If you work in an office, chances are you or the person sitting next to you has grumbled about it being too hot or cold. No one likes rugging up on a summer’s day to contend with the air-conditioning. Or having to shed one too many layers in winter to compensate for stifling heat indoors.
According to a paper published today in the journal Nature Climate Change, this scenario is more likely if you’re a woman. Climate control systems in office buildings are often set according to an old formula based on men’s thermal comfort. This gender bias, the authors argue, is wasting energy.
What is thermal comfort?
Keeping office workers from feeling too hot or too cold is no simple task. While most office air conditioners control only air temperature, the way we exchange heat with the environment depends on a suite of environmental factors. And so does our thermal comfort.
Engineers need to consider:
- the humidity
- the movement of air (wind speed)
- the radiation temperature (the temperature of everything the body can “see”)
- the temperature of everything we touch.
In the 1970s, Danish engineer Ole Fanger developed a model to determine the combination of environmental variables that we find comfortable.
Because heat exchange also depends on individual factors such as body size (and therefore body surface area), metabolic rate (that determines metabolic heat production), tissue insulation (related to the amount of body fat), and clothing, Fanger’s own experiments showed that no office thermal environment ever would satisfy everyone.
Even before Fanger, we knew that, at the low wind speeds typical of offices, radiant heat exchange mattered more than convective heat exchange. In other words, radiation temperature is more important for thermal comfort than air temperature. You could argue that offices should have wall conditioners, rather than air conditioners.
In today’s Nature Climate Change paper, Dutch researchers Boris Kingma and Wouter van Marken Lichtenbelt show that if the thermostat is set for men, as it usually is, the air temperature will be too low for women.
Because women are smaller, the authors explain, they generate less metabolic heat than men, and so will not feel comfortable in winter at office temperatures set for men.
By the same logic, if the thermostat is set for Europeans, it will be too low for Asians, who weigh, on average, 30% less than Europeans.
In countries such as Australia and South Africa, where air conditioning generally is used for cooling, setting the thermostat to satisfy large people in summer will leave smaller people feeling too cold.
But while Fanger’s equations predict thermal comfort – how satisfied we are with the thermal environment – that is only one of the body functions relevant to the question of where we set the thermostat.
More than just comfort
Heat exchange also affects our body temperature control (how hot our bodies are), thermal sensation (how hot or cold we feel the environment to be), and our performance (how well we do on a particular task).
Those body functions are not necessarily correlated. In a hot bath, for instance, body temperature rises and we feel hot, but we meet Fanger’s criterion for thermal comfort: we wouldn’t want the temperature to be any different.
Performance at some tasks drops off when body temperature rises, even if we do not feel the environment as warm. For that reason, and those outlined in the Nature Climate Change paper, children probably underperform on learning tasks in classrooms that teachers assess as feeling just right. Perhaps the smaller children should set the thermostat.
As if all that complexity weren’t enough, Australian researchers have challenged Fanger’s 1970s thermal comfort model on the basis of the concept of adaptive thermal comfort. Should we set the thermostat at the same level in winter, they asked, when we are acclimated to colder outdoor environments, as in summer?
Some occupants of offices in the tropics want the thermostat set higher than Fanger predicts. Thirty years ago, people of European ancestry living in Darwin rejected air conditioning in the “the Dry” (July and August) because they felt overcooled. Though it’s unclear whether modern Darwinians, many of whom use air-conditioning at home, would say the same.
So, what can we do?
We certainly could maintain thermal comfort and simultaneously relax the demands on the thermostat if we were prepared to wear warmer clothes in our offices in winter and cooler clothes in summer. Selecting clothing also would solve the dilemma of providing thermal comfort to both men and women in the same office.
In the new Nature Climate Change paper, the authors estimate that energy consumption of residences and offices “adds up to about 30% of total carbon dioxide emissions”.
It’s true, we could substantially reduce the energy required for acceptable thermal environments in offices and consequently reduce greenhouse gases. But that approach would require us to abandon the compulsion to create a shirt-sleeve thermal environment in offices, and to vary the thermostat between summer and winter.
We would also need to switch to wall-conditioning rather than air-conditioning and use green engineering to get the thermal design of the office building right. We can be comfortable without it costing the earth.