Human Thermal Comfort- Six must know Variables
A vast majority of mankind often enjoy jointly with each other a thermal comfort level between Sixty-Eight degrees Fahrenheit, up to Eighty-Five degrees Fahrenheit with a relative humidity level at Fifteen to Seventy-Five Percent. There are six reasons that are related to and affect our thermal comfort levels and will fall under two types of conditions (Environmental and Personal Conditions).
There are four main factors associated with environmental conditions (1) Air Temperature (2) Relative Humidity (3) Air Motion and (4) Mean Radiant Temperature. So lets break them down even further for a complete understanding.
A. Air Temperature: The temperature of the air flowing around an individual and is measured in degrees whether it is measured in Celsius (°C) or degrees Fahrenheit (°F).
When air is flowing around us and is relatively at a much higher temperature than our normal comfort zone requires we tend to become very uncomfortable.
B. Relative Humidity: A measurement of the amount water vapor in a water to air mixture compared to the maximum amount possible.
Most of us will feel a lot cooler on a hot day as long as the relative humidity is low along with a breeze, but if the air is humid and still we will feel a lot warmer.
C. Air Motion: Air behaves in a fluid manner, meaning particles naturally flow from areas of higher pressure to those where the pressure is lower. Atmospheric air pressure is directly related to your altitude, temperature, and composition
Basically the air in warmer temperatures will rise in an upward direction, but the air in cold temperatures is denser and will move in a downward direction to replace the warmer air temperatures. This important factor is what causes wind.
D. Mean Radiant Temperature: Is defined as the uniform temperature of an imaginary enclosure in which the radiant heat transfer from the human body is equal to the radiant heat transfer in the actual non-uniform enclosure
If our own body temperature is higher than the surrounding temperatures of other surfaces, our bodies will radiate heat to those surfaces regardless of the air temperature (surrounding surfaces are cold). If our body temperature is lower than the surrounding temperatures of other surfaces an opposite reaction would take place. This is why standing next to a wood burning stove feels so good on a cold day. So the process of our bodies are constantly trying to maintain a internal temperature of about 96° F which is done by either burning fat or “dumping” heat.
There are two main factors associated with personal conditions (1) Clothing Insulation Value and (2) Metabolic Rate.
A. Clothing Insulation Value: Is the capacity of the particular clothing material which balances out the moisture and heat between our bodies and the type of environment we are exposed to.
Depending on the type of materials that the clothing is made of can create and maintain the proper moisture and thermal comfort levels of our bodies. So selection of the type of material will significantly affect the comfort of the wearer. Different clothing fibers hold individual properties that are suited for different environments.
For Example: Natural fibers such as cotton, linen, wool, and silk are breathable and absorbs moisture. There are also some more “Exotic” natural fibers to choose from such as alpaca, camel hair, cashmere, llama, mohair, hemp, jute, and ramie. Synthetic fibers such as polyester, acrylic, nylon, rayon, acetate, spandex, latex, orlon and Kevlar will repel moisture but do not allow air to pass.
Remember: Different environments will demand a certain selection of clothing materials if you want to cause your Thermal Comfort to change.
B. Metabolic Rate: Approximately the minimum number of calories a person needs to burn in order to sustain their basic life functions throughout the day of rest. Proper Breathing and Circulation are of Great Importance.
All six variables should be taken into account when attempting to create an atmosphere of Human Thermal Comfort. By paying close attention to the following factors mentioned in this article will give you more control over your individual comfort levels.
Creating the ideal temperatures during particular Heating or Cooling Seasons are critical to a Healthy and Safe Environment.
So What Should The Ideal Conditions Be? I’m Glad You Asked!!
1. Heating Season: 68 degrees Fahrenheit, between 20% to 40% Relative Humidity.
2. Cooling Season: 75 degrees Fahrenheit while keeping the Relative Humidity below 60%.
If you ever have any questions, feel free to leave them below and I will be more than happy to help you out.
All the best,
Troy Halstenson (Founder of Performance Air and Heating)