal tax credits are available forroofing reduces peak demand for electricity, some cool materials, including asphalthelping to lower costs and avoid power shingles and metal products.outages. • Many state agencies and utility programs offer incentives for installing cool roofs. • Cool roof credits are offered in sustainable building programs such as the U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED).Cool roofing for residential buildings need not be white; cool colors areavailable for a number of roofing materials.