The term "green building" is wide-ranging and applies to many different facets of environmentally conscious building. The terms zero-carbon and zero net-energy, however, are much more specific. Both zero-carbon and zero net-energy homes seek to use only as much energy as they can produce. Zero-carbon (or carbon neutral) homes also use no fossil fuels and therefore produce no greenhouse gasses. The U.K. has set a goal to require all new homes to be zero-carbon by 2016, and the city of Copenhagen is gunning to become the world's first carbon neutral capital by 2025.
To achieve either carbon neutrality or zero net-carbon status, builders must employ a number of green building techniques. External walls are made of thick, insulated concrete to protect interiors from temperature fluctuations. Geothermal wells capture water warmed by rooftop solar panels and circulate it through radiant heated floor systems or closed-loop cooling systems. Roofs may be V-shaped to collect water and hide solar panels from view.