Ask anybody you know if they would prefer to have an operable window in their office, and chances are you’ll find few people who would say “no.” So why are so many office buildings designed with sealed glass facades that don’t allow occupants to open a window and get a (real or imagined) “breath of fresh air”? Has our culture become addicted to air conditioning? How can we encourage engineers to be truly innovative, to move beyond conventional practice by designing or retrofitting buildings that provide the benefits of both natural ventilation and air conditioning?

Most contemporary office buildings are designed with sealed envelopes, are internal-load dominated, and are cooled by mechanical air conditioning systems. The hvac system is designed to maintain constant, uniform conditions throughout the interior, but at a significant cost in terms of capital, energy consumption, and associated environmental impact. Mechanical cooling and fan energy accounts for approximately 20% of commercial building electrical consumption in the United States.

Furthermore, occupants have little opportunity to adjust the systems for their personal comfort preferences (which can vary significantly), leaving the centralized controls to satisfy, at best, only a percentage of the occupants at any one time. It is, therefore, no surprise that many studies have found that the top complaint in office buildings is typically related to thermal comfort.

In contrast, naturally ventilated buildings are generally skin-load dominated, rely on operable windows for both ventilation and cooling, and require user interaction to maintain comfort conditions in the building. Naturally ventilated buildings tend to use much less energy than air-conditioned buildings. Occupants often have significant control over their personal comfort conditions, and there is a distinct connection between the outdoor and indoor environments. The thermal environments in naturally ventilated buildings are typically more variable and less predictable than those found in air-conditioned buildings, but not necessarily less comfortable.

Recent ASHRAE-sponsored research conducted by co-author Dr. Gail Brager demonstrated that occupants of naturally ventilated buildings are comfortable over a much wider range of temperatures compared to occupants of air-conditioned buildings, primarily because the higher degree of personal control shifts their expectations and preferences. The next revision of ASHRAE Standard 55 will incorporate a new “adaptive model” of thermal comfort based on this research, but as yet it will only be applicable to naturally ventilated buildings without mechanical cooling systems.

While nearly all engineers would probably agree that occupants prefer operable windows, most of them are unwilling to relinquish the tight control over interior temperatures that a mechanical system provides. So why not use both, and design a “mixed mode” building?

The test building in Palo Alto, CA uses a VAV system and allows individual control of thermostats. With its "changeover" system, micro-switch sensors in the windows shut off a zone VAV box if the corresponding window is open.

What is a Mixed-Mode Building?

Mixed-mode refers to a hybrid approach to space conditioning that combines natural ventilation with mechanical ventilation and cooling. A well-designed, mixed-mode building often incorporates sophisticated controls that allow the building to alternate between these two modes during different times of the day or season, to maximize comfort while avoiding the significant energy use and operating costs of year-round air conditioning.

Mixed-mode is appropriate both for the design of new buildings, as well as the retrofit of air conditioning into older naturally ventilated buildings in which internal gains have increased significantly due to higher occupant densities or equipment loads. Mixed-mode cooling and ventilation is most applicable for relatively small, low- and medium-rise commercial buildings in temperate climate zones, where mechanical cooling may be necessary only a small fraction of the year.

In these situations, natural ventilation has clear advantages over air conditioning, yet air conditioning may be desirable to address seasonal peak cooling loads and/or highly loaded zones. Table 1 contrasts typical approaches to naturally ventilated, air-conditioned, and mixed-mode buildings for several key building design issues.

Mixed-mode cooling strategies can take many forms, but generally will involve an intelligent control strategy and a building envelope that becomes a critical part of the system. William Bordass has created a useful taxonomy of mixed-mode strategies, as shown in Table 2, based on a classification scheme originally proposed by Max Fordham Partners.

Concurrent mixed-mode operation with occupant-controlled operable windows is the most prevalent design strategy in practice today, although changeover and zoned mixed-mode designs are becoming increasingly common. Changeover and concurrent systems perhaps offer the greatest potential for gaining “the best of all worlds” by mixing the different ventilation and cooling strategies in the same space at the same time. These systems, however, may also yield “the worst of all worlds” if air conditioning and natural ventilation systems are not coordinated in an efficient manner that is understandable to occupants, operators, and automated control systems.

There does not seem to be a “standard” mixed-mode approach in practice today – each building continues to be unique. For many mixed-mode buildings, operating conditions have deviated somewhat from their original design intent (e.g. a building originally designed for seasonal changeover between air conditioning and natural ventilation may, in practice, operate both systems concurrently).

Potential Benefits

Mixed-mode buildings offer three broad advantages over sealed-air conditioned buildings:

Reduced hvac energy consumption. A well-designed and properly operated mixed-mode building can scale back or eliminate the use of mechanical cooling and ventilation systems throughout much of the year, with associated reductions in pollution, greenhouse gas emissions, and operating costs. Ventilation with cool outside air can reduce a commercial building’s energy use by 15% to 80%, depending on climate, cooling loads, and building type. Concurrent mixed-mode schemes, however, may result in wasted energy if air conditioning and natural ventilation conflict with one another.

