A team of scientists participated in the "Passive House Preparatory Research Project" – an international cooperation in which Bo Adamson and Gerd Hauser were also involved. With funding by the State of Hesse, a systematic research on the requirements for energy-efficient buildings has been done and prototypes of new building components were developed and produced: including insulated window frames, reduced thermal bridges and CO2-regulated ventilation. In 1990/91, based on plans by Prof. Bott/Ridder/Westermeyer, four terraced house residential units were built by for families in a joint venture. The houses have been occupied since 1991. An accompanying monitoring programme provided information about super-insulated building components, windows, ventilation heat recovery, user behaviour, indoor air quality, amount of internal heat sources and much else.