This paper presents and analyses the performance of an integrated system with diffuse ceiling ventilation and a thermally activated building construction. A full-scale experiment is carried out in a hot box with an office setup. The performance of the integrated system is evaluated under different boundary conditions, considering different weather conditions, internal heat loads, TABS activation modes and with/without diffuse ceiling. The measurement results indicate that the diffuse ceiling plays a beneficial role improving thermal comfort in the occupied zone. However, the diffuse ceiling has the opposite effect on energy performance when TABS is activated in heating or cooling mode. Finally, the air temperature distribution in the plenum and the surface temperature distribution of the diffuse ceiling point out that the air does not perfectly mix in the plenum, the air is not evenly distributed throughout the entire ceiling area and the radiation cooling potential of diffuse ceiling is not sufficient. Thus, a further study should be conducted on optimizing diffuse ceiling and plenum design.