Higher occupant satisfaction. Mixed-mode buildings generally offer occupants higher degrees of control over their local thermal and ventilation conditions, which should lead to increased occupant satisfaction and reduced potential for IAQ problems. Several researchers have found that building occupants are more tolerant of fluctuations in interior conditions when they are provided with some measure of personal control. Under some conditions, however, natural ventilation may be undesirable due to airborne pollutants and allergens, or outdoor noises.

Highly “tunable” buildings. Mixed-mode strategies provide inherent flexibility and redundancy in the space conditioning systems of a building, resulting in potentially longer life, greater adaptability to changing uses, and reduced lifecycle costs. With the careful application of mixed-mode cooling and ventilation, one can anticipate somewhat smaller mechanical systems and extended hvac equipment life.

Occupants in this 208,000-sq-ft facility in Sacramento, CA may have the most "sophisticated" of the three systems. It employs a "changeover" system that reacts when windows are open; workers also enjoy the flexibility of reconfigurable floor air diffusers.

Barriers to the Approach

Over the last 10 years, an increasing number of office buildings have successfully integrated air conditioning and natural ventilation, particularly in continental Europe, the United Kingdom, and Japan. Given the potential benefits of mixed-mode cooling strategies, the question remains: Why aren’t we seeing more of these buildings in the United States? There are several potential barriers (real and perceived) to more widespread adoption of mixed-mode schemes. The problems we have identified fall into the following four inclusive categories.

Building Design Issues. The U.S. building design industry is generally unfamiliar with mixed-mode cooling strategies, and there is a lack of available design tools and case studies to facilitate their education. Existing design standards (particularly ASHRAE Standards 62 and 55) leave little flexibility for unconventional or innovative hvac designs.

Building Operations and Controls Issues. Mixed-mode buildings may require integrating automatic and manual control strategies for both hvac and building fenestration systems, which can be significantly complex. Commercial building designers and operators also share a general lack of familiarity with operable windows (and other permeable building envelopes) and a concern about their associated maintenance requirements.

Fire and Safety Concerns. The potential for smoke migration in a commercial building designed to incorporate wind-driven or stack-driven ventilation is at odds with many local building codes. There may be further concerns about building security and occupant safety for commercial buildings with operable windows.

Energy Code Concerns. California Title-24 and other energy codes tend to limit designers to fairly conventional hvac systems. Standards generally frown upon installing operable windows and mechanical cooling systems for the same zone.

This test building is located in San Rafael, CA. The smallest of the test buildings (75,000 sq ft), it operates a "concurrent" system, so that the status of windows had no effect on the mechanical system operation.

Studying at Home and Abroad

In recent years, the international building industry has focused more attention on buildings explicitly intended to operate as mixed-mode, or what is also referred to as “hybrid ventilation.” In 1999, The International Energy Association (IEA, launched a three-year “research annex” to develop control strategies and performance prediction methods for hybrid ventilation in new and retrofitted buildings. Participants from 15 countries are carrying out research on control strategies and analysis methods, and conducting field studies of existing mixed-mode buildings to investigate their IAQ, thermal comfort, and energy consumption. The main objectives of the project are:

  • To develop control strategies for hybrid ventilation systems in new and retrofitted office and educational buildings;
  • To develop methods to predict ventilation performance in hybrid ventilated buildings;
  • To promote energy and cost-effective hybrid ventilation systems in office and educational buildings; and
  • To select suitable measurement techniques for diagnostic purposes to be used in buildings with hybrid ventilation systems.

Recognizing that the U.S. building industry has not yet widely embraced mixed-mode strategies, the Center for the Built Environment (CBE, a university-industry collaborative research center located at the University of California, Berkeley) has undertaken research to find out what makes a mixed-mode building successful and what designers can do to implement cost-effective, energy-efficient, and user-friendly mixed-mode strategies. The research is ongoing, and the full results will be available from CBE in fall 2000. However, we can briefly summarize the nature of the project and some of the key findings here.

Methods. This project focused on investigating occupant comfort, control, and satisfaction in three mixed-mode office buildings located in Northern California’s temperate climate. At each site, CBE administered an occupant survey and interviewed the architects, engineers, and facility managers to learn about how each building operates and how well it lives up to occupants’ expectations. The web-based survey is based on the “Indoor Environmental Quality Assessment” tool that CBE has developed for benchmarking how satisfied occupants are with various aspects of their indoor environment ( The survey includes questions about satisfaction with various aspects of the indoor environment, as well as access, use patterns, and satisfaction with operable windows and personal hvac controls. The survey is announced via e-mail and filled out by building occupants online through their web browser. Survey responses and comments are then recorded anonymously in a database maintained by CBE.

