Occupant perceptions of an indoor thermal environment in a naturally ventilated building.
Air movement in a naturally ventilated space is affected by a number of building and environmental parameters. The principal causes of air motion are pressure gradients resulting from wind and buoyancy effects, which result from temperature differences. Other parameters that may affect the air motion in buildings include the complexity of the pathways that the air must traverse while entering, passing through, and leaving; the occupant usage behavior in the building spaces; and any active system operation. All these factors combine to produce wide-ranging magnitudes of influences under different conditions. As a result, air motion behavior in naturally ventilated spaces varies considerably. While, the flow patterns for forced air systems are better understood, the patterns associated with natural ventilation are far more complex. Hence, in order to identify possible or common air motion behaviors, it is generally necessary to measure the behaviors while also measuring the influencing parameters.
In an interior space, heat is generated by: the occupants, lighting and process equipment. If the accumulated heat is not diluted by the addition of cooler air from the exterior, the affected space will become overheated and result in a thermally uncomfortable environment for the occupants. Moreover, building occupants, process equipment, model-making materials and art supplies etc. are all sources of contaminants. The introduction of fresh air can dilute the concentrations of these contaminants and expel them from the affected space, thereby maintaining an acceptable level of indoor air quality as well.
A number of studies have associated poor ventilation rates in a space to the prevalence of building related illness symptoms. The most comprehensive investigation of air quality in U.S. office buildings began with the Building Assessment Survey and Evaluation (BASE) study, which was undertaken by the U.S. Environmental Protection Agency (EPA). Following the completion of data gathering in 100 office buildings, researchers at the Lawrence Berkeley National Laboratory (LBNL) undertook extensive analysis of these data. In one set of analysis involving a subset of 41 buildings, correlations were found between elevated indoor carbon dioxide concentrations and health measures that are associated with sick building syndrome symptoms (Apte, Fisk, and Daisey, 2000). Another study investigated the effects of varying ventilation rates in a normal office building while maintaining constant and neutral thermal, acoustic and visual conditions (Wargocki, et al., 2000). The results showed that the prevalence of sick building syndrome symptoms amongst the occupants decreased and their productivity increased for higher ventilation rates.
There are fewer such data available for naturally ventilated buildings. Only a few studies have assessed the performance of these buildings in terms of their thermal environment and occupant thermal comfort. One such study investigated how personal control of operable windows in office buildings affects the thermal environmental conditions and occupant comfort (Brager, Paliaga, and Richard, 2004). Conducting occupant surveys and physically measuring thermal conditions at the occupant workstations accomplished this. In recent years, several research studies have used computer based Computational Fluid Dynamics (CFD) techniques to estimate and visualize airflow in naturally ventilated buildings (Horan and Finn, 2008; Evola and Popov, 2006; Visagavel and Srinivasan, 2009). Some studies have also used small-scale physical models in wind tunnels (Larsen and Heiselberg, 2008; Jiang, et al., 2003). However, there is less extensive research that involves measurement of airflow patterns and properties in full-scale naturally ventilated buildings.
METHODS OF EVALUATION
The utilization of natural ventilation in Architecture Hall presented an opportunity to measure airflow behaviors in a full scale occupied building. Over a period of two years, [CO.sub.2] levels were measured in occupied design studios, while air velocities and ventilation rates were also measured under varying conditions. Occupant surveys, based on a U.S. EPA study, were also administered in all naturally ventilated spaces. The measured data was then compared to occupant perceptions of their thermal environments and the prevalence of any building related illness symptoms.
The following experimental methods were developed during the course of the study to systematically evaluate the performance of the naturally ventilated spaces in Architecture Hall.
1. [CO.sub.2] concentrations were measured using transmitters mounted on two instrumented carts in occupied spaces.
2. Air exchange rates were measured using tracer gas concentration decay analysis in unoccupied spaces using a portable infrared ambient analyzer.
3. Air velocities were measured using hot-sphere anemometers mounted on two instrumented carts in both occupied and unoccupied spaces.
The instrumented carts also measured and recorded instantaneous dry-bulb air temperature, globe temperature and relative humidity. The globe thermometer had a diameter of 6 inches (0.15 m).
