Urban Heat Islands and Cool Roofing


    urban heat islands
    • Thermal satellite image of Atlanta Georgia.
    • Dark colored roads, parking lots and roofs cause a raise temperature as much as 12 degrees Fahrenheit and are shown in red islands.
    • An increase in ambient temperature causes an increase in ground level Ozone.

     

    Sketch of an Urban Heat-Island Profile

    urban heat island profile

    Temperatures in urban areas can increase 8 to 12 ºF because
    of dark colored roads, parking lots and roofs.

     

    What's Hot and What's Not!


    Sacramento

    Aerial thermal photograph of Sacramento showing cool areas and hot spots.

    key to colors

     

    urban environment albedos

    What can be done to reduce the Heat Island Effect?
    • Plant more canopy type trees
    • Use lighter colored materials for roads and parking lots
    • Use high Albedo materials for roofing

    What is Albedo?
    • Reflectivity + Emissivity = Albedo
    • Reflectivity: the percentage of incident light or
    electromagnetic radiation that is reflected.
    • Emissivity: the rate at which absorbed energy is
    radiated from an object.

    Dark Asphalt has a very low Albedo which means
    that it absorbs more heat than an object with a high Albedo
    such as a highly reflective white roof.

     

    A High Albedo roof system reduces cooling energy costs by reflecting away most of the solar energy that strikes it's surface.

     

    Cap Sheet Roof

    cap sheet roof


    Thermal photograph of a Cap Sheet roof.
    The upper portion has been coated white and appears blue
    indicating a lower surface temperature.

     

    Billions of metric tons of carbon
    global carbon dioxide emissions

    Data collected by Oak Ridge National Laboratory and published in "Trends '93: A Compendium
    of Data on Global Change" shows an overall increase in global carbon dioxide emissions from fossil fuels in the air.

     

    Ground Level Ozone


    Ground level ozone is a chemical reaction between VOC's (Volatile Organic Compounds) and oxides of Nitrogen
    (NOx) in the presence of sunlight and heat to create Ground Level Ozone.

    Also Known As SMOG

    color of roof surface chart

    The graph depicts surface temperatures of various
    roofing materials taken on a 90ºF day in central
    Texas. Thermocouples were placed below the surface
    of various roofing materials This test shows that a black
    roof can be as much as 100 degree hotter than
    a white colored roof.

    A High Albedo roof system reduces
    cooling energy costs by reflecting away most of the
    solar
    energy that strikes it's surface.

     

    ASTM Standards for Solar Reflectance
    • 0=White. 100=Black
    • Calculates for booth reflectivity and emissivity
    ASTM 1980 is the standard for calculating the
    Solar Reflectance index. Used in the field to
    measure larger surfaces such as roofs.
    • ASTM laboratory standards for measuring a sample materials Albedo
    • ASTM E903 Thermal emissivity
    • ASTM E40 Solar reflectance Index

    ASHRAE Absorptivity Ratio
    • White materials would provide an Absorptivity ratio of
    approximately 0.2 indicating that only 20% of the solar energy
    that reaches the surface is absorbed. And 80% is radiated
    or reflected into the surrounding air.

    EPA Energy Star Roofs

    Must meet EPA's specifications for solar reflectance
    and reliability to get this label.