Research sites. All three buildings were built in the 1990s and include air conditioning systems and occupant-controlled operable windows. They range in size from 75,000 to 200,000 sq ft, and they are unique in terms of the design of their hvac systems, space layout, and organizational culture. Two of the buildings allowed occupants to control the thermostats, while the third provided occupant control of floor air diffusers. Two of the buildings could be classified as “changeover” schemes, where micro-switch sensors in the windows shut off a zone VAV box if the window is open. The third building could be characterized as a “concurrent” scheme, since opening and closing the windows have no direct impact on the operation of the mechanical system.

Key findings. Some key findings include the following: Frequency of use of windows was influenced by ease of access (related to interior space planning, the window handles, and the placement of desks and partitions), outdoor climate, organizational culture, and availability of alternative control over the hvac system.

The most frequently offered reason for why people open their windows was “to bring in fresh air,” followed by “to create more air movement,” and then “workspace too warm.”

The most frequently offered reasons for why people close their windows was “outdoor noise,” followed by “workspace too cool,” “too much air movement,” and “outdoor dust, dirt, and odors.”

There was a strong correlation between having access to operable windows and satisfaction with air movement, ventilation, and air quality. The correlation was not as strong for satisfaction with temperature. These patterns were slightly different in the three buildings, however, and need to be investigated in more detail.

The building that offered the highest degree of control (i.e., highest percentage of people with easy access to windows and thermostats in every office) produced the highest levels of occupant satisfaction with nearly every aspect of the indoor environment (temperature, air movement, ventilation, humidity, and odors). This trend was particularly strong for temperature satisfaction. It is likely that a responsive facility management team that addressed thermal comfort complaints quickly also influenced this pattern.

When occupants had access to both hvac controls (thermostats or floor diffusers) and operable windows, they tended to be more satisfied with the windows, and also use them more often, as compared to the hvac controls.


Mixed-mode buildings offer great promise for reducing energy operating costs while maximizing comfort and providing occupants with a sense of personal control and connection to the outdoors. Other potential advantages include reduced lifecycle costs and increased flexibility and adaptability of building use. Although mixed-mode buildings are becoming increasingly popular in Europe and Japan, there are relatively few examples in the United States. What can we do within both the research and engineering communities to encourage the more widespread adoption of this innovative design strategy, and overcome the many barriers that exist? We feel that the following activities are needed:

  • Theoretical and experimental research to quantify the benefits of mixed-mode building;
  • Building energy simulations to evaluate the energy savings potential for mixed-mode buildings in different climate zones;
  • Detailed field studies which combine subjective surveys with field measurements of thermal conditions and ventilation levels in mixed-mode buildings;
  • Development of design tools and guidelines;
  • Revisions of ASHRAE Standards 55, 62, and 90.1 to encourage more alternative environmental control strategies; and
  • Greater collaboration between researchers and the professional community.

      Side Bar: Interesting times at the CBE

      The Center for the Built Environment (CBE) was established in May 1997 at the University of California, Berkeley. The research is funded by annual contributions from industry partners, while the National Science Foundation underwrites the administrative costs. The goal is to provide timely, unbiased information on promising new building technologies, operating strategies, and design techniques.

      Currently there are 15 industry partners participating in CBE: Armstrong World Industries; California Department of General Services; California Energy Commission; Henningson, Durham & Richardson (HDR); International Facility Management Association (IFMA); Johnson Controls; Lucent Technologies; Ove Arup & Partners; Pacific Gas & Electric Co.; Skidmore, Owings & Merrill (SOM); Tate Access Floors; U.S. Department of Energy (DOE); U.S. General Services Administration (GSA); York International Corporation (York, PA); and a Webcor Builders Team that includes its subcontractors Alfa Tech Consulting Engineers, Critchfield Mechanical, and Rosendin Electric. This diverse set of industry partners has a common interest in sharing information on improving the design and operation of commercial buildings. Industry partners help direct CBE research and get early access to research progress and findings.

      Faculty, research staff, and graduate students in the Building Science Group within the Department of Architecture at UC Berkeley conduct the majority of CBE’s research. The core research staff includes a mix of engineers and architects with a wide variety of academic research, applied research, and other professional experience. CBE also has affiliated faculty in other departments at UC Berkeley, at other universities, and at Lawrence Berkeley National Laboratory.

      CBE focuses its research into two broad categories concerning commercial buildings:

      Tools for Improved Building Performance: Research methods that “take the pulse” of operating buildings —- looking at how people use space, asking them what they like and do not like about the interior environment, and linking these responses to physical measurements of indoor environmental quality.

      New Building Technologies and Design Strategies: Research on how designers can make buildings more environmentally friendly, more productive to work in, and more economical to operate.

      There are currently seven ongoing CBE research projects, and a host of smaller, short-term efforts. Three of the current projects are developing new tools for improved building performance. They address improving the occupant feedback process, benchmarking IAQ levels, and the correlation between ventilation control and occupant productivity. The remaining four projects are exploring emerging building technologies and design strategies. These include underfloor and task/ambient systems along with the study of team space design and use, as well as the mixed-mode concept.