[CO.sub.2] is produced as a result of the human respiration process. If the ventilation rates in a space are inadequate, levels of odor causing bio-effluents and [CO.sub.2] will increase. Since [CO.sub.2] can readily be measured, it can serve as a good indicator of ventilation rates and acceptability of a space in terms of human produced bio-effluents (Godish, 1995). Therefore, the [CO.sub.2] concentrations in a space primarily depend on the number of occupants and the air exchange rates.
The natural ventilation function in Architecture Hall is climate driven. During much of the school year from November through March, outside temperatures are often quite low and very few or no windows are opened by the occupants. Figure 1 shows a rough correlation between the outside dry-bulb air temperatures ([T.sub.out]) and minimum amount of window open areas ([Area.sub.min]) recorded for each experiment. As a result, [CO.sub.2] concentrations increase when the studios are occupied and consistently reach steady state levels that substantially exceed the maximum acceptable in the design studios (IIyas, Emery, and D. Heerwagen, 2010). A [CO.sub.2] criterion for outdoor air monitoring for the studios was established using a methodology presented in an ASHRAEJoumal article (Lawrence, 2008). Based on this criterion, it was determined that the [CO.sub.2] concentrations exceeded the steady state levels on 40% of the test days, indicating poor ventilation rates.
[FIGURE 1 OMITTED]
Air Exchange Rates
A principal task of any ventilation system is to control the indoor thermal environment. One way this is achieved is by admitting fresh outside air to cool a space and to dilute indoor contaminant concentrations. A naturally ventilated space provides fresh air through passive means, for example, by opening windows. The amount of fresh air required can be quantified in terms of hourly air exchange rates (ACH).
ACH's were measured in unoccupied design studios using tracer gas analysis under varying conditions. The results indicated that air exchange between the interior and exterior does occur provided adequate amount of windows were open. When all windows were closed, the measured ventilation rates were in the range of 0.20 to 0.70 ACH. Table 1 shows a range of measured ventilation rates per person in the design studios for those experiments when all windows were closed, and how they compare to the minimum required ventilation rates calculated using ASHRAE Standard 62.1-2004. The results suggest that the ventilation rates in the design studios were considerably below industry standards when windows were closed, which were the case on 27% of the test days.
Table 1. Comparison of Measured and Calculated Ventilation Rates Studio G20 G30 G60 210 220 230 260 CFM/person (L/s/person) Calculated per ASHRAE 18 18 21 18 18 19 19 62.1-2004 (8.5) (8.5) (9.9) (8.5) (8.5) (9.0) (9.0) Measured Minimum 1.9 2.0 2.7 2.1 2.4 2.8 2.8 (0.9) (0.9) (1.3) (1.0) (1.1) (1.3) (1.3) Measured Maximum 6.6 6.8 9.3 7.3 8.1 9.7 9.7 (3.1) (3.2) (4.4) (3.4) (3.8) (4.6) (4.6) Studio 270 Calculated per ASHRAE 20 62.1-2004 (9.4) Measured Minimum 3.0 (1.4) Measured Maximum 10.5 (4.9)
Since the tracer gas analysis could only be conducted in unoccupied spaces due to the use of sulfur hexafluoride ([SF.sub.6]) as the tracer gas, and the rooms were in use most of the academic year, only a small number of such experiments could be completed. Hence, it is difficult to ascertain what the optimum amount of window open area is in order to achieve adequate ventilation rates under varying influencing parameters.
Air Velocity Measurement
Air movement in a building affects occupant thermal comfort and rates of heat gain or loss through the building envelope (by infiltration; but not by transmission). Also it is a major determinant of whether the indoor air quality will be satisfactory (D. Heerwagen, 1996). An occupant would feel thermal comfort or discomfort depending on how rapidly or slowly the air moves. A uniform distribution of air at acceptable rates would ensure a healthful indoor environment by diluting indoor contaminants such as dust, allergens and other toxic substances.
Air velocity measurements were done in occupied design studios using hot-sphere anemometers mounted on instrumented carts. The velocity was measured in a central location at approximately 5 ft (1.5 m) above floor level. Typically, very low air velocities were recorded in the design studios (Table 2), which can be attributed to the insufficient window open areas. Studios where occupants had a tendency to open relatively more windows recorded higher velocities, but still on the lower end.
