9 Thermal Comfort
Though so far we have spent time studying heat flows and thermal balancing through the building envelope, buildings are not designed and built just to shield ourselves from the external environment and its temperature. In fact, it can be argued that buildings are erected to improve the quality of life (or at least the productivity, in the case of commercial buildings) of its occupants. From that perspective, maintaining a proper temperature is but one of the many aspects required to achieve the greater objective. As such, the study of the quality of the indoor environment, including the acoustic, visual and thermal environment and its objective and subjective impact on the occupant is of great importance. This field is generally referred to as Indoor Environment Quality (IEQ), and includes sub-fields such as Indoor Air Quality (IAQ) that deal with explicit aspects of the environment.
In ways that are similar to how buildings regulate their internal temperature, our bodies do so as well. In particular, our bodies need to balance the heat that they generate (a byproduct of using up the chemical energy we ingest in the form of food) by dissipating excess heat to the environment and thus avoid overheating. This balance is the basis for human thermal comfort.
There are multiple mechanisms at play to achive this balance and self-regulate, but they all boil down to modifying the heat transfer rate from the body to the environment to match the rate at which heat is being transferred from the environment to the body. Though we have multiple such mechanisms (e.g., constricting blood vessels near the skin, sweating, shivering, etc.) they have limited capacity, as you may have discovered by venturing into the cold or hot outdoors without proper clothing. So it is no surprise that we need technologies (clothing, housing, etc.) to allow extend our bodies’ self-regulation capabilities and be able to continue living in naturally harsh environments.
The total heat production rate for our body is the sum of the rate at which we produce work, and the rate at which we produce heat. Of these two, the heat production rate is the one that matters for thermal comfort. That said, there are many other external factors that affect comfort too including temperatuere, humidity, (solar) radiation and wind velocity.
The total heat (\(\dot{Q}\)) and work (\(\dot{W}\)) production rates together need to be equivalent to the rate at which heat is disspated through our skin. This equivalence is summarized in the following equation:
\[\dot{Q}+\dot{W} = \dot{M} A_{sk}\]
Where \(\dot{M}\) is the metabolic rate, expressed in units of met (1 met = 58.2 \(W/m^2\)), and \(A_{sk}\) is the total surface area of the skin. Of those values, the most important one for thermal comfort is \(\dot{Q}\). Furthermore, \(A_{sk}\) is constant, so most of the change in this heat production rate is caused by the activities that the person is engaged in since they change the metabolic rate from, say, \(0.7\) mets for sleeping, to up to \(250\) mets or more when dancing or engaging in excersise. This rate of heat generated \(\dot{Q}\) is then dissipated into the environment through the surface of the skin and clothing. Though the same kind of heat flow mechanisms we learned about before (namely: conduction, convection and radiation) are at play in this dissipation process, we should also take into account other heat flows due to evaporation (through skin) and other sensible and latent heat flows due to respiration. In other words, the major heat ransfer modes that account for the overall heat transfer rate \(\dot{Q}\) are: \(\dot{Q}_{\text{skin conduction}} + \dot{Q}_{\text{skin radiation}} + \dot{Q}_{\text{skin evaporation}} + \dot{Q}_{\text{respiration - sensible}} + \dot{Q}_{\text{respiration - latent}}\). Here in this chapter we will almost exclusively discuss radiation and convective (sensinble) heat flows through the skin and leave the evalporation and (sensible + latent) heat flows of respiration out of the explanation. The reader is encouraged to review Chapter 3 of Reddy et al. (2016).