Center for the Built Environment

“Mixed-mode” refers to a hybrid approach to space conditioning that uses a combination of natural ventilation from operable windows (either manually or automatically controlled), and mechanical systems that provide air distribution and some form of cooling (air-conditioning, radiant cooling, etc.). By utilizing mechanical cooling only when and where it is necessary to supplement the natural ventilation, 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. But there is limited information about the performance of mixed-mode buildings, particularly with regard to occupant satisfaction, and this can potentially be a powerful part of the argument to avoid or minimize the use of air-conditioning.

This paper describes the results of web-based surveys conducted in 12 mixed-mode buildings, in comparison to our overall benchmarking survey database of 370 buildings, with over 43,000 individual responses. The survey focuses on seven areas of indoor environmental performance, including thermal comfort, air quality, acoustics, lighting, cleanliness, spatial layout, and office furnishings. The data shows that only 11% of the 370 buildings, most of which have conventional air-conditioning systems, are meeting the intent of the thermal comfort standards to achieve 80% satisfaction in the buildings. In comparison, the mixed-mode buildings are performing exceptionally well compared to the overall building stock, especially with regard to thermal comfort and air quality. Among the mixed-mode buildings, the best performers were those that were in more moderate climates, were newer, had radiant cooling or mechanical ventilation only (instead of an air-cooled system), and allowed high degrees of direct user control without changeover window interlock systems.