The Heat of the Matter Behind Design for Thermal Comfort
In this installment of our series on high-performance design features, we’re exploring the effects views, daylighting, thermal comfort, and indoor air quality have in producing significantly positive impacts on workplace productivity. Though we are exploring each feature individually in these installments, we consider these design features holistically in our high-performance design practice for workplace, which focuses on complementary evidence-based design strategies to improve the health and wellness of users. And as part of an integrated strategy, thermal comfort is a multi-faceted feature that profoundly affects user contentment with the built environment.
How do we define thermal comfort? According to ASHRAE Standard 55, it is the state of mind that is satisfied with the perceived temperature of the environment, and is assessed by subjective evaluation. Factors that affect thermal comfort include clothing, metabolic activity, indoor air temperature, humidity, radiant temperature, and air speed. In addition to these conditions, acceptable thermal conditions can also shift if the space is naturally conditioned with user-controlled operable windows. This shift or method is also called the adaptive thermal comfort model that relates acceptable indoor temperature ranges to outdoor climate parameters.
Factors affecting thermal comfort. Image © DLR Group.
A user’s temperature perception is based on heat exchange with the immediate environment through convective and radiant heat exchange. This is why someone working close to a window with sun beating down on him or her may feel warmer in the summer and colder in the winter months, even though the air temperature consistently remains at a comfortable 72°– 75° Fahrenheit.
How Much Does Thermal Comfort Matter?
Thermal comfort is a topic of discussion in offices everywhere you go. From the Washington Post to the BBC to the Wall Street Journal, potentially contributing factors such as age, gender, and clothing choices have been explored. As architects and engineers of the workplace, we use evidence-based design tactics to enhance thermal comfort in spaces. Here are some of the most informative studies on the topic.
A 2007 study by Tanabe, Haneda, and Nishihara found a group of call center workers and a group of college students had differing levels of performance during simulated office tasks – such as typing, reacting to choices, and doing math – depending on the temperature in which they worked.1 One part of the study determined that higher temperatures – at or above 33° Celsius, or 91° Fahrenheit – decreased oxygenated blood supply. It is at these same temperature levels that participants experienced a decline in performance, and complained of fatigue and discomfort.
Pennsylvania Academy of the Fine Arts in Philadelphia by DLR Group. Photo by Kevin G. Reeves.
Not only does task performance decrease in uncomfortably warm environments, in 2011, Lan et al also found that people assessed a building to have worse air quality and be less healthy, in general, when temperatures rose above 30° Celsius, or 86° Fahrenheit. These reactions were not linked to indicators of stress, implying that it is the body itself sending signals of unhealthiness and discomfort – it is not “just in your head.”
Recently, Li Lan and Zhiwei Lian, along with other collaborators, have published a series of studies attempting to quantify lost productivity due to temperature, and to link these changes to physiological effects on the human body.2, 3, 4 They examined various aspects of performance and productivity, including ability to perceive a change, memory, critical thinking, decision making, motivation, and accuracy on math and typing. Three sets of studies indicated that a neutral to cool temperature range – around 22° Celsius, or 71° Fahrenheit – to slightly warm – around 24° Celsius, or 75° Fahrenheit – was optimal for task performance. Below and above this range, performance dropped off, as did motivation and comfort.
Attempts to summarize the far-reaching impacts of thermal comfort on work and mood are difficult, since so many personal and environmental factors come into play. In a 1996 presentation to the International Society of Indoor Air Quality and Climate, Dr. David Wyon, a leading researcher in the field, synthesized multiple studies by his own team and others to speak about the effect of thermal conditions on a variety of factors in the workplace.5 Most studies in Wyon’s presentation indicated that cooler temperatures increased productivity, while more notable declines in performance were seen in warmer temperatures – an effect that was often more pronounced for men than for women. In addition, control over a personal temperature zone through windows or ventilation systems was linked to improvements in not only satisfaction, but also productivity. From Wyon’s own work, he estimates that having even slight control over temperature, just ~3° Celsius, improved performance by “2.7% for logical thinking, 7.0% for general office work, 3.4% for very skilled manual work, and 8.6% for very rapid manual work.”
How Can We Design Spaces for Better Thermal Comfort?
The top five strategies that can show improvement in thermal comfort include the following.
Case Western Reserve University in Cleveland by DLR Group. Photo by Kevin G. Reeves.
