It's midsummer, and you came to work dressed appropriate for the weather outside, but you had to bring a scarf and sweater to survive a day in the meat locker otherwise known as "the office." Meanwhile, the guy next to you is doing fine. He even has his sleeves rolled up. It's like the building was designed to make you uncomfortable.
And in fact, maybe it was. A new study suggests that the insulation that's supposed to make buildings more energy-efficient doesn't keep them warm enough---that the standards are biased toward the metabolic rates of men, who tend to burn hotter than women.
Is the question of which sex gets to control the AC more complicated than that? Guess. Just guess.
Buildings are meant to do more than just keep out the rain. They're supposed to maintain comfort: Cool enough so you can sleep without sweating through your sheets, warm enough so you don't shiver through your work day. At least, that's the ideal. Finding that sweet spot has advantages way beyond a lower complaint-to-person ratio---the right temperature keeps morale up, HVAC costs down, and a building's carbon footprint small.
To make all that happen, engineers follow standards published by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. ASHRAE standards determine the amount of insulation a building should have by calculating ambient air temperature, heat radiating off surfaces and devices, humidity, and air speed, along with two human factors: metabolic rate and clothing. These numbers all go into an equation called the Predicted Mean Vote---or PMV. In any given building, the solution to this equation will fall between -3 (for cold) to +3 (for hot). 0 is perfect. The ASHRAE standards recommends that a building have a PMV that solves to plus or minus 0.5.
So, easy-peasy, right? The problem, say researchers in a paper published today in the journal Nature Climate Change, is PMV calculation only uses the standard metabolic rate of dudes.
Metabolism is the biochemistry of turning calories into energy. The rate of a person's metabolism has a lot to do with how much energy that person needs to expend to stay at a comfortable temperature. Women typically have lower metabolisms, so as you'd expect they tend to be cold at a temperature men find comfortable.
The new study found that women doing normal office work have a significantly lower metabolic rate than the one office space engineers use. The authors put 16 women one-by-one into a sealed chamber for 45 minutes while they studied, read, and emailed. You know, office stuff. All the while, the chamber measured oxygen and carbon dioxide---inhalations and exhalations---which are good proxies for metabolic rate. "For example, to burn one molecule of glucose, you require six oxygen molecules and produce six carbon dioxide," says Boris Kingma, who studies thermal comfort at Maastricht University Medical Center in the Netherlands and co-wrote the new paper. After collecting their data and running the numbers, Kingma and his co-author estimated that the ASHRAE standard may overestimate female metabolic rate by up to 35 percent.
Come on! Not only do offices have a glass ceiling, but it lets all the warm air out? "In the last 20 years, a lot of effort has been put into insulation," says Kingma, in order to cut down on artificial heating and cooling. But this work is for naught, because half of the people have to crank on the central air to get comfortable. "What's basically happened is there's now a behavioral aspect that's caused this building to consume more energy than expected," he says.
Now, this finding bears a little skepticism. Even if anecdotally is sure does seem like ladies in offices always have to keep a sweater in their desks, critics say the new research isn't perfect. "They did study 16 females, but not 16 males doing the same thing. That’s the question mark," says Bjarne Olesen, a civil engineer at the Technical University of Denmark. Olesen was involved in the original research for PMV in the 1960s and 70s. "The important thing in the basic standard is we studied a couple of thousand people, not just 16, and almost half were women," he says. He's not the only one who noticed that too-small sample size; an accompanying commentary in Nature Climate Change points it out, too.
But that same commentary also calls the original PMV research on the carpet. Even though research showed that women were more sensitive to temperature changes, in large groups sex differences are ironed out. The PMV assumes for the same metabolic rate---58 watts per square meter---applies to every human, regardless of sex. Olesen says that's not a flaw, that metabolic rate differs more between individuals than it does between sexes.
The argument boils down (sorry) to the way people perceive temperature. The current view is that the body has two types of temperature-sensitive neurons, one for hot and one for cold. These collect sensations and send them to two areas in the brain. In one, a region called the hypothalamus, the information informs the body's autonomic thermal responses. "That's where the body decides if it should shiver or sweat," says Kingma. Other signals arrive in an area called the parietal insula, which is how you become conscious of whether you are too cold or too hot, and drives decisions like "put on a sweater," or "buy a fan."
To Kingma and his co-authors, these conscious impulses are triggered when your body senses (below the level of consciousness) that the external temperature is putting pressure on your metabolic rate---it's having to regulate the effect of releasing too much or too little heat. (Normalized by the amount of clothes you're wearing, of course.)
Olesen doesn't dispute that women may have lower metabolic rates than men, but he says this is balanced by the fact that women are usually smaller, and therefore have less surface area from which to lose heat. He says the anecdotal differences we hear about women shivering next to a perfectly comfortable man come down to clothing choices. "In a lot of offices, especially in summertime, men still wear pants and a dress shirt, while women dress lighter, in skirts or short sleeve shirts," he says. (The women in the current study were wearing T-shirts and sweatpants.) The difference in comfort levels isn't a design problem, Olesen says. It's a matter of men being in control of the thermostat. PMV is error free, he says, and the ASHRAE standard is unlikely to change.
Kingma, you can imagine, disagrees. He admits his sample was small, but says he and his colleague were aiming to point out a flaw in the current standard. "I totally agree that more research and continuing measurements are always necessary," he says. And while the flaw is singular---the metabolic rate is wrong---Kingma says it extends across more than just sexual boundaries. The elderly might have different metabolisms, as might people with different body types. "How much did body composition in the population change relative to the 1960s? In Europe there are a lot more obese people," Kingma says. "I think in America you have the same thing?" I couldn't tell if he was being sarcastic.
And while you're waiting for building engineers to figure it all out? Well...maybe that window opens? Just a crack?