Thermal comfort: a key element to improve our buildings
The concept of thermal comfort in buildings takes into account two thermal systems: the human body and its habitat. The habitat serves the human by providing favorable climatic conditions. It is in constant interaction with this system, which on average occupies 90% of an individual's time. Thermal comfort is a complex concept to define because it involves both quantifiable physical phenomena and unconscious phenomena. It is through physiological, physical, and psychological definitions that one can attempt to understand this concept. With regard to the individual, it can be said that an occupant is in a state of thermal comfort when they do not feel the need to increase or decrease the ambient temperature: they have a feeling of well-being. Thus, the concept of neutral temperature can be introduced: it refers to the situation in which an individual's thermal environment allows them to have a stable energy balance.
What is the heat exchange between an individual and its environment?
Man is a homeothermic animal with energy needs that result in exchange of matter and heat with his external environment. Thus, thermal exchanges persist within buildings. First, as shown in figure 1, respiration is a source of exchanges, as are evapotranspiration (24%) and food intake (6%). Three other mechanisms also contribute to thermal exchanges:
• Radiant transfers (35%): infrared exchanges between the individual and the building surfaces, which can be warm or cold.
• Convection transfers (35%): heat exchanges due to the movement of the surrounding air around the individual.
• Conduction transfers (1%): exchanges between the individual and the surface with which he or she is in contact.
A healthy human body has a temperature between 36.7°C and 37.1°C (96.8°F - 98.6°F), which is generally higher than the ambient temperature of the individual's environment. In order to maintain this body temperature and keep the individual in a state of thermal comfort and good health, it is necessary to have a balance between:
• thermal exchanges that exist in the external environment that we just mentioned (convection, conduction, radiation, evapotranspiration...)
• physical work
• and internal heat production: metabolism.
What are the differences between summer and winter comfort?
To obtain satisfactory modeling, it is necessary to have good correlations between the values measured on site and those obtained from various calculation models. This is the case for winter conditions where the calculation models are based on a stationary approach ( an hypothesis where the temperature outdoor is the same anytime and everywhere) and the use of a thermal sensation scale as the basis for the thermal comfort scale. However, for summer conditions, due to the observed difference between the comfort temperatures determined on site and those theoretically found via the different models, it is necessary to work on summer comfort differently. There are two reasons for the temperature differences: the first reason is related to various physical phenomena. Indeed, in summer, the physical phenomena that occur during exchanges are more complex than those taken into account in winter, which makes the models less efficient. Sweating also complicates these exchanges. Another factor that makes the use of these models for summer difficult is directly related to the stability criterion: in winter, the environment is stabilized thanks to various heating systems, but conversely, there may not necessarily be air conditioning systems in buildings, which prevents a stable environment in summer. In addition, the individual is also less stable in summer than in winter, as he or she may resort to various actions to improve his or her thermal comfort (changing rooms, modifying his or her activity, acting on openings, etc.). The second reason for the temperature differences between on-site values and theoretical values is related to the psychology of the individual. These psychological parameters are numerous and difficult to take into account. The phenomenon of adaptation must also be added: an occupant of a site is more likely to be tolerant in an uncomfortable situation if he or she knows that it will be relatively short-lived.
Methods for simulating the thermal environment
ASRAE 55 Standard
In 2004, the American Society of Heating, Refrigerating and Air-Conditioning Engineers proposed a new method for defining the comfort zone. The comfort zone is defined based on the average monthly temperature. The model of this standard is an adaptive model that is based on the influence of the outdoor climate and the adaptive nature of human beings. The adaptive graph links the dominant outdoor temperature and the indoor comfort temperature and defines two satisfaction zones (an 80% satisfaction zone and a 90% satisfaction zone).
In 1963, the architect brothers Victor and Aladar Olgyay developed a bioclimatic approach in which building design was based on the local climate in order to reduce heating and cooling costs. This approach led to the creation of "bioclimatic diagrams". These diagrams are constructed using local meteorological data and allow for the identification and characterization of a climate relative to an established comfort zone. This is then a diagnostic tool that can anticipate potential discomfort for a given climate.
In conclusion, thermal comfort is a relatively complicated notion to describe. This difficulty is partly due to the difference in seasonal phenomena (winter and summer) but also because of the multidisciplinary nature of this notion (physical, psychological, physiological). In order to quantify thermal comfort, various adaptive diagrams have been created, whether they are normative (such as ASHRAE 55) or descriptive (such as the various bioclimatic building diagrams)