What Makes a Roof a Cool Roof? The U.S. Department of Energy considers many factors when deciding whether a roofing product meets its definition. Specific factors are:
* High Reflectance – at a minimum 25% of the sun’s energy must be reflected off of the surface. Standard asphalt shingles in use today have a 10% reflectance rate.
* Long-lasting Reflectance – as roofs weather they begin to lose their reflectance power. A roof that’s cool should still be reflecting at a 15% rate after three years.
* High Thermal Emissivity – this describes the shingles tendency to release heat instead of storing it. The higher the emissivity, the more heat it releases. In climates that are very hot and sunny you would want a high emissivity rate whereas in colder, snowy climate you would want a roof with low emissivity so the roof holds the heat during your cold spells.
The next question we should ask is where or in what part of the United States should have cool roofs. Obviously, the greatest savings that can be achieved would be in our hotter climates. This type of roof should be beneficial in all but the northern most United States according to the Environmental Protection Agency (EPA).
This is based on the fact that energy reflected in the winter when days are short and sunlight is at a minimum, is usually less than the unwanted energy absorbed during the hot days of summer. If the climate you reside in has three months of cold weather you would be a good candidate for this type of roof. A cool roof might also be a good idea for a home that has their duct system in an unconditioned attic or have a house with a roof area that is 25% or more of the total exterior surface. Houses that are extremely hot in the summer or find that their shingles age prematurely because of the sun might also be in line for a new roof.
What are the trade-offs if you install this type of new roof? Cool roofs in hot, humid climates may find that their shingles are experiencing mold growth as well as algae growth. This results because the roofs do not reach temperatures necessary to kill off this type of growth. The algae growth may also cause black streaks on your light-colored cool shingles. Manufactures of cool roofs are combating this by including copper or zinc additives that helps prevent the mold and algae growth.
These types of roofs are a little or no more expensive than traditional roofs. In some situations a cool roof will cost no more than $1000 to $2000.