In Cold Offices, It’s All About Your Feet
By Sarah Zhang
30 September 2016
Photos courtesy Getty Images, Stefano Schiavon
When a tech company recently approached College of Environmental Design Assistant Professor of Architecture Stefano Schiavon to test an air-conditioning system for its office, his mind went straight to flip-flops. The system, which would blast cool air from the floor rather than the ceiling, made Schiavon realized he couldn’t use the same models researchers have been using since the 1970s to study thermal comfort. So he decided to test people wearing flip-flops.
Feet, as it turns out, are very sensitive to temperature. When you feel cold, the blood vessels in your extremities are the first to constrict, which is the body’s way of preventing heat loss. “You feel uncomfortable because your feet get numb or close to numb,” said Professor Emeritus of Architecture and Director of the Center for Environmental Design Research Edward Arens. So researchers at CED are now focusing on thermal comfort from the feet up.
Thermal comfort researchers have long had a cache of data on how well different types of clothing insulate the body, but not about shoes. “Shoes haven’t gotten quite as much attention as clothing,” said Arens. “Ten years ago, about all they had data for was men’s shoes with thick socks, going back to the 60s when everyone was wearing suits.”
The office trend now for warming up your feet is space heaters under desks, Arens explained. The problem is they are fire hazards and hugely inefficient because they blow hot air. Blowing air and heat have opposite effects; to warm your feet by blowing air, the heater has to blow extra hot air. And because that air mixes with cooler air in the room, it has to blow extra, extra hot air, leading to fire hazards and wasted resources.
Arens solution was to build an energy-efficient foot warmer that radiates heat onto feet with a light bulb. In preliminary tests among UC Berkeley librarians across campus, the foot warmers kept librarians warm even when the room temperature dropped four degrees Fahrenheit. Not only do these units cut costs in half, the rest of your body can compensate for the four-degree loss by burning more calories without much discomfort.
Another way to direct heat to the feet is through battery-operated heat insoles -- tiny batteries draw wireless power from charging coils in a mat and only use two watts of power, compared to the 1,000 watts of hot air personal heaters use. The challenge in selling office workers on these personal heating devices is their clunkiness.
The tech company requested that Schiavon test an office-wide system that blew cool air at foot level. He set up a test system in the Center for the Built Environment’s controlled environment chamber, a simulated office space where he could control the entire chamber’s temperature, airflow and humidity. Recruiting sandal-wearing students, he placed them in front of computers for three hours and repeatedly surveyed them about their comfort levels.
The conclusion? Feet and ankles are even more sensitive to blowing air than previously thought. This was an especially important finding for women, who are more likely to wear sandals and have bare ankles. To keep people from feeling too cold, foot-level air-conditioning requires fine tuning, like getting a diffuser to slow cold air before it hits ankles. From an energy perspective, it’s good to relax the dress code.
Read Assistant Professor Schiavon’s study on floor-level air conditioning systems in full here.