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For over four decades Professor Richard de Dear has focused his research career on defining what occupants want and need from their built environments, and assessing the performance of buildings in terms of meeting those requirements. He is currently the most highly cited active researcher in thermal comfort, with over 230 peer-reviewed papers in the SCOPUS database, as well as several monographs on the subject. Within that body of research, it is his adaptive model of thermal comfort that’s had the greatest impact, not just on the research community but also on the design and operation of actual buildings.
His adaptive model was in 2004 incorporated in ASHRAE Standard 55 and in 2007 in EN 15251 (from 2019 – EN 16789-1). In June 2024 he was awarded the Pettenkofer Gold Medal by ISIAQ in recognition of his outstanding contribution to our understanding of thermal comfort in the built environment.
RdD: I can explain it by saying what it’s not. It is not PMV (Predicted Mean Vote), the invention of an engineer’s mind, with energy inputs balanced against energy outputs.
Human perception is not so deterministic, it’s not linear, it’s circular. It’s a feedback loop. If we feel something that we don’t enjoy or don’t like, we will kind of respond to it, do something to adapt. This response will change the physics of the heat balance. We can adapt clothing or metabolic heat production, but we also adjust our expectations. This feedback loop is what I’m referring to and that’s what we mean by adaptive comfort. We adapt to the thermal situation. We don’t just sit there passively and suffer an uncomfortable environment.
RdD: Psychological. First, I thought the physiological one would be much bigger. Physiologist call it acclimatization. In your first interview [2] Wouter (van Marken Lichtenbelt) talked about it. In my experience of thermal comfort, expectation is the most important thing of all. And I get reminded of it as I travel around the world. And I still get surprised at what people call comfortable and acceptable, in disparate places like India, China, Sweden, for example.
RdD: It is the $64 million question how to nudge comfort expectations. I’m using the language of behavioral economists here, and we actually wrote a paper about it.
RdD: I don’t have all the answers other than to say that indeed it is possible to shift comfort expectations. The best example in my travels was in Japan. Cool Biz, have you heard that expression?
RdD: That’s exactly it. Koizumi was the Prime Minister (in 2005) and in a last-ditch effort to get Japan to meet its Kyoto Protocol commitments, he recognized that probably the most efficient strategy would be just to shift set points in air-conditioned buildings during summer. And so, they really pushed the limits up to 28°C. They had a campaign in which the Prime Minister and the Minister of Environment were telling office workers what they could do to adapt, and clothing was their focal point. It was hugely successful, but perhaps not100% successful. Later they pulled it back from 28 to 27 degrees.
Result of Cool Biz Campaign (source: Wikipedia): After web-based questionnaire survey on September 30, 2005, survey results indicated that 95,8% of respondents knew Cool Biz and 32,7% respondents answered that their offices set the air conditioner thermostat higher than in previous year. Based on these figures, Ministry of Environment estimated that the campaign resulted in a 460,000-ton reduction in CO₂ emission. The results for 2006 were even better, the estimation was 1.14 million-ton reduction in CO₂ emission.) |
RdD: From what I remember, I don’t think it’s a part of a standard, but it is still practiced every summer. They have a winter-time counterpart now too – “Warm Biz” – with wintertime heating setpoints dropping well below levels that we normally find in the Anglosphere.
RdD: Yes, this is something we’ve learned about the adaptive comfort model in the 20 years of its existence as a standard. ASHRAE 55 had to be built on strong empirical evidence and because we had the evidence mostly from naturally ventilated spaces, it was said that we didn’t have enough evidence to generalize beyond naturally ventilated buildings.
In this paper that you’re referring to, 20 years after the original ASHRAE adaptive comfort standard, we did the analysis in a different way, on a vastly enlarged database, and instead of using mean monthly outdoor temperature we took the mean indoor temperature as the X-variable instead. And we did that for all of the buildings we had data from – naturally ventilated, air-conditioned and even mix-mode buildings. Building type became a parameter in the regression model and the data just lined up beautifully, even more strongly when it did for when we used outdoor temperature as the predictor (X-variable). And so, the implication is that actually it’s the indoor temperature that we’re exposed to that actually driving our comfort expectations. We are indoor creatures after all! (Figure 1 (reprint from [3])).
