Occupant Satisfaction in Mixed-Mode Buildings

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. Introduction In current commercial buildings in the U.S., cooling and mechanical ventilation account for over 30% of total energy use, approximately 20% of electricity use, and approximately 40% of peak demand. However, prior to the 1950s, air conditioning and mechanical ventilation were not yet commercially viable, and so commercial buildings had little choice but to utilize natural ventilation for cooling. Buildings typically had extended perimeter zones so that every office could have access to windows that would open to the outdoors, and provide the primary source of light and fresh air. But the availability in the 1950’s of large-scale mechanical ventilation and cooling, along with other technologies such as curtain walls and fluorescent lighting, led to the more common commercial building forms of today that are typically all-glass, flush-skin buildings with large floor plates and no operable windows. These buildings miss out on the large number of documented benefits of operable windows – thermal comfort over a wider range of temperatures based on the adaptive comfort zone (Humphreys, 1975; deDear and Brager, 1998), reduced energy consumption compared to conventional air-conditioned buildings (Emmerich and Crum, 2005), and fewer Sick Building Syndrome symptoms (Seppänen and Fisk, 2001). But even with all these potential benefits, there are a variety of concerns and design challenges associated with operable windows. The ability to rely solely on natural ventilative cooling is limited by loads and climate. And given our modern day expectations, engineers are often uneasy about the lack of predictability and control over indoor thermal conditions in naturally ventilated buildings. As a result, many innovative engineers are exploring “mixed-mode” buildings – a way to combine the best features of naturally ventilated and air-conditioned buildings, and essentially extend the range of climates in which operable windows are feasible even when they can’t provide acceptable comfort year round. “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. A well-designed mixed-mode building allows spaces to be naturally ventilated during periods of the day or year when it is feasible or desirable, and uses mechanical cooling only as necessary for supplemental cooling when natural ventilation is not sufficient. The goal is to maximize comfort while minimizing the significant energy use and operating costs of air conditioning. While mixed-mode buildings are much more common in Europe, it is a relatively newer concept for American engineers. The U.S. building design industry is generally unfamiliar with mixedmode cooling strategies, and there is a lack of published case studies or design and analysis tools to facilitate their ability to chart new territory. To address this need, CBE developed a webbased library of mixed-mode building case studies, covering a range of climates, design approaches, and control strategies (CBE, 2006). The library offers two levels of information: 1) a database with a broad list of buildings and basic project information, and 2) more detailed case studies. The database includes approximately 150 mixed-mode buildings, with over 60 of them in North America. It is downloadable as an Excel spreadsheet to allow for easy sorting, and includes basic information about each project including location, year built, type of building, owner, architect, engineer, brief comments about the mechanical system, operable windows, and control & operation strategies and web links for more information. The 8 case studies provide more detailed narrative and graphic descriptions obtained from literature reviews, drawings and photographs, and interviews with building owners, architects, engineers, and facility managers. The case studies include information about the windows, HVAC system, control strategies, building design process (design tools used, commissioning, relevant code issues), cost (where available), and additional green features of the building. The Resources section of the website also includes a more recent report with 23 new case studies that focus on control algorithms (Brager et al., 2007). What motivates building owners and the design team to move beyond conventional airconditioning and design a mixed-mode building? Without question, it is absolutely crucial to reduce energy consumption in buildings, and help avoid the potentially devastating impacts of climate change. But in terms of the building owner’s pocketbook, energy costs are still relatively small compared to worker salaries, which represent over 90% of the total operating costs of a commercial building. In addition, the cost of worker recruitment and retention is significant. So from the building or company owner’s point of view, perhaps the most persuasive argument for sustainable design in general – and operable windows in particular – is one that makes the connection between a higher quality indoor environment, and increased comfort, health and productivity of the workers. If we can demonstrate that occupant satisfaction is higher in buildings with operable windows, then that can be a powerful part of the argument to avoid or minimize the use of air-conditioning. So how does one learn about the quality of the indoor environment? Sadly, very few architects or other members of the design team are likely to know how well their building is working after it’s completed and occupied, the fees have been paid, and they’re on to another project. Without learning from experience in an objective way, building industry professionals are less likely to make design or economic decisions that will truly enhance the performance and experiential quality of their buildings. Physical measurements can be valuable, but by themselves they also need to be interpreted in terms of how they impact the occupants. Buildings occupants themselves are a rich yet underutilized source of direct information about how well a building is working, but the challenge is how to collect both the positive