nef occupies a purpose built low energy building designed to demonstrate some of the features that it espouses. built in 2002-3, its office is heated by a ground source heat pump, with hot water being partly provided by an evacuated tube solar water heating system. there is a winter back-up heating system in the form of a wood pellet stove. the building is naturally ventilated, with "brise-soleils" to prevent overheating in summer, and use of daylight is encouraged through light tubes ('sunpipes'). around a fifth of the office's energy is supplied by a 6.47kwp photovoltaic array. as well as energy features, its environmental credentials include the use of rainwater for toilet flushing and a low thermal mass sustainably sourced timber frame structure.