How and Why Cool Roofs Work
There are two things that keep a roof cool. High reflectivity and high emissivity. Cool roof coatings reflect 70%- 90% of the sun’s energy when newly installed. Most of the energy is radiated away. Conventional roofing materials have reflectivity of 5% to 25%, meaning they absorb 75% to 95% of the sun’s energy. A typical “white” cap sheet roof only reflects 25% of the sun’s energy. The absorbed energy is transferred to the building.
Emissivity is the ability to radiate away absorbed heat. Most cool roof coatings emit 90% or more. Even though aluminum coated roofs are highly reflective they are not considered a cool roof because they only emit 50% to 60% of the absorbed energy.
In South Florida conventional roofs can get up to 180 degrees on hot summer days. A cool roof coating keeps the roof at just 10-15 degrees above the air temperature. This means a cool roof can be 80 to 90 degrees cooler than a conventional roof.
Cooler roof surfaces means less heat is transferred into the building. If you air condition your building you will save money on cooling energy. How much you will save depends on many factors including roof insulation, cooling system equipment and other variables. In the right situation, a reflective roof can bring cooling savings of up to 50% with a reduction in peak cooling demand of 10%-15%.
Your Roof Will Last Longer With A Cool Roof
Two enemies of long roof life are heat and ultraviolet light.
In the cool of the evening, a roof contracts. During the day it expands due to the heat it absorbs. This constant movement is called thermal shock. Roofs with a cool roof coating do not experience large temperature fluctuations so there is less stress on the roof membrane.
On a cloudy day at the beach you can get severe sunburn because of the ultraviolet rays of the sun. Your roof is affected by ultraviolet light even on cloudy days. Heat from the sun accelerates the breakdown process of your roof. An acrylic coating not only reduces heat dramatically, it blocks 100% of ultraviolet light.
A Cool Roof Coating Pays For Itself Over Time!
A considerable sum of money will be saved each year on lower energy costs with a cool roof. Roof life will be significantly increased. In fact, according to an article written by an expert on cool roof research, Dr. Lisa Gartland; “Cool coatings are assumed to greatly slow down, if not stop, the again process of the underlying roof materials…they can applied indefinitely for decades before the old roof must be torn off and replaced.”
Are All Cool Roof Coatings Equal?
Some coatings start out as very reflective but after a few short years, they lose much of their reflectivity. CRRC (Cool Roof Rating Council) compiled a list of field applied roof coating products. This list gives an unbiased rating on the performance of competing materials. The full list can be examined online at www.coolroofs.org.