Table 2. Statistical Summary of Air Velocity Measurements in Occupied Spaces Statistical Measure Air Velocity (ft/sec) Air Velocity (m/s) Average 0.26 0.08 Standard Deviation 0.15 0.05 Minimum 0.10 0.03 Maximum 1.51 0.46 Mode 0.16 0.05
In other experimentation, velocity probes were placed on the window sill and also directly in the path of incoming air, in order to observe the behavior of incoming air. Figure 2 shows a plot of one such experiment where velocity was measured at the open window (Location A) and 8 ft (2.4 m) downstream (Location B). The graph shows 5-minutes averaged data over a 4-hour period. According to wind data obtained from a nearby weather station, the window was receiving direct wind with an average wind speed of 3.3 ft/s (1.0 m/s). We note that the magnitude of the air velocity decreased by 4 to 5 times after traveling a distance of only 8 ft (2.4 m). Similar results were obtained in other such experiments.
[FIGURE 2 OMITTED]
ENVIRONMENTAL QUALITY ASSESSMENT
In addition to physical measures of air quality, it is important to assess how occupants perceive the air and thermal conditions and whether they experience discomfort or illness symptoms. Results from a decade of research in Europe and the US show high levels of symptoms associated with air quality in typical office buildings (i.e., not identified as "sick buildings"). Symptoms include headache; eye, nose and throat irritation; upper and lower respiratory irritation; and fatigue. The environmental causes have been difficult to identify, in part because of the high level of potential sources that are present in very low indoor concentrations (Stolwijk, 1991).
Data from the Building Assessment and Survey Evaluation (BASE) study (U.S. EPA, 2003) as well as findings from European research show that 20% or more of workers experience illness symptoms at work. Analysis of 41 buildings in the BASE sample with 1,970 subjects found that 27% experienced irritation of the eye, nose and throat and that 16% reported headache and fatigue (Apte, Fisk, and Daisey, 2000). Given the indications in this study that air quality in Architecture Hall was compromised for significant amounts of time, we administered a survey assessing the nature and frequency of symptoms.
The prevalence of illness symptoms in the occupants of Architecture Hall was compared to the BASE study, for which the U.S. EPA conducted a systematic survey of 100 U.S. public and private office buildings from 1994 to 1998 (Brightman, et al., 2008). The buildings were randomly selected from 201 cities with populations greater than 100,000 people. A paper-based survey was used for this study and was modified from the BASE study questionnaire to suit the specific requirements of Architecture Hall. Results relevant to occupant air movement preferences and illness symptoms are presented and discussed in this paper. The survey was administered during a two-week stretch in late November and early December when most of the students spend a significant amount of time in the design studios.
Description of Questionnaire
The questionnaire used for the survey was divided into two main parts: (1) perceptions and comfort and, (2) environmental experiences. The first part asked questions about the occupants' interaction with the studio thermal environment. The second part of the questionnaire asked the occupants to identify any health symptoms they had experienced in the past week and those that they were presently experiencing. The entire questionnaire took approximately 10 minutes to fill out. A total of 116 surveys were completed and based on the average number of occupants present in the design studios, the response rate was 87%.
Occupants were asked to rate their individual thermal perceptions of temperature, daylight, noise level, air movement, electric light and humidity at their respective workspaces on a scale of 1 to 5. Only temperature and air movement are relevant to the discussion here. Figure 3 shows the distribution of temperature and air movement satisfaction votes for a total of 116 respondents. 64% of all respondents were satisfied with the temperature in the space. A majority of the occupants (53%) were not entirely satisfied with the air movement in the space. Most of them felt that the air movement was too little (43%), whereas 11 occupants (10%) felt that the air movement was on the higher side.
[FIGURE 3 OMITTED]
Some questions in the survey inquired about the occupants' control over their environment. In one question, the occupants were asked about their ability to open windows. Results showed that 41% of the occupants were satisfied with the control they had over window operation. Majority of the occupants who opened a window in the past week did so to let fresh air in (57%) or to cool down the room (33%). For those that didn't open a window cited the cool incoming air as one of the primary reasons for not doing so (22%) or inability to operate the windows (21%). A large number of occupants felt no need to open any windows (20%).
