How does airflow under a metal roof further enhance energy savings?
Metal roof coatings with solar reflectance can help building owners save substantially in annual cooling costs. Research has confirmed that creating an air space under a metal roofing system will increase energy savings during both summer and winter months.
A study was funded by the U.S. Department of Energy and conducted by the Oak Ridge National Laboratory to document energy savings of metal versus asphalt roofs. Scientists selected an asphalt shingle and a stone-coated metal shake for field-testing—both had comparable solar reflectance and thermal emittance levels. The asphalt shingle was directly nailed to the roof deck, with no air space underneath, while the dark-gray metal shake was attached to a batten/counter-batten system that allowed for airflow underneath. The result yielded a 45-percent reduction in heat flow for the metal shake; about 15 percent of that reduction was attributed to solar reflectance, and an additional 30 percent of the heat flow reduction was due to above-sheathing ventilation.
In cool or cold weather conditions, scientists confirmed that direct-nailed asphalt shingle roofs had significantly larger heat loss than attics with metal shakes and above-sheathing ventilation. The air gap appears to serve as an insulating layer, reducing heat transfer by 50 percent as compared to asphalt.
The study results have been corroborated by Florida Solar Energy Center and accepted by the ASHRAE SSPC 90.1 subcommittee, indicating that any type of metal roof can save a home or building owner up to 25 percent in annual cooling energy costs compared to a dark-gray asphalt shingle.
