I've got a green innovation for you: drywall. The stuff may seem about as low-tech as it gets, but take a look around you — we use a lot of it. Drywall production is energy-intensive, requiring gypsum and a lot of heating — enough to emit 20 to 25 billion pounds of CO2 a year, estimates Kevin Surace, CEO of the green construction company Serious Materials. That's why his company invented EcoRock, an environmentally friendly drywall whose production requires little heat and emits vastly less carbon. It's one of many products Serious Materials manufactures to help reduce the energy consumption associated with the built environment, which is estimated to account for almost half the greenhouse gas emissions in the U.S. "The average consumer thinks his car is the big carbon emitter," says Surace. "But the built environment is the big kahuna." It's another sign that carbon savings can be found in unlikely places.