Urban Heat Islands and Cool Roofing
- 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
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!
Aerial thermal photograph of Sacramento showing cool areas and
hot spots.
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
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
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
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.
