Daylighting systems in commercial buildings can produce various benefits such as maximizing daylight penetration, optimizing visual comfort and reducing energy consumption. Optimizing and balancing the desirable and unpleasant effects of a daylighting system can be a challenge and requires a comprehensive study, with both measurements and simulation. This study examines the effectiveness of installing a controlled semi-silvered reflective louvre system in the clerestory portion of a direct solar (north) facing facade system in a deep cellular office space. On-site measurements were made to evaluate the performance of the system. Simulation and correlation studies have been carried out to identify the daylight contribution of the louvre system. The energy-saving potential and cost benefits of the daylight-guiding system were predicted using energy simulation. Experimental results show that the reflective louvre system can provide up to 70% additional illuminance at the workplane level under clear sky conditions. However, the system failed to produce a reasonable cost saving to the office space and has the drawback of creating contrast and casting light patterns onto the room surfaces at different times of the day. (C) 2013 Elsevier Ltd. All rights reserved.