What causes mold to grow? And what are the true best indoor tactics for preventing it?
Mold concerns became highlighted in the 1920s, when commercial and military needs for safe storage and transport of supplies focused attention on preventing biological deterioration of materials. To prevent this, basic conditions that had been used for decades as guides for food storage in small, agrarian communities (damp: bad for grains, good for root vegetables) were applied to warehouse construction and management with the expectation of preserving high material values. When mechanical control of indoor climates linked temperature and humidity requirements to energy consumption, tighter setpoint values were adapted from these early food preservation practices.
Instead of food preservation, let’s look at buildings and mold. Mold is extremely lazy compared to cells which use photosynthesis or glycolysis to create energy for metabolizing foodstuff. For mold to grow, it needs to be supplied with the proper water level, temperature, oxygen, pH, and nutrients. If any of these are withheld, growth decreases.
Of all the components needed for growth, water is the easiest to restrict in buildings. Consequently, building managers often reduce indoor rh to desert-like levels of 15% to 20% with the rationale that decreasing atmospheric rh will decrease available water for mold growth. This thinking is erroneous! Contrary to this popular belief, fungal organisms cannot extract water from even very humid air to use for growth.
Besides spills or leaks, the liquid water in buildings which mold can use comes from gas-phase water molecules encountering cold surfaces and then condensing into liquid water. The culprit here is the cold surface, not the air. In poorly insulated buildings, the dew point at which condensation forms is easily reached, as shown in Figure 1.
This means that humidifying your indoor air to healthy levels of 40% to 60% will not lead to mold growth unless the building shell or other materials such as cold water pipes are not properly insulated.
Scientists have known since the 1930s that liquid water in materials determines mold growth, not the rh of the atmosphere. Experimental evidence from as far back as the 1930s shows a linear relationship between moisture in materials and the hydrolytic deterioration from fungal growth. In all chemical reactions, the concentration of reactant (the water concentration in the material cell wall) controls the rate of a reaction. Conversely, reaction kinetics show that atmospheric rh is not related to the rate of degradation. A report of the U. S. National Bureau of Standards (1947) states, “RH permitting growth of molds varies from 76 to 96 percent for different species; thus protection of materials is assured when the atmospheric RH is below 75 percent.” Even the strict Smithsonian Museum building codes to prevent fungal growth on wooden artifacts recommend a rh range between 45% and 65%.
HVAC systems are notorious for amplifying fungal growth and distributing microorganisms, and they are an excellent source of water through condensation, water droplets, and wet insulation material. In addition, limited visual access means that often HVAC systems are ignored until there is an outbreak of building associated illnesses. The solution to this problem is to prevent leaks and condensation through proper insulation — not by creating a desert-like atmosphere. Drying out indoor air to unhealthy levels is analogous to stopping a fatal hemorrhage by draining out the patient’s the blood. It is better to simply bandage the wounds. ES