Nowadays, discussions of “green building” are ubiquitous, and it would be hard to find a city in the developed world that doesn’t feature multiple active green and sustainable building organizations, both public and private. Taken collectively, it’s a complex and expensive prospect that can involve every aspect of building: materials, processes, design, energy use, appliances, heating, venting and air conditioning. But “green building” projects always feature one element that has been used since prehistory: cool roofs.
Cool roofs work in the most basic way possible: they are made of white or light-colored materials that reflect sunlight and heat, rather than absorb it, keeping both the roof – and the building it covers – cool. This, in turn, reduces the need for air conditioning, which helps save energy. A trip to Greece or the Caribbean will remind you that this is not a new concept: white-washed houses with light-colored roofs to facilitate cooling is an old idea.
History aside, black roofs have been the norm in U.S. cities since the Industrial Revolution. Groups of black roofs, in particular in urban areas, absorb heat and combine to cause significant increases in ambient air temperature – temperatures in cities can be five degrees higher than those in the suburbs because of this “urban heat island effect.” This can lead not only to heat build-up, but a reduction in air quality due to increased accumulation of ozone: with every one degree rise in temperature, there is a three percent increase in smog. The urban heat island effect can also reduce water quality by causing warmer run-off rain water to mix with colder city waterways. (Interestingly enough, the urban heat island effect can actually increase rainfall downwind of cities.) The effect can be exacerbated by the radiation of waste heat from poorly insulated buildings that do not recycle their heat. In addition, the increased heat resulting from a black roof can accelerate the rate of deterioration of the building materials, which means they need to be replaced more often.
Dark roofs can absorb up to 90 percent of the solar energy that radiates onto them, and temperatures on those roofs can reach up to 150 degrees. A white or light-colored roof, on the other hand, will absorb less than 50 percent of energy. The good news is that municipalities are starting to realize how easy the fix can be, and how generous the benefits.
In a recent memo, U.S. Energy Secretary Steven Chu of the U.S. Department of Energy announced details of a plan for a cool roof initiative in DoE operations buildings. Future DoE buildings, or any existing buildings being refurbished, will be constructed with cool roofs. The memo noted that building operations account for 40 percent of the nation’s energy use, and about 35 percent of the nation’s greenhouse gas emissions. Chu also decided to spread the cool roof love by encouraging other government entities to do the same. In a letter he wrote to the heads of other federal agencies, Chu stated, “By demonstrating the benefits of cool roofs on our facilities, the federal government can lead the nation toward more sustainable building practices, while reducing the federal carbon footprint and saving money for taxpayers.”
The Cool Roof Rating Council is a non-profit organization that maintains a third-party rating system for the radiative properties of roof building materials. The ratings system takes into account two areas: solar reflectance and the emission of thermal energy (heat). Materials are rated on a scale from 0 (the least efficient) to one (the most efficient). The CRRC offers a prime “before and after” case study example of a 100,000 square foot building in Austin, Texas that had been initially built with a black roof, but was later refurbished with a white roof. The study, conducted by the Ernest Orlando Lawrence Berkeley National Laboratory, measured weather conditions, inside building temperatures and temperatures throughout the layers of roofing material, the use of air conditioning and total building energy consumption before and after the roof replacement and compared the two sets of results.
There were significant differences between the two roofs. With the black roof, the average maximum temperature in the summer was 168 degrees. With the white roof, it was 126 degrees. The white roof allowed the building to use 11 percent less air conditioning to maintain the same comfort levels, and peak hour energy demand was 14 percent lower. In one year, the building with its white roof saved $7,200 in energy costs over the “black roof” years (a saving of seven cents per square foot per year).
Another form of a cool roof is the so-called “green roof,” which is essentially a roof that has been covered by grass or live plants, and includes an irrigation system. Green roofs can help absorb and recycle rainfall, mitigate heat absorption and help insulate the building (not to mention look attractive to humans and birds).
Once again, the idea of green roofs is not new – sod roofs have been used since prehistoric times in Scandinavia, and were standard practice for the Vikings. Builders would cover the rough wooden plank roof of a house with a layer of water-resistant birch bark, and then plant sod on top of it. In addition to being cheap and plentiful, the sod, as it grew, would actually tighten the roof, serving to keep the building sturdy, and provide natural insulation from both winter cold and summer heat, and the birch bark underneath the sod provided water-proofing for the dwelling.
A recent study showed that use of a green roof can help cut a building’s need for cooling by a minimum of 50 percent, and in some scenarios, as much as 90 percent. They can also filter out pollutants and carbon dioxide from the air, and pollutants and heavy metals from rainwater, and provide a kind of natural sound insulation.
Green roofs are also quite handy at countering an urban problem known as combined sewage overflow (CSO). During periods of heavy rain in cities, the rain water lands on roofs, roadways and parking lots, picking up a toxic soup of contaminants from paving and building materials. The flow overwhelms cities’ sewage processing systems, causing untreated sewage to overflow into the city’s rivers, bays and estuaries. Planted, green roofs retain as much as 75 percent of the rainwater that falls onto them, which eases the strain on a city’s treatment facilities and can help prevent CSO. The city of Toronto concluded that by greening a mere six percent of the roofs in the city, it could retain as much rainwater as could a storage tank costing $60 million, while at the same time reducing the heat island effect, improving air quality and saving energy.
Beyond the local effects, studies conducted by the Lawrence Berkeley National Laboratory have concluded that the use of green roofs in cities around the world has the potential of essentially canceling the heating effects of as much as two years of global carbon dioxide emissions.
– Tracey E. Schelmetic