Thermal Comfort - PMV

Expressed as ‘that condition of mind which expresses satisfaction with the thermal environment’, thermal comfort is inherently subjective and based on individual physiological and psychological experience. Therefore, it is impossible to design the perfect thermal comfort solution for a building as it is always variable.

Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD)

International industry standards have adopted calculable indices, such as Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD). These useful approaches enable us to undertake building user comparisons that apply to most people and statistically predict the number of individuals who would express dissatisfaction with certain interior conditions.

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Using air temperature, air movement, humidity, mean radiant temperature, metabolic rate and occupants’ clothing, we can predict the likelihood of occupant comfort. These six variables have specific characteristics and effects on thermal comfort:

Air temperature	Indoor air temperature affects the rate of heat loss from our skin. Although it can be measured precisely and objectively, our perception will differ depending on the outdoor air temperature, the amount of solar radiation at different times of day, activity levels.
Air movement	Air movement affects thermal comfort by conducting heat from a warm surface, such as our skin, to the colder room air and surfaces, and it helps the evaporation of perspiration from the skin. Therefore, to maintain comfort, we respond by adjusting one of the variables we can control, such as the air temperature or clothing.
Humidity	Humidity is the measure of the amount of water vapour in the air, defined as relative humidity (RH). Under humid conditions, the rate at which perspiration evaporates through our skin is lower than under dry conditions. Therefore we usually find overly humid air to be uncomfortable. Also, overly dry air should be avoided due to negative health outcomes, so we design for levels ranging between 30 – 60 % RH.
Mean radiant temperature	Mean radiant temperature is the measurement of the energy radiated by objects and surfaces. Sunlight is one of the greatest sources of heat for comfort. Because radiant energy acts independently of air temperature, we may feel discomfort from the radiant energy even though the air in the room is at a normally comfortable temperature.
Metabolic rate	Metabolic rate, the measure of the amount of energy expended and generated by our bodies, can fluctuate depending on our physical characteristics and activity levels. Our metabolic rate, when engaged in strenuous exercise, will be higher than that when sitting at a desk. Therefore, we experience different sensations of comfort in the same environment.
Clothing	Clothing forms a thermal barrier that creates a layer of trapped body heat between the skin and the clothing and is the easiest to ensure higher levels of comfort.