In the second part of the questionnaire, the respondents were asked if they experienced a specific symptom either while working in the studio at the time of the survey or at any time during the preceding week. This approach was one of the modifications made versus the way the question was asked in the U.S. EPA BASE study. The building-related symptoms include lower respiratory (e.g., wheezing, shortness of breath, or coughing), upper respiratory and mucosal membrane (e.g., dry eyes, stuffy or runny nose, sore throat, or sneezing), unusual fatigue, difficulty concentrating, dizziness or lightheadedness, and skin symptoms (e.g., dryness, irritation, or itching). Respondents were asked to check all that apply.
85 occupants (out of the 116 respondents) reported at least one or more illness symptoms (during the past week or while taking the survey). On average, a respondent claimed 5 illness symptoms. Figure 4 shows a distribution of the number of symptoms reported by a person at the time of the survey and in the preceding week. We note that most of the respondents had more than one symptom, whereas, 9 people also reported at least 10 illness symptoms. Results indicate that the occupants were experiencing pronounced rates of illness symptoms.
Figure 4 Occupant symptom occurrences in Architecture Hall Number of Symptoms past week Right now 1 10 15 2 10 14 3 14 11 4 9 5 5 10 1 6 12 3 7 7 5 8 2 0 9 3 1 10 4 1 >10 4 0 Note: Table made from bar graph.
Table 3 shows the prevalence of illness symptoms in Architecture Hall (reported for the previous week and at the time of the survey) and U.S. EPA BASE study buildings. We note that the prevalence of health symptoms in Architecture Hall (for past week) is considerably higher than the BASE buildings for almost all symptoms (except shortness of breath). The prevalence for the "Right now" category either exceeds or is similar to the results from the BASE buildings for most symptoms.
Table 3 Comparison of Prevalence (%) of Illness Symptoms in Architecture Hall and BASE Buildings Illness Symptom Architecture Architecture BASE Hull Hall Building Past Week Right Now Tired or strained 50 26 22 eyes Dry, itchy or 26 9 19 irritated eyes Headache 37 15 15 Unusual fatigue or 34 13 15 tiredness Stuffy or runny now 31 16 13 Sneering 25 11 11 Sure or dry throat 27 9 7 Cough 17 6 5 Difficulty 19 6 5 remembering things Dry or itchy skin 21 14 5 Difficulty 38 11 5 concentrating Dizziness or 11 2 3 lightheadedness Nausea or upset 6 6 3 stomach Wheezing 6 4 2 Shortness Of breath 2 2 2
Another study (Apte and Erdmann, 2004) used the U.S. EPA BASE study data to quantify the relationship between indoor [CO.sub.2] concentrations and upper respiratory and mucosal membrane (MM) and lower respiratory (LR) building related symptoms. The study found statistically significant correlations of MM and LR building related symptoms with increasing indoor [CO.sub.2] concentrations. The small sample size of 116 limits the application of similar statistical methods to find correlations in Architecture Hall. However, a high frequency of occurence of MM (54% past week and 24% right now) and LR symptoms (21% past week and 9% right now), and the presence of elevated [CO.sub.2] levels in the building is a cause for concern.
The results from this study suggest that the air quality in the naturally ventilated design studios is questionable. Although the data do not support a casual relationship between high [CO.sub.2] levels, low air movement, and illness symptoms, the results are consistent with results from other studies which have used similar survey methods and environmental measures (e.g., U.S. EPA BASE study). It would be useful to have results from experimental studies that clearly show symptoms related to specific environmental parameters. Nonetheless, this study shows that there is reason to believe that an increasing use of natural ventilation to reduce building energy consumption may be questionable in some circumstances. When the natural ventilation strategy relies on occupants to open windows (rather than automated controls), we can expect to find a conflict between comfort maintenance and ventilation when outdoor temperatures are low. Under these conditions, opening windows creates discomfort that is resolved by closing the windows again. This leads to build up of [CO.sub.2] levels and reduced air movement that are associated with illness symptoms.
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Salman Ilyas, PE
Ashley Emery, PhD
Judith Heerwagen, PhD
Life Member ASHRAE
Salman Ilyas is a Mechanical Engineer at Arup, Los Angeles. Ashley Emery is a professor in the Department of Mechanical Engineering, University of Washington, Seattle, WA. Dean Heerwagen is a professor in the Department of Architecture, University of Washington, Seattle, WA. Judith Heerwagen is a Program Expert at US General Services Administration.
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|Author:||Ilyas, Salman; Emery, Ashley; Heerwagen, Judith; Heerwagen, Dean|
|Date:||Jul 1, 2012|
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