Center for the Built Environment

‘Mixed-mode’ refers to a hybrid approach to space conditioning that uses a combination of natural ventilation and some form of mechanical ventilation and/or cooling. This study focuses on mixed-mode in buildings with operable windows (as opposed to natural ventilation through vents). By utilizing active cooling only when and where it is necessary, a well-designed mixed-mode building offers the potential to improve the indoor environmental quality while minimizing the significant energy and operating costs of air-conditioning. However, limited information exists on the performance of mixed-mode buildings, particularly with regard to occupant satisfaction, and evidence can be a powerful part of the argument to minimize the use of air-conditioning. The results of web-based surveys conducted in twelve mixed-mode buildings are described and compared with a benchmarking database of 370 buildings, with over 43 000 individual responses. The survey focuses on seven areas of indoor environmental performance: thermal comfort, air quality, acoustics, lighting, cleanliness, spatial layout, and office furnishings. The mixed-mode buildings are performing exceptionally well compared with the overall building stock, especially with regard to thermal comfort and air quality. The best performers were those that were newer, in more moderate climates, had radiant cooling or mechanical ventilation only, and allowed high degrees of direct user control without changeover window interlock systems.