‘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. 

“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.

Designs for low-energy office buildings increasingly incorporate operable windows for the benefits of personal control, environmental quality, and architectural value. However, integrating operable windows with mechanical systems to achieve their full benefits is an unresolved energy challenge. If operable windows are left up to the control of the occupants, designers run the risk of putting unpredictable or unnecessary loads on the HVAC system, causing air pressure balancing issues, or causing unreliable or unwanted air change rates. However, if windows are automated for natural ventilation, the building design loses the comfort benefits, amenity, appeal and robustness of manually-controlled windows.

New buildings with operable windows situate themselves in an ongoing debate about the efficiency benefits of manual versus automatic building control. The decision to use manually-operated windows rests on assumptions about their value and a faith in the idea that occupants will actively participate in maintaining indoor environmental quality (compared to a conventional, sealed office environment). Concurrent with this debate, there has been growing interest in occupant behavior as the new low- hanging fruit for improving a building’s energy performance and closing the gap between predicted and actual performance. Emerging research that is relevant to these topics includes studies that better characterize and model behavior, as well as those that look at the impact of information systems and feedback on energy-using behaviors. However, these research questions are seldom considered together. This literature review draws together a number of topics that contribute to our understanding of building-occupant control interactions in mixed-mode buildings, beginning with the known (and as- yet unresolved) benefits and limits of operable windows when combined with mechanical cooling.