“Cool Roof” Coatings
The application of coatings to dark roof surfaces is certainly not a new concept. Aluminum coatings have been long used to protect asphalt based roof systems against UV degradation. In recent years roof coatings have become increasingly popular as an attempt at retrofitting an existing roof system into an energy efficient “cool roof”.
Several government officials are strongly encouraging the application of cool roof coatings. In addition to the regulations passed by local governments, including New York City and Philadelphia, the Federal government has also offered support. US Energy Secretary Steven Chu has required the installation of white reflective roofs on all new construction and roof replacement for Federal government buildings. Secretary Chu has also endorsed coating all existing roofs white. While we all agree that energy conservation is a fiscally and socially intelligent endeavor, it requires more thought and consideration then wholesale roof painting.
As we are all aware the climate in Southern Florida is vastly different from Northern Maine. To a lesser extent the weather experienced in the Washington DC area differs from that of the NY metro area. So, to state that a white roof is best for the entire country is misguided. Secretary Chu’s requirements do however call for a minimum thermal resistance of R-30 for all roof systems. The new ASHRAE standards are also calling for increasing the R-value of a roof system, which will likely be adopted by the next International Building Code and trickle down locally from there. Increased insulation is a wise mandate for the entire country as it will prevent the loss of conditioned air and the migration of outside air on both hot and cold days.
Points to Consider
A white, or reflective, roof surface will prevent much of the sun’s UV radiation from transferring heat into the building, On a hot summer day this is an obvious benefit, however in the winter the additional heat gain realized by dark roofs are a welcome relief. Darker roofs will also aid in melting snow, which reduces the structural impact and potential water infiltration that result from significant snowfalls.
To truly determine the best way to improve the energy efficiency of a roof, the condition of the entire existing roof system, including insulation and deck composition, should be assessed. Deteriorated roof systems, even if not allowing leaks into the building’s interior space, may be allowing air infiltration as well as saturation of the existing insulation, significantly reducing the insulation’s thermal resistance. Ponding water is also an issue for many roof coatings.
When considering the application of a roof coating the following items should be analyzed:
- The compatibility between the proposed new coating and the existing roof. Not all roof coatings are created equal, some will have deleterious effects on various roof systems, leading to premature failure.
- The manufacturer’s warranty for the existing roof should be examined to determine if the coating will void the warranty.
- The anticipated maintenance should also be considered. After all there is no point going to the trouble and expense of coating a roof white, if it will be dark with dirt and debris in several months.
- The expected lifespan of the coating is important to understand, to identify how often re-application will be required.
In some cases, application of a roof coating may be the best option to improve energy conservation; however installation of an overlay roof or full roof system replacement should also be considered, as they may in fact have a reduced life cycle cost and provide the greatest energy conservation improvement.