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1、 BUILDING TECHNOLOGIES PROGRAM Guidelines for Selecting Cool Roofs July 2010 Prepared by the Fraunhofer Center for Sustainable Energy Systems for the U.S. Department of Energy Building Technologies Program and Oak Ridge National Laboratory under contract DE-AC05-00OR22725. Additional technical suppo
2、rt provided by Lawrence Berkeley National Laboratory and the Federal Energy Management Program. Authors: Bryan Urban and Kurt Roth, Ph.D. Table of Contents Introduction . 1 Why Use Cool Roofs . 1 What Is a Cool Roof . 1 How Cool Roofs Work . 2 What Qualifies as a Cool Roof . 3 Reading Cool Roof Prod
3、uct Labels . 4 Not All Cool Roofs Are White . 4 Heat Gains, Heat Losses, and Thermal Insulation . 5 Types of Cool Roofs . 5 Low Sloped Roofs. 5 Steep Sloped Roofs . 6 Low and Steep Sloped Roofs . 7 Should You Use a Cool Roof. 7 Project Requirements . 7 Is Cool Roof Required or Encouraged . 8 Cool Ro
4、of Economics . 8 Online Tools to Predict Energy Savings . 12 Cool Roof Selection and Applications . 15 Precautions the amount of insulation in your roof; how your building is used; energy prices; and the type and efficiency of your heating and cooling systems. Cool roofs can also provide benefits to
5、 the environment. Cool roofs can reduce local air temperatures, which improves air quality and slows smog formation; reduce global warming by reflecting more sunlight back into space; reduce peak electric power demand, which can help prevent power outages; and reduce power plant emissions, including
6、 carbon dioxide, sulfur dioxide, nitrous oxides, and mercury, by reducing cooling energy use in buildings. Policymakers may issue regulations requiring cool roofs to provide these benefits to society. What Is a Cool Roof Cool roofs are roofs that are designed to maintain a lower roof temperature tha
7、n traditional roofs while the sun is shining. Sunlight is the primary factor that causes roofs to become very hot. 2 Figure 3: Understanding Thermal Emittance Potato skins (left), like most roofing materials, have high thermal emittance. A potato wrapped in foil (right) stays warmer longer since its
8、 aluminum surface has low thermal emittance. Image Source: Wikipedia, Free Clipart Images How Cool Roofs Work Cool roofs have surfaces that reflect sunlight and emit heat more efficiently than hot or dark roofs, keeping them cooler in the sun. In contrast, hot roofs absorb much more solar energy tha
9、n cool roofs, making them hotter. Solar reflectance and thermal emittance are the two key material surface properties that determine a roofs temperature, and they each range on a scale from 0 to 1. The larger these two values are, the cooler the roof will remain in the sun. Since most dark roofs abs
10、orb 90% or more of the incoming solar energy, the roof can reach temperatures higher than 150F (66C) when its warm and sunny. Higher roof temperatures increase the heat flow into the building, causing the air conditioning system to work harder and use more energy in summertime. In contrast, light-colored roofs absorb less than 50% of the solar energy, reducing the roof temperature and decreasing air conditioning energy use. Reducing the
