You notice the difference when you walk in. Some buildings feel comfortable and clean, while others are too hot, have paint peeling off the walls, and are generally permeated by unpleasant odors. This is probably due to a difference in the humidity control in those buildings.
The importance of humidity control in buildings is increasingly reflected in legislation (In ‘Building Codes and Indoor Air Quality’, for the US Environmental Protection Agency: “In “Warm-Humid” climates, equipment shall be installed with sufficient latent capacity to maintain indoor relative humidity (RH) at or below 60%”. This is also part of ASHRAE Standard 62.1.), comprising a component of indoor air quality (IAQ) standards across the globe.
Generally, legislation and literature suggests that the relative humidity of a building should be between 20 % – 60 % (at an ambient temperature of 15 – 25 °C), with a dew point of 7 – 9 °C. The inclusion of humidity control in buildings in legislation and modern building design is because it plays a vital role in comfort, health, safety, and the structural integrity and longevity of building materials or objects within buildings (furniture, art, etc.).
The purpose of humidity control in buildings is therefore threefold:
People are generally more concerned with temperature than with humidity when it comes to their comfort. For this reason, many buildings only regulate temperature, without considering humidity control. However, the relative humidity is a large component of comfort. This is partly t
hrough high humidity leading to poor IAQ because of microbial growth (see below), but also through the effect that relative humidity has on thermal comfort; air temperatures that would be comfortable at low relative humidity can become uncomfortable at high relative humidity because the efficiency of evaporative cooling drops significantly. As a result, humidity control serves to optimize the comfort of occupants, thereby improving concentration, mood, and work ethic.
Health and safety
Naturally, as mentioned above, health and safety are part of being comfortable. However, they are also issues beyond comfort, including the fact that they are a financial concern; workers taking off for being sick can accumulate to a considerable monetary loss. Moreover, similar to comfort, healthy occupants will perform better.
The control of relative humidity in buildings strongly influences health and safety through the direct relationship between humidity and microbial growth (and, therefore, IAQ). Controlled relative humidity can significantly lower the propagation of microbes that damage building materials, release spores that lead to allergic reactions, or cause illnesses (some pathogenic bacteria thrive in high humidity).
For instance, studies have shown that dust mites (a significant source of allergens) can be effectively controlled through maintenance of an indoor relative humidity below 50 %. In general, the contribution a lack of humidity control makes towards poor IAQ is a component of what is commonly referred to as ‘sick building syndrome’ (“eye, nose, and throat irritation; headaches, lethargy, difficulty concentrating, and sometimes dizziness; nausea, chest tightness; and other symptoms.” Apter et al.1994).
Maintenance of building materials
The health of the occupants is not the only concern when it comes to the relative humidity in buildings – there is also the health of the building. As Lü (2002) states, “moisture damage is one of the most important factors limiting a building’s service life”. Interestingly, this comes from poor control of humidity in both directions; either too high or too low.
For instance, in the case of museums, relative humidity too low risks causing fracturing in some materials, while relative humidity that is too high risks causes erosion to artworks and artifacts.
Generally, maintaining a relative humidity that is too low can also result in the incursion of water into buildings (through creating a high enough water vapor deficit between inside and outside a building to allow water to pass through the outer ‘envelope’), damaging building materials through corrosion, and allowing the growth of microbes as discussed above.
However, maintaining a relative humidity that is too high can warp wood, cause metal corrosion, and raise problems with condensation. Effective humidity control (such as through the use of a heating, ventilation, and air conditioning (HVAC) system with an integrated dedicated outdoor air system (DOAS), enthalpy wheel, and radiant cooling system) effectively deals with and removes these issues.
Overall, the purpose of humidity control in buildings is made up of three, non-mutually exclusive, reasons:
- Health and safety
- Maintenance of building materials.
Obviously, each of these are not solely determined by relative humidity. Indeed, relative humidity only comprises a part of international codes for IAQ. HVAC systems are increasingly integrative and complex, controlling humidity (latent energy), temperature (sensible energy), and ventilation. However, the contribution of humidity control is a vital one, for the building and its occupants.