What is the ideal relative humidity indoors in our homes?

Ideal relative humidity in indoor spaces

The human body is prepared to react to climatic changes. The normal temperature of the body is 36.5 °C. The thermal sensation, or the human body's reaction to the set of environmental conditions, is also very sensitive to changes in temperature and humidity. Sitting in a room with light clothing and engaging in light activity, the sensation of thermal comfort is reached between 21° C and 25° C. When it comes to interacting with environmental humidity, the body is not so demanding. In fact, the range in which the ideal relative humidity is established is quite broad, between 20% and 75%. The body is also very sensitive to radiation changes. If the temperature is below 18 °C but the sky is clear and solar radiation is received, then the sensation of hygrothermal comfort increases. This is the principle used in slab heating systems or underfloor heating.

To understand what conditions the well-being and its relation to architecture, it must be assumed that the human body produces heat and exchanges it with the environment around it. Within the human organism, chemical transformations occur as it interacts with the environment. This is known as the metabolic index, which varies according to the activity level of individuals.

Functioning of metabolism in the human body

• Basal metabolism: this is the energy required for maintaining vegetative life (in fasting or at rest). It is equivalent to 81 W for an average height male.
• Resting metabolism: this is the minimum metabolism to meet basic physiological needs: 104 W
• Working metabolism: in addition to the functions of basal metabolism, it includes energy expenditures for physical activity. Its value varies depending on the type of activity: from 104 W for an intellectual task to 812 W for intense physical effort.

In order to maintain a constant temperature, the body develops mechanisms to evacuate residual heat. These mechanisms are:

• Convection: this is the transmission of heat from the skin to the surrounding fluid or vice versa. The heat flow is proportional to a convection coefficient and the temperature difference between the air and the skin. The speed of the surrounding air accelerates convection.
• Conduction: this is the transmission of heat between the surface of the body and contact elements. This heat flow depends on the thermal conductivity coefficient of these elements.
• Radiation: this is the unidirectional transmission of heat from the organism to the environment through skin and respiratory evaporation. This heat flow is proportional to the universal radiation constant, the absorption power of the skin, and the temperature difference between the skin and radiant walls.
• Evaporation: this is the unidirectional transmission of heat from the organism to the surrounding air through skin and respiratory evaporation. This loss of heat from the organism depends on the amount of sweat or water evaporated. In turn, evaporation depends on the speed of the surrounding air, its temperature, and the partial vapor pressure of water.