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.
"For at least one-third of the building we have really strict requirement for temperature and humidity control," he explained. "The collection zones and the exhibit areas need to be maintained at 72°F and 50 percent relative humidity, and that can be +/- 3. That’s a really narrow band to be working within. Then you open the doors in the morning and all of a sudden you have this herd of people come in and it sends the system out of kilter. So you’re gunning the AC in the summer, and in the winter in those areas we’re literally cooling and heating at the same time, introducing humidity or extracting humidity from certain areas and running air conditioning chillers to draw out the moisture in the air in some spaces."
The green roof could spare the museum some replacement expenses down the road, too. The trade group Green Roofs for Healthy Cities suggests that green roofs help lengthen the life of the roof membrane underneath it and the facility’s waterproofing from temperature differentials, and the resulting expansion and contraction that normally shorten a roof’s life.Melissa Muroff, president of Philadelphia architectural design firm Roofmeadow, estimates green roofs add at least $10 to $15 per square foot to the cost of a roof, but that can be offset by incentives from cities that recognize their other important benefit: managing stormwater runoff.
"Green roofs capture rainfall and prevent a minimum of 60 percent to 70 percent of that rainfall from entering the city sewer system," Muroff noted. "For many cities struggling to comply with the Clean Water Act, like Philadelphia, diverting rainwater from the overtaxed sewer systems is a priority. Philadelphia and other cities offer a variety of permitting, zoning and tax incentives to incorporate nature-based stormwater practices, including green roofs, as part of their citywide stormwater management plans."
Rainwater that the museum’s green roof doesn’t suck up is collected in an underground retention basin for reuse as makeup water in the cooling tower. "In a very hot spell in July," McDevitt said, "we’re saving 18,000 gallons a day."
The museum's green roof and innovative stormwater management and heat-recovery systems last year earned it the coveted Gold LEED (Leadership in Energy and Environmental Design) rating system, developed by the U.S. Green Building Council (USGBC), the nation’s most widely recognized and accepted green building rating system.