Using an extensive trove of thermal comfort research data, CBE’s research team recently published a set of ‘nudges’ to the existing adaptive comfort standards to improve comfort in commercial buildings while potentially reducing energy use. This work updates the landmark study from 1998 by UC Berkeley’s Gail Brager and Richard de Dear of The University of Sydney on the Adaptive Comfort Model (ACM), which demonstrated that people in naturally ventilated buildings were more comfortable with seasonal temperature variation compared to people in air-conditioned buildings.
The explanation for the earlier findings is based on the idea that we adapt to our environments. We often refer to three types of adaptation — physiological, behavioral, and psychological — that are all strongly influenced by context, and shape our ability to adapt, for example, varying clothing or opening a window. Adopted by the ASHRAE Standard 55 in 2004, the ACM represented the best knowledge of adaptive comfort at that time. But approaches to building operation have evolved in the 20 years since the original ACM was developed, so we wanted to know…is it still valid? Or have the comfort requirements and expectations of building occupants changed enough to warrant a new approach?
To answer these questions, we recently conducted a meta-analysis of the largest database of thermal comfort field-study data. Using the Global Thermal Comfort Database II, we expanded upon the original adaptive comfort analysis using over 60,000 comfort measurements from buildings around the world. There were many interesting results, and here we summarize three high-level findings which we call nudges. We encourage you to read the recent publication if you are interested in the full details.
The relationship between the indoor comfort temperature, or the ‘neutral temperature,’ and mean monthly outdoor temperatures for three types of buildings.
Nudge 1: Allow the adaptive comfort model to be used in mixed-mode buildings.
Our first finding is related to mixed-mode buildings, those that rely on both operable windows and mechanical systems for cooling. There was not enough data in the original analysis to test the comfort relationship in mixed-mode buildings. However, analyzing the larger database showed that adaptive comfort responses in mixed-mode buildings align more closely with naturally ventilated buildings than with those that were air-conditioned, as shown in the graph above. We believe that this nudge will enable broader use of the ACM, enabling a new generation of comfortable and low energy mixed-mode buildings.
Nudge 2: Change the ASHRAE Standard 55 to allow warmer temperatures in Asian countries.
Our second finding is that the original adaptive comfort model remains valid in estimating the thermal comfort of occupants in naturally ventilated buildings. However, analysis of the larger and more geographically diverse dataset revealed that mean indoor temperatures in buildings in Asia were typically warmer than other regions, and led to occupants in those buildings being better adapted to warmer conditions. While this finding is not shown in the above figure, a detailed analysis can be found in the full publication.
Nudge 3: Allow the air conditioning setpoint in your office to vary seasonally.
The original ACM used outdoor climate to describe the environmental context shaping the thermal experience of occupants in naturally ventilated buildings, in which the indoor conditions are closely tied to the natural rhythms outdoors. Our earlier findings were based on analysis of the relationship between indoor comfort temperatures and outdoor climate. But this doesn’t tell the whole picture of thermal comfort in buildings, so we repeated the analysis using indoor climate as the independent variable. We found that people adapt most strongly to prevailing indoor temperatures, regardless of the building conditioning strategy (naturally ventilated, mixed-mode, or air conditioned), and confirmed that people’s adaptive processes and expectations are shaped by exposures to both indoor and outdoor temperatures. This leads to our third nudge — a call to practitioners and building operators to think creatively about air conditioning setpoints. If indoor temperatures were to be nudged upwards slowly enough to allow for adaptation, we could avoid the persistent over-cooling we see in our buildings, and improve energy performance without sacrificing occupant comfort.
We’d like to end with a challenge to all of us. Are you in an office building that feels a bit too cool in the winter? If so, we encourage you to start raising the thermostats during the cooling season and start a movement!
References
de Dear, R.J., and G.S. Brager (1998). Developing an adaptive model of thermal comfort and preference. ASHRAE Transactions, 104 (1), pp. 145-167.
Földváry Ličina, V., et al. (2018). Development of the ASHRAE Global Thermal Comfort Database II. Building and Environment, 142, 502–512.
Parkinson, T., de Dear, R., & Brager, G. (2020). Nudging the adaptive thermal comfort model. Energy and Buildings, 206, 109559.