and negative feedback in a systematic way. Detailed thermal comfort field studies that include both physical measurements and subjective surveys are the most revealing, but are also time consuming and expensive, and therefore the number of buildings that can be investigated is inherently more limiting. Web-based surveys are an effective way to study building performance from the occupants’ point of view. They can be used as a diagnostic tool to help designers, building owners and operators, and tenants evaluate how well their office buildings are working from the occupants’ perspective, and to help prioritize investments to improve performance. The surveys can also be used as a research tool for specific projects requiring the assessment of occupant response, or for broader benchmarking and comparative analysis of the performance of particular building design, technologies, and operation strategies. It was with these dual purposes in mind that the Center for the Built Environment (CBE) developed their survey. Methods CBE Survey In 2000, CBE began developing a web-based indoor environmental quality (IEQ) survey and accompanying online reporting tools. Advantages of the web-based format are 1) it is quick and inexpensive to use; 2) it allows for branching questions to get more detailed information where appropriate (in particular, when the occupant indicates dissatisfaction with a certain area), thus avoiding making the survey too long for everyone with overly detailed or inappropriate question; and 3) survey results can be accessed using an automated, advanced reporting tool that allows users to filter, aggregate, compare, or benchmark their data. In addition to basic questions about demographics and workspace descriptions, the core CBE survey measures occupant satisfaction and self-reported productivity related to nine environmental categories: office layout, office furnishings, thermal comfort, air quality, lighting, acoustics, cleanliness and maintenance, overall satisfaction with the building, and with the workspace. Satisfaction questions use a consistent 7-point scale ranging from “very satisfied” (coded as 3) to “very dissatisfied” (-3), with a neutral midpoint (0). We use a secure SQL (standardized query language) server database for collecting and recording the responses. It takes approximately 5-12 minutes to complete the survey, depending on the number of branching questions one receives, and the number of open-ended comments one writes in. Additional, custom survey modules can be added which gather data about additional topics, depending on available building features or the client’s particular issues. Examples of existing modules include accessibility, safety and security, daylighting, and operable windows. In addition to the occupant survey, a representative of the building owner or design team fills out a building information form to provide descriptive information about the building and its systems, such as the age of the building, the number of occupants, the type of air-conditioning and whether the windows are sealed or operable. CBE also developed an automated web-based reporting tool that researchers and clients can use starting approximately one week after the survey is completed, allowing time to create a final data set where responses of participants who answer less than 15 questions are removed. The reporting tool allows one to produce standardized summaries of the responses in a particular building, compare them to the overall benchmarking database, or do more in-depth data mining to compare responses from selected subgroups of people or explore relationships between questions. The CBE Survey benchmarking database represents the portion of buildings we’ve surveyed that meet certain quality control criteria, such as the number of responses or % response rate. At the time of our analysis, the CBE Survey benchmarking database included over 370 buildings, with over 43,000 individual responses, and 3.8 million data points. More information about the CBE Survey can be found in Zegreus et al. (2004). One previous study focused on comparing the performance of green and LEED buildings to the overall database (Abbaszadeh et al, 2004), where it was found that there was not necessarily a correlation between buildings with a large number of LEED IEQ points, and the IEQ performance from the occupants’ perspective. Another focused on the role of air movement and personal control in influencing thermal comfort and perceived air quality for the database overall (Huizenga et al, 2006). Not surprisingly, it was found that satisfaction with both thermal comfort and air quality increases significantly in buildings that provide people with some means of personal control over their environment, such as thermostats or operable windows. Mixed-mode buildings The purpose of this analysis was to examine occupant satisfaction in mixed-mode buildings, with the aim of showing that we do not need to rely on sealed, air-conditioned buildings to maintain good indoor environmental quality. The 12 mixed mode buildings that were analyzed for this study were identified from the CBE Survey database. A representative of the building fills out a “building characteristics” form, which helps us identify basic descriptive information about the building. Unfortunately, this form is not always filled out fully or consistently. So while there are most likely additional buildings in the CBE database that may have operable windows, we only included those in the study for which we had sufficient information about the building from the characteristics form, and where we could find additional available case study material 1 LEED stands for Leadership in Energy and Environmental Design, and is a green building rating system developed by the U.S. Green Building Council. confirming that they all were mixed mode buildings. Also, we only included surveys that had a response rate of over 50% for buildings with fewer than 50 occupants, or a response rate of over 25% for buildings with 50 or more occupants. Table 1: Building Characteristics Mixed Mode vs. Rest of Database