Thermal comfort is a complex topic that combines an understanding of human behavior and physiological response to external environmental conditions. There is a myriad of thermal comfort metrics in existence ranging from simple objective metrics to complex metrics which combine thermal systems and survey results. It should be noted that there is a difference between thermal comfort and thermal risk. Thermal risk refers to issues of immediate life safety. Thermal stress metrics such as humidex, wind chill, and wet bulb globe temperature are used to convey extreme climate conditions that pose varying levels of health implications. For example, wind chill is often used to predict the risk of frostbite or hypothermia.
Thermal Comfort Metrics
Thermal comfort metrics are more nuanced and presented at a higher level of granularity. They are used to convey varying levels of thermal discomfort but generally exclude safety concerns. A space might be deemed uncomfortable using a thermal comfort metric but not be flagged as a safety risk if a thermal risk metric is used. The metrics presented herein correlate comfort to six primary factors.
Four of these factors are related to the ambient environment:
- Dry bulb temperature;
- Humidity;
- Air speed; and
- Mean radiant temperature (temperatures of surfaces around the individual and solar radiation).
The other two are related to the individual. These personal or subjective factors are:
- Garment thermal resistance/insulation; and
- Metabolic rate (The heat generated by the human body resulting from physical activity).
Standard Predicted Mean Vote Modified (SPMV)
In an outdoor environment, the “Standard Predicted Mean Vote Modified” (SPMV*) metric is used as it can effectively account for the physics associated with elevated solar exposure and humidity ranges present outdoors. SPMV* is a modified version of the ASHRAE PMV metric which better accounts for the larger range of outdoor conditions.
Thermal comfort is a useful concept and SPMV* provides a method to communicate this concept. Still, it should be noted that the SPMV* index is based on surveys and perceptions of individuals. It is based on 80% of respondents agreeing on the condition they are feeling. It is, in no way, a definitive measure of an individual’s thermal comfort. Also, the variety of conditions and activities that impact the perception of comfort are large and varied.
Any thermal comfort analysis should therefore be viewed as a guide and not as a quantitative assessment. Finally, with any numerical scoring, a definition of good or “acceptable” is required. While the SPMV* index can be used to define the human thermal balance, and a neutral rating (0) might be comfortable indoors, people generally find a wider range of conditions acceptable outdoors and are willing to tolerate some measure of “discomfort” as being acceptable.
Comfort Categories
In temperate environments, the index is typically broken up into multiple ranges:
Comfortable (-1 to 1): People are generally comfortable. The conditions are perceived to be similar to indoor conditions.
Acceptable (1 to 2 or -1 to -2): This represents a comfortable outdoor environment (allowing for greater variability).
Uncomfortable (2 to 3 or -2 to -3): People will accept these conditions outside but will seek cooler or warmer spots when possible.
Intolerable (> 3 or < -3): People will not linger and will attempt to move to protected environment