Sourced from KYW News Radio 1060
In Ben Franklin’s time, a green roof would have been the sod roof that a random Brandywine Valley Swedish settler might have installed; common in Scandinavia, and not widely adopted by colonists here. So our Founding Science Geek would likely have been fascinated by the plantings atop today’s cutting-edge museum devoted to his (and his colleagues’) finest hour: The Museum of the American Revolution.
It’s not just about bragging rights to the biggest meadow in Old City (although that’s a fun fact.) Operations Director John McDevitt explained that, like the sod roofs of old, the four planted sections on the Museum roof serve as an added thermal barrier.
"In each there's six inches of moist soil and all this plant material," he said. "So the sun is not hitting the roof directly and then transferring that heat to the spaces below, it’s being absorbed by that cool soil, and it gives us an added barrier along with what we have in the building as basic insulation."
Insolation — no typo — is what creates the heat island effect in cities. Roads and building materials absorb solar radiation and warm the air around them, making our Philly streets sizzle in the summer and blow-torching the air conditioning budget. It’s no wonder it’s also a synonym for "sun stroke."
Plant matter on a roof absorbs less heat from the sun and cools the air by evaporating water. One study estimates a cooling effect of 6 °F to as much as 20 °F. It’s hard to care about cooling in midwinter, but lowering heat absorption by a building in the summer reduces energy consumption, a year-round concern for the museum, which McDevitt says is a very expensive building to maintain because of the air quality requirements for preserving its historic artifacts.