A major concern of those wishing to limit the energy use in buildings is the growing trend towards installing air-conditioning in new and refurbished buildings. Building design, high thermal loads, and a desire for perceived comfort, contribute to an ever-growing demand for full air-conditioning. Therefore, to counter the impact on building energy use, it is essential that building design and operation is developed to minimise the use of air conditioning systems. The Airlit - PV project has taken this challenge and has designed an innovative faade unit incorporating the latest thinking in natural ventilation, night cooling, fuzzy control and photovoltaic power. This part funded DETR Partners in Innovation project has developed an integrated modular faade unit that can reduce, or possibly eliminate, the need for air conditioning in appropriate buildings. The Airlit - PV unit adopts an integrated approach to the role of the building envelope by: Providing controlled natural ventilation; Optimising night cooling; Controlling solar gain; Utilizing PV power to operate the local intelligent control unit. The design of the first prototype was reached by adopting conventional components and adapting them as required. The main intention was to produce a working unit from which development could proceed. This first prototype unit and results of its performance have already been reported (1) . The main conclusion of the testing carried out over the summer of 1999 was that by a combination of daytime and night-time cooling, based on internal and external temperatures and occupant control, the Airlit-PV unit could improve internal office conditions. However, these results, together with simulation studies, showed that the design could be enhanced by a number of modifications, both to the physical construction and the control and operation. This paper reports the developments made to the first prototype unit and the results of further testing.