"Passive" houses are quickly becoming the houses of the future in many parts of Europe, and are slowly growing in popularity in North America.

Germany pioneered the Passivhaus (passive house), which dispenses with conventional central heating and consumes minuscule amounts of energy. Thousands have been built.

Another German house builder is taking the Passivhaus concept one ambitious step further. Baufritz is making a "carbon positive" house - a house it claims actually reduces the amount of carbon dioxide in the atmosphere. Baufritz does not boast it builds the greenest houses on the planet - a grass hut in the tropics might qualify - but its products must come close.

Baufritz says its house is carbon positive to the extent that it stores at least 50 tonnes of CO2; the equivalent of a modern family car doing around 6,000 miles per year, every year for 20 years. The house is built entirely from bio-constructive, efficient and natural materials that have not been chemically treated so that it is 100% biodegradable.

Baufritz was founded by a carpenter, Sylvester Fritz, in 1896 in Erkheim, about an hour outside of Munich. In the late 1920s, it began building prefabricated timber buildings. In the 1970s, when his granddaughter contracted cancer, the company decided it should build houses "that provide a healthy living environment that promotes well-being."