If you close your eyes for a moment, imaging you are at home in your ideal space, how would you define the environmental conditions in terms of your senses? What would you smell, taste, feel and hear? Could you describe the room temperature, humidity, and air draft?  If you were sitting next to a large window on a cold day or standing on a cold floor or perhaps you are fortunate enough to have radiant heating how would you describe your level of comfort?

Six Primary Comfort Factors

We know it's possible to feel cool even cold when the thermostat says otherwise. That tells us the typical wall mount thermostat is a poor representative of our feelings. The phrase we use to describe what the body actually perceives is  operative temperature which is a combination of the air temperature (what the thermostats says) and the average of all surface temperatures (what your skin feels) or what is called the mean radiant temperature.

Air motion (draft), humidity (% RH), air temperature and mean radiant temperature are the 'environmental factors' influencing your comfort.  In addition, your metabolic rate (met rate) and the clothing you wear (clo value) defined as your 'personal factors' make up the primary comfort factors.

The Role Of Senses

Our five senses collectively represent what our body is experiencing and the mind establishes the basis for our perception of (dis)comfort based on the way we experience the four methods of heat transfer (Figure 1). These four mechanisms can be likened to the four elements (earth, wind, water and air). Convection is the movement of air, evaporation (respiration) is release of water, conduction is contact with a solid - earth, and radiation is rays of heat - sun. 

Figure 1 - Our Mind Interprets What Our Body Feels

Our Skin Sensors

Our body has tens of thousands of sensors that pick up temperature and pressure and they are distributed across our skin as shown in Figure 2. Make note that roughly 21% of our sensors are located on our head which is why when your mom said to wear a hat she knew what she was talking about.

Figure 2

Our Thermoregulatory System

The body produces more heat than it needs, roughly 400 Btu/hr is released in the percentages shown in figure 3.  Of course ones activity and environment will influence these values but it safe to say that radiation plays a major role in controlling comfort. Table 1, Representative Rates at Which Heat and Moisture Are Given Off by Human Beings in Different States of Activity, from Chapter 29, Nonresidential Cooling and Heating, Load Calculation Procedures, 2001 ASHRAE Fundamentals Handbook provides the percentages of radiant losses from a body during various activities and air velocity rates. click here

Figure 3

Comfort Conditions and Humidity

We can thank Willis Carrier for developing a method to help us visualize the influence of humidity and temperature on comfort. The 100 year-old tool is called a Psychrometric Chart. The graphic below is a portion of the chart, which represents the comfort region for a sedentary person during the summer or winter assuming a variance on clothing. You can see if there was one temperature that humans would be comfortable year round, it would be 74 deg F (23 deg C). If you want to experiment with temperature and humidity on your comfort, click this link to use the thermal comfort calculator.

Psychrometric Chart

Our Senses + Sensors + Regulatory System = Survival Mechanisms

If the body loses more heat than it can generate the blood will begin flowing away from the skin towards the internal vital organs and in extreme cases where the bodies core temperature continues to drop, hypothermia can set in and if untreated can lead to death.  If the environment is such that the body is unable to shed its excess heat the blood will flow away from the internal organs toward the skin. If the bodies core temperature continues to rise hyperthermia can set in and if left untreated can lead to death. When our bodies are able to shed excess heat at the same rate that it is generated it is in a state called homeostasis (Figure 4).

Hypothermia - Homeostasis - Hyperthermia
Figure 4

Skilled architects and HVAC designers create environments which allow the body to exist in 'thermal harmony'...when 80% to 85% of the occupants are satisfied it can be said that the architectural/mechanical systems have conditioned the body to acceptable standards.