Figure 1. Plot shows the very clear relationship between the mean temperature inside a building and the temperature that people find most comfortable in that building. Each dot represents a building comfort study. These studies have been done in hundreds of buildings all over the world over the last couple of decades, and this graph is a meta-analysis of a database of building comfort studies. A building comfort study consists of a sample of people inside a building filling in a comfort questionnaire while the indoor climate of their room was simultaneously measured. When the questionnaire has been completed by a large enough sample of building occupants across a period of time (often a month), we can analyse the results and statistically identify the optimally comfortable temperature for that particular building for that month in that specific city - we call it “neutral temperature”. The graph shows that, when a building is warm (x-axis), the temperature that its occupants regard as neutral (Y-axis) is also warm, and vice versa; cool buildings tend to have cool neutral temperatures. The most remarkable aspect of the graph is that this dependence of optimally comfortable temperatures on the average temperatures prevailing inside the building occurs in A/C buildings, free running buildings, and mixed-mode buildings.
RdD: It’s so logical and you are absolutely right, but it’s not the way air-conditioned buildings are operated, at last not here. They just have a single set point here in Sydney, and it’s the same set point all year around because it’s too complicated for facilities managers to adjust their delicately balanced building’s controls. Crazy I know!
RdD: It is inevitable this was going to happen. I have to bring ‘the gender wars’ into the discussion now. Women are much more adaptive in their clothing behaviour. It’s been demonstrated over and over again. If it’s hot outside, they will dress more lightly (wearing lighter clo-values). Men are not so responsive, and especially in the finance sector. Men are all wearing suits because they think they look more professional, more credible (stifled laughter).
RdD: Exactly! Huge amounts of greenhouse gas go into the atmosphere so that men can wear their business suits. That’s a very cynical interpretation, but it’s probably closer to the truth than we’d like to admit.
RdD: I wrote it together with a colleague here in Australia, Fan Zhang, and a psychologist in Florida, Peter Hancock. There was a 10 questions paper published in Building and Environment in 2017 [5] which said that people can adapt to their comfort expectations across wide-ranging temperatures, but the costs in terms of lost productivity will be enormous. On that logic the authors asserted that we must keep indoor temperature at 22 degrees.
Peter Hancock is a very famous psychologist - authoritive voice on cognitive performance, and he has mountains of empirical evidence that people are comfortable across a wide range of temperatures and productive as well. So, in that conversation piece you are referring to, we were arguing that there are no serious effects on productivity until you get to the edges of the fairly broad adaptive comfort temperature range.
RdD: That’s a question I’ve received many times. To be honest, thermal comfort is almost a meaningless word. It means different things to different people. A lot of people think that thermal sensation is thermal comfort. I kind of regard comfort as an all-encompassing word that includes thermal sensation, thermal preferences, thermal pleasure, thermal acceptability. It’s just a research domain that includes all of those different dimensions. But once we get inside the domain of thermal comfort, we can start splitting it down into pleasure, acceptability, satisfaction, preference, and lastly, thermal sensation, which is the one that everybody knows about, because that’s what Fanger wrote about. When we say something is slightly cool, it doesn’t say anything about whether we like it or not. Thermal sensation is not synonymous with thermal acceptability or thermal preference. They are really different. And thermal pleasure is the trickiest of them all. That’s where alliesthesia comes in.
RdD: First, let me tell you how I first came across it. I was a PhD student (early 1980s). My supervisor prof. Andreas Auliciems, had been to a conference in Copenhagen (organized by Fanger) and he came back with proceedings (those days in hardcover). I avidly read all the papers and there was one paper by French Canadian physiologist, Michel Cabanac, on human thermal perception and alliesthesia. I read it, I knew it was important, but I couldn’t quite make out what to make of it, so I kind of ignored it and so did everybody else apparently. And then, many years later I rediscovered it through my own experience. Suddenly, I felt what alliesthesia is all about. And the context was very memorable.
