This paper is based on measured environmental conditions and ventilation rates during a summer period in the occupied naturally ventilated and passively cooled Environmental Building at the Building Research Establishment, Garston, UK. Initially, the comfort and air quality environmental conditions in the first (typical) floor of the building are discussed. These are found to be satisfactory. The paper then goes on to investigate the cooling effectiveness of the various ventilation flowpaths and associated thermal mass. In particular, the paper focuses on the south façade passive stacks and their relative contribution to cooling through increased ventilation. It was found that due to the relatively narrow plan of the building, the cross-ventilation flow paths could satisfy most of the summer cooling ventilation requirements. Passive stacks can enhance the cooling ventilation of the space during warm and still days, as intended by the design, and may also have the potential to assist night-time cooling due to their thermally massive structure.