On Unst, northernmost island of the Shetland Isles is what may be the world’s first zero carbonbit of a slippery fish. It tends to mean that a building uses no carbon (oil, coal, etc) to heat it (meaning in a 'net' way). It usually ignores the carbon which goes into building it (the embodied energy). See the page on Zero Carbon? house. The owner/self builders, Michael and Dot Rea have a web site giving details of the project and there is a video on the Guardian web site.