Thermal Comfort

  • 21 February, 2020

What makes the human body thermally comfortable? It's not just temperature alone. A common misconception is that thermal comfort-the state in which the mind is comfortable with the thermal environment-is strictly due to air temperature. However, our bodies are complex organisms and as such, it takes a lot more to make us comfortable or uncomfortable.

In addition to air temperature-that is just one puzzle piece-factors such as relative humidity, activity, mean radiant temperature, and air motion all play into comfort. Air temperature is the temperature of the air measured as the dry-bulb temperature-the temperature of the air without humidity or moisture. Relative humidity accounts for the moisture, or humidity, in the air. The mean radiant temperature measures the temperature radiating off of surfaces. Then there is the movement of air. Oftentimes, the weather seems a lot cooler due to increased wind speeds (wind chill) on a cool or cold day (or vice versa). In addition to the above factors is our individual bodies' metabolism, which may make some people feel warmer or cooler than others.

Because everyone is different and has their own thresholds of thermal comfort, it is nearly impossible to assign a thermal comfort level to all people. A classic example is air conditioning within an office environment. When it is turned on, some people may still be too warm, others may be just right, and others may resort to wearing a sweater (another factor considered for thermal comfort)!

In fact, the standards for setting thermal comfort only account for most of the occupants of a particular space. ASHRAE (the American Society of Heating, Refrigerating and Air-Conditioning Engineers) sets the thermal comfort standards for office spaces and other programs. This is outlined in the ASHRAE 55 standard, which is based on the predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD)-a method devised from a percentage of people who are thermally dissatisfied. From this, the thermal comfort of a space is essentially geared toward about 80% of the population of a certain space. The range of thermal comfort in temperature is about 68° to 72°F but can be shifted due to the other factors noted above.

With thermal comfort, however, physical comfort is not the only consideration in the design of spaces and integration of building systems, such as mechanical systems. The body's thermal comfort can play a substantial role in the physical and mental well-being of a person, especially in promoting a healthy indoor environment and experience. A hot or stuffy indoor environment due to poor air circulation or high humidity can have adverse effects, making the indoor air quality poor and subsequently affecting the performance of those occupying that space. It is because of this that many healthy building standards promote a high-quality indoor environment. Thermal comfort and a person's well-being are intertwined and, therefore, involves a lot more than simply air temperature.

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