In 2003 the new extension of the German Museum of Technology (Deutsches Technikmuseum, Berlin), which comprises around 12,000 m?? of exhibition space was completed as a low-energy building. Planning and building this construction took almost 20 years (from 1987 to 2003). Validation measurements have been conducted since 2004. The building has been monitored by the Fraunhofer Institute for Building Physics IBP and IBUS (Institut fuer Bau und Umwelt, Berlin) during construction, in the commissioning phase, and during the period of validation measurements. The inner leaf of the building's external walls and the interior walls were built from materials (KLB bricks) that easily absorb humidity from the indoor air to rerelease it back into the room when the level of indoor-air humidity has decreased. In this way it was not necessary to provide an expensive airconditioning system which would also have caused high operational costs. For structural reasons, a crawlspace had been designed beneath the thermally insulated floor slab. Part of the supply air is conducted through this crawl space before it is transferred into the museum building. In this manner, the air is preheated in winter and it is cooled in summer. As the validation measurements have proved, this procedure results in substantial energy savings.