Thermal Comfort:Seven Factors Influencing Thermal Comfort

Seven Factors Influencing Thermal Comfort

You are a person, so you already know a lot about thermal comfort. You have a lifetime of experience. You know that physical exertion makes you “hot and sweaty.” You know you can be more comfortable in a cooler space if you wear more clothes, or warmer clothes. You know that the air temperature matters and that the radiant heat from a fire can help keep you warm and comfortable. You have likely experienced feeling hot in a very humid space and been aware of a cold draft. You have anticipated that a space will be warm and comfort- able or cool and comfortable when you get inside.

As a result, you have personal experience of the seven factors that affect thermal comfort.

Personal

1. Activity level

2. Clothing

Individual Characteristics

3. Expectation

Environmental Conditions and Architectural Effects

4. Air temperature

5. Radiant temperature

6. Humidity

7. Air speed

1. Activity Level

The human body continuously produces heat through a process call “metabolism.” This heat must be emitted from the body to maintain a fairly constant core temperature, and ideally, a comfortable skin temperature. We produce heat at a minimum rate when asleep. As activity increases, from sitting to walking to running, so the metabolic heat produced increases.

The standard measure of activity level is the “met.” One met is the metabolic rate (heat output per unit area of skin) for an individual who is seated and at rest. Typical activity levels and the corresponding met values are shown in Figure 3.1.

2. Clothing

In occupied spaces, clothing acts as an insulator, slowing the heat loss from the body. As you know from experience, if you are wearing clothing that is an effective insulator, you can withstand, and feel comfortable in lower temperatures.

To predict thermal comfort we must have an idea of the clothing that will be worn by the occupants.

Due to the large variety of materials, weights, and weave of fabrics, clothing estimates are just rough estimates. Each article has an insulating value, unit “clo.” For example: a long-sleeved sweat shirt is 0.34 clo, straight trousers (thin) are 0.15 clo, light underwear is 0.04 clo, ankle-length athletic socks are 0.02 clo, and sandals are 0.02 clo. These clo values can be added to give an overall clothing insulation value. In this case, the preceding set of clothes has an over- all clothing insulation value of 0.57 clo.

Typical values for clothing ensembles are shown in Figure 3.2. All include shoes, socks, and light underwear.

Later in this Chapter we will introduce a chart, Figure 3.4, that illustrates comfortable conditions with 0.5 clo and 1.0 clo. As you can see from Figure 3.2, 0.5 clo is very light clothing, and 1.0 clo is heavy indoor clothing.

3. Occupants’ Expectations

People’s expectations affect their perception of comfort in a building. Consider the following three scenarios that all occur on a very hot day:

e A person walks into an air-conditioned office building. The person expects the building to be thermally comfortable.

e A person walks into a prestigious hotel. The person expects it to be cool, regardless of the outside temperature.

e A person walks into an economical apartment building with obvious natural ventilation and open windows. The person has lower expectations for a cool environment. The person anticipates, even hopes, that it will be cooler inside, but not to the same extent as the air-conditioned office build- ing or the hotel.

Standard 55 recognizes that the expectations for thermal comfort are significantly different in buildings where the occupants control opening windows, as compared to a mechanically cooled building. To address this difference, Standard 55 provides different criteria for naturally ventilated buildings, as compared to the criteria for mechanically cooled, air-conditioned buildings.

This difference in expectations also shows up in buildings where occu- pants have a thermostat to control their zone. In general, if occupants have a thermostat in their space, they are more satisfied with their space, even when the performance of the thermostat is very restricted or non-existent (dummy thermostat). This is discussed in the Section 3.3, “Conditions for Comfort.”

4. Air Temperature

When we are referring to air temperature in the context of thermal comfort, we are talking about the temperature in the space where the person is located. This temperature can vary from head to toe and can vary with time.

5. Radiant Temperature

Radiant heat is heat that is transmitted from a hotter body to a cooler body with no effect on the intervening space. An example of radiant heat transfer occurs when the sun is shining on you. The radiant temperature is the temperature at which a black sphere would emit as much radiant heat as it received from its surroundings.

In an occupied space, the floor, walls and ceiling may be at a temperature that is very close to the air temperature. For internal spaces, where the temperature of the walls, floor and ceiling are almost the same as the air temperature, the radiant temperature will be constant in all directions and virtually the same as the air temperature.

When a person is sitting close to a large window on a cold, cloudy, winter day, the average radiant temperature may be significantly lower than the air temperature. Similarly, in spaces with radiant floors or other forms of radiant heating, the average radiant temperature will be above the air temperature during the heating season.

6. Humidity

Low humidity: We know that, for some people, low humidity can cause specific problems, like dry skin, dry eyes and static electricity. However, low humidity does not generally cause thermal discomfort. Standard 55 does not define minimum humidity as an issue of thermal discomfort, nor does it address those individuals who have severe responses to low humidity.

High humidity: Standard 55 does define the maximum humidity ratio for comfort at 0.012 lb/lb. This level of moisture in the air can also cause serious mold problems in the building and to its contents, since it is equivalent to 100% relative humidity at 62°F.

7. Air Speed

The higher the air speed over a person’s body, the greater the cooling effect. Air velocity that exceeds 40 feet per minute (fpm), or cool temperatures combined with any air movement, may cause discomfort —a draft. Drafts are most noticeable when they blow across the feet and/or the head level, because individuals tend to have less protection from clothing in these areas of their body.

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