Control solar gains along perimeter spaces with external shading devices to limit the temperature delta through the depth of the room. If you’ve ever sat in a space with direct sun hitting your body, you will tend to experience the space differently from a person who is not in direct sun. Controlling direct solar gains reduces the difference in temperature experienced by people within one space, keeping the operative temperature consistent.
Building load balancing is key in keeping people comfortable. Building envelope strategies, including external shading and insulation, can help reduce the variance in the temperature through the space. This allows the HVAC system to supply air at temperatures that can keep all zones comfortable merely by varying the amount of supply air, typically through variable volume systems.
Pathfinder Kindergarten Center in Everett, Washington by DLR Group. Photo by Chris Roberts.
Providing thermally variable spaces lets users elect their workspace based on thermal preference. Our comfort expectations also change based on whether we are in an outdoor space or indoor space that is naturally ventilated. Alliesthesia, or a positively or negatively perceived change to a sensory experience, is used to differentiate thermal pleasure from thermal neutrality and acceptability. We seek an environment in which our body holistically feels comfortable not in spite of but because of varying conditions.
Canyon View High School in Waddell, Arizona by DLR Group. Photo by Tom Reich.
Individual or personal thermal controls can enhance how thermal comfort is perceived. These controls can allow a person to control their comfort set-point. Desk fans ora user-controlled thermal chair gives individual control over the immediate thermal environment without affecting the environment and comfort of other occupants.
AC Hotel Raleigh North Hills in Raleigh, North Carolina by DLR Group. Photo by John Woodcock.
Zone cooling and heating systems, decoupled from ventilation, that can cycle on and off can improve thermal comfort. Cooling and heating through radiant systems that focus on operative temperature, or the average between air temperature and the mean radiant temperature rather than solely air temperature, can improve comfort for people. Using a thermal slab for heating, when the users are closer to the ground such as in a kindergarten, can enhance thermal comfort through operative temperature. And because hot air is not forced into a space heating requirements are not met through hot air alone, humidity levels are less likely to decline. ASHRAE 55 also establishes an acceptable operative temperature range for naturally conditioned spaces.
The 2013 edition of ASHRAE 55: Thermal Environmental Conditions for Human Occupancy specifies a number of methods to help determine indoor thermal environmental conditions that a significant number of occupants would find acceptable, and details thermal comfort for mechanically and naturally ventilated spaces. Both the U.S. Green Building Council’s LEED and the International Well Building Institute’s WELL building standard refer to ASHRAE 55 for compliance. The WELL building standard also includes thermal comfort as a precondition for certification, with radiant thermal comfort and individual thermal comfort as optimizations for higher levels of certification. The USGBC LEED standard, in addition to ASHRAE 55, provides an option to use International Organization for Standardization (ISO) and European Committee for Standardization (CEN) standards. LEED includes one credit for meeting the thermal comfort requirements per ASHRAE 55, and providing individual controls for 50 percent of the individual occupant spaces.
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- Tanabe, S. I., Nishihara, N., & Haneda, M. (2007). Indoor temperature, productivity, and fatigue in office tasks. HVAC&R Research, 13(4), 623-633. Accessed at: https://www.researchgate.net/profile/Shin-ichi_Tanabe/publication/233338619_Indoor_Temperature_Productivity_and_Fatigue_in_Office_Tasks/links/5567239308aefcb861d38217.pdf
- Lan, L., Lian, Z., Pan, L., & Ye, Q. (2009). Neurobehavioral approach for evaluation of office workers' productivity: The effects of room temperature. Building and Environment, 44(8), 1578-1588. Accessed at: https://doi.org/10.1016/j.buildenv.2008.10.004
- Lan, L., Lian, Z., & Pan, L. (2010). The effects of air temperature on office workers’ well-being, workload and productivity-evaluated with subjective ratings. Applied ergonomics, 42(1), 29-36. DOI: 10.1016/j.apergo.2010.04.003
- Lan, L., Wargocki, P., & Lian, Z. (2011). Quantitative measurement of productivity loss due to thermal discomfort. Energy and Buildings, 43(5), 1057-1062. DOI: 10.1016/j.enbuild.2010.09.001
- Wyon, D. P. (1996, October). Indoor environmental effects on productivity. In Proceedings of IAQ (Vol. 96, pp. 5-15). Accessed at: https://www.researchgate.net/profile/Shin-ichi_Tanabe/publication/233338619_Indoor_Temperature_Productivity_and_Fatigue_in_Office_Tasks/links/5567239308aefcb861d38217.pdf