In the commercial building space, thermal comfort is the holy grail—both incredibly important and notoriously difficult to achieve. When thinking about comfort, it’s easy to make a swift association with temperature, but there are, in fact, several things at play. ASHRAE’s Graphic Comfort Zone Method considers six key factors to find that magical, happy place: metabolic rate, air temperature, air speed, radiant temperature, clo factor, and humidity.
Metabolic rate is the measure of energy produced by the human body, and is always in a state of flux, but like anything else, it can be math-ed. The energy produced per unit surface area of an average person seated at rest is 1 MET, equal to 58.2 W/m2.
Harvard’s School of Public Health made the MET more relatable by dividing activities into light, moderate, and vigorous modes. Imagine a typical day at work—light activity, like browsing through your email, answering said emails, and saying “I hear you, but I feel like...” in meetings, are 1.5 METs. Moderate activity, like walking to the bathroom or washing your lunch dishes, caps out at 2 or 2.5 METs, and vigorous activity like jogging to make your train, or chasing down your CEO to sign a contract, equals out to 7 METs.
Air temperature and speed, the dynamic duo. Measured with a dry-bulb thermometer, temperature is averaged from the air surrounding your body—incorporating ankle, waist, and head heights. So, yes, there would be a slight measurable difference between sitting in your ergonomic chair and using your standing desk.
Air speed is the average rate and flow to which your body is exposed, with respect to location and time. For example, think back to the last time you were in a meeting room without enough air circulation. Then, how glorious was it when you had a moment to open a window? The simple joys in life.
Radiant temperature has nothing to do with beaming smiles, beach tans, or beauty, but everything to do with heat transference. Transference varies depending on the type of material—its ability to absorb or emit heat, and how much of it is exposed. Meaning that if you are sitting in the sun, your radiant temperature is dependent on how much of your body is exposed to light and how much you perspire. Radiant temperature can have a huge impact, even something as small as a skylight can cause a 7 degree perceived difference in temperature.
Clo factor is exactly what you might think it is—thermal insulation aided by layers of clothing. Lowest clo value (0) would be bare-bummed birthday suit, while a full winter outfit would register a high clo value (4). Generally speaking, thicker clothes have a greater insulating ability, unless we’re talking about Under Armour, then that’s some magic voodoo a trade secret.
Humidity is the relative ratio of water vapor in the air to the amount of water vapor that the air could hold. New Orleans in the Summer, or a sauna at 24 Hour Fitness, that’s high HR. Low HR: Las Vegas in 1953, when the city received only 0.56 inches of rainfall. Recommended levels of indoor humidity range from 30% to 60%. Anything below or above feels either too dry or too clammy.
Next time you think about comfort at work, realize it’s a full on MARCH to get there. Lower the shades when radiant temperature gets you heated. Keep clo factor close to heart and bring an extra layer when meeting in a drafty room. And lastly, because your building operators’ day-to-day challenge is to ensure thermal comfort, be sure to say “thanks!”
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