Comparison of window performance at different temperatures from “Window Performance for
Human Thermal Comfort,” by Center for the Build Environment, Nov. 2005. (www.bit.ly/WindowThermal-Comfort).
It’s Not Just the Thermostat!
Joanne Coons
When I was in elementary school, if my friend Wendy was cold in her house, her mother would have her run outside without a coat and come back in rather than turn up the thermostat. In late winter, if we have a 45-degree day and I need to slog back to the compost pile, I skip the coat, but if it was the same temperature in the middle of summer, I would throw on a sweat shirt. As I get older, I seem to be more sensitive to my thermal comfort and think about these scenarios. I was happy to attend and learn more about this topic at Efficiency Vermont’s Better Buildings by Design in early February and registered for the session, “Lessons Learned, Thermal Comfort, The Next Energy Frontier” by Brian Just, Engineering Manager.
Statistically, one half of the people in the United States struggle to stay warm. There are six factors that influence thermal comfort. The first two are “personal factors,” metabolic rate and clothing insulation. The last four are environmental factors, air temperature, radiant temperature, air speed and humidity. Building science can assign values for the metabolic rate such as sleeping (0.7) to walking at 2 miles per hour (2.0) to sitting (1.0), The clothing insulation is a compilation of all that you are wearing with assigned values from underwear to outerwear. Next, the environmental factors are considered. The first one, air temperature, is controlled by the thermostat or the “universal compensator.” The second factor, the radiant temperature, is the “hot” and “cold” of surrounding surfaces that can make you feel uncomfortable. The radiant temperature factor is the one factor out of the six which is set at building construction time and is not variable. This is why it’s possible to feel cold in a 72F room when seated near a cool window or wall and why insulated or radiant heated floors make you feel more comfortable. Simply put, heat travels from hot to cold.
The third factor, humidity, which is ideally controlled to a target of 35% to 55% (+/-5%) and keeps things like air quality, bacteria, fungus and viruses in check, provides respiratory eye and skin comfort and decreases condensation on windows and drywall. Lastly, air motion affects thermal comfort if the moving air is three degrees below the room temperature or is moving faster than thirty feet per minute. Thermal comfort is affected when the vertical temperature is greater than a five-degree difference from ankle to head. Scientific studies show that a comfortable floor temperature while wearing shoes is between 66.2 degrees to 84.2 degrees F. The goal here is to make 80% the people in the building happy.
What does this all mean? Here is an example. When you are in a room set at 66 degrees and your goal is to make 80% of the occupants thermally comfortable, the thermal comfort factors that need to be changed if you want to achieve your goal are: put on two thick sweaters (clothing insulation), walk at a rate of two miles per hour (increase metabolic rate), or manipulate the environmental factors by increasing the floor temperature to 90 degrees F and the air temperature to 73 degrees F. Measuring, monitoring and adjusting all these variables is a difficult task, and most people will just reach for the thermostat and crank it up. (Personally, I vacuum when I am cold.) None of this would be necessary if our buildings were built to a higher standard. A code level-built home scores a D or the worst that you can legally build to. Brian Just points out that buying a new code-level home “is like buying a new car, but the car was built in 1985.” Sure, it is new, but none of the current technology or benefits of building science has been applied. Brian Just concluded his presentation with these recommendations: When designing a building, use at least two inches of continuous insulation which means no thermal bridging, triple pane windows, 1.0 air exchange per hour at 50 pascals pressure and a 24/7 high efficiency ventilation system with a Minimum Efficiency Reporting Values (MERV)13 filtration system. (Mr. Just stressed that if you don’t purchase a high efficiency ventilation system, an aluminum core ventilation system will blow cold air on you.)
Joanne Coons teaches at Hudson Valley Community College, TEC-SMART facility teaching. Locally, Joanne advocates for sustainability as a member of the Town of Clifton Park’s GREEN (Government Re-Thinking Energy & Environment Now) and is active in NY-GEO and NYSES. Prior to her current endeavors, she taught high school science for 28 years.
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