You know the Sydney Opera House. In summertime, they often have concerts on the forecourt, at the back of the house. Everybody sits on the steps of the Opera House to listen to classical music (Figure 2). And then it happened. I was there and it was a perfect summer evening. The temperature was perfectly balmy, the sunset behind the Harbour Bridge was perfect. I was thinking to myself, I know the music, I’ve heard this piece before, inside the Opera House, where the acoustics are supposedly much better, but this outdoor concert was next level. I realized that it’s not just the music I’m enjoying, it’s the whole package, the whole ‘vibe’ was perfect, including the bats in the night sky, and of course summer breeze – goosebump stuff! This wonderful gentle, warm breeze coming in off the harbour. And then it came to me; this is what Michel Cabanac was telling us about all those years ago. Thermal pleasure, a million miles away from neutral!
Figure 2. Opera house. (https://www.boudist.com/news/2013/documentary-photography/sydney-opera-house-40th-anniversary)
There are layers of thermal perception. The most basic layer is thermal sensation. It’s cool, it’s warm. And then there are other layers that we don’t often deal with because it gets a bit complicated. And thermal pleasure is being completely ignored.
RdD: It’s a reasonable point you make. Let’s look at a cold climate example. The ambient temperature there might be lower than neutral and then you use some localized heating and that gives you thermal pleasure. Most of the personalized comfort systems are really alliesthesia devices in my mind.
There are also some traditional ones, like an open fire, where you feel the radiation but also aromatic smells, and you hear and see the flames, but of course can be problematic in relation to air quality. In eastern cultures, for example in Japan, they’re very familiar with personal comfort systems. The Japanese vernacular architecture is very lightweight, but Japanese winters can be quite cold. They don’t heat the whole house but they have traditional, localized heating sources, which don’t provide a thermally neutral environment, but they provide a thermally pleasant environment. The contrast is where the pleasure resides.
I think the conventional wisdom in HVAC engineering is to homogenize everything and I think it’s high time to seriously question that. I think variability is not such a bad thing after all. Variability can be through time. Like when we enter a space from outdoors (or vice versa). That’s what we call temporal alliesthesia, with the change through time. That’s what we enjoy.
In most standards is a section called local discomforts. One of them is vertical temperature gradient – cold feet, warm head. And actually, if you flip the gradient, it’s not uncomfortable at all. It’s quite pleasant – cool head and warm feet. That is what we call spatial alliesthesia (change through space instead of time). You can keep it going for much longer time exposures than temporal alliesthesia.
RdD: It’s conflicting. There’s not a really clear picture, but I believe it’s true. And one of the realms in which it could be applied very effectively is in so-called “ultra-long-haul” air travel. They’re now talking about Sydney to London, Sydney to New York non-stop flights. It is very interesting but that’s a whole other topic.
[1] de Dear, R., Brager, G. and Donna, C. (1998). Developing an adaptive model of thermal comfort and preference. ASHRAE Trans., vol. 104, no. Part 1, pp. 1–18. [Online]. Available: http://repositories.cdlib.org/cedr/cbe/ieq/deDear1998_ThermComPref.
[2] van Marken Lichtenbelt, W. D. (2024). The Concept of Comfort and Health May Be Related but Are Not Synonyms. REHVA J., vol. 61, no. 2, pp. 52–54 [Online]. Available: https://www.rehva.eu/rehva-journal/chapter/new-series-of-interviews-the-concepts-of-comfort-and-health-may-be-related-but-are-not-synonyms.
[3] Parkinson, T., de Dear, R. and Brager, G. (2020). Nudging the adaptive thermal comfort model. Energy Build., vol. 206, p. 109559. https://doi.org/10.1016/j.enbuild.2019.109559.
[4] Zhang, F., Hancock, P. and de Dear, R. (2019). Why the ’perfect’office temperature is a myth. The conversationhttps://theconversation.com/why-the-perfect-office-temperature-is-a-myth-111823.
[5] Wargocki, P. and Wyon, D. P. (2017). Ten questions concerning thermal and indoor air quality effects on the performance of office work and schoolwork. Build. Environ., vol. 112, pp. 359–366. https://doi.org/10.1016/j.buildenv.2016.11.020.
[6] de Dear, R. (2011). Revisiting an old hypothesis of human thermal perception: alliesthesia. Build. Res. Inf., vol. 39, no. 2, pp. 108–117, Apr. 2011. https://doi.org/10.1080/09613218.2011.552269.
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