Picture that monthly envelope from the electric company and imagine that it contains not a bill but a statement of credit-every month. That's the future of homes and other buildings, as seen by the U.S. Department of Energy. The agency hopes that its new road map for building-technology research and development will help make this green vision a reality by 2020.

Developed in conjunction with the building industry, the road map sets goals for improving building "envelopes"-walls, windows, foundations and roofs. According to Mark Ginsberg, the agency's deputy assistant secretary in the Office of Building Technology, shortcomings or defects in a building's envelope can be responsible for as much as half of its energy consumption: poor insulation wastes heat, for example, and air leaks make air conditioners work overtime. The department's funding of research for the next two decades, Ginsberg says, is meant to produce "the next generation of insulation, roofing materials and building products that will perform significantly better than what we have today."

Instead of replacing a roof, for example, you might someday be able to simply spray a plastic foam over the existing shingles that provides not only waterproofing but also an additional layer of insulation. Intelligent lighting and climate control systems could learn your preferences and adjust each room to suit your needs as you move through your home-making the most efficient use of heating, cooling and electricity. And solar cells integrated into not only roofs, but exterior walls as well, could help a building generate its own power.

"My own personal goal," Ginsberg says, "is 120 percent energy efficiency-buildings that use so little energy, and produce their own, that they give back to the grid." And, come bill time, they'd give back to their owners as well.