Two forms of moisture typically affect buildings in warm, humid climates: liquid and vapor. Common liquid water sources are roof and plumbing leaks, leaks around windows and doors, and condensation. Common water vapor sources are air, clothes dryers, bathing and other family activities. In these cases, liquid water is turned into a gas where it can then move freely through planned and unplanned openings in buildings.
Air, as we know it, contains some moisture. A phenomenon with “moist” air is that the amount of moisture the air can hold depends on the temperature of the air. As air is heated, it can hold more moisture. As air is cooled, it can hold less moisture. The amount of moisture air holds is commonly stated as “relative humidity” or the relative amount it is holding compared to the maximum amount it can hold, at that temperature. For example, air at 70 degrees and 50% relative humidity (RH) is holding 50% of the moisture air can hold at 70 degrees. Air at 100% is saturated, and cannot hold any more moisture.
As a hunk of air is cooled, its capacity to hold moisture decreases, so it’s RH goes up. If cooled enough, it reaches 100% RH and becomes saturated. If cooled further, the water vapor turns into liquid water; it becomes condensation. (An air conditioner helps dehumidify air because it cools the air below the air’s condensation or dew point temperature, and condenses some of the water out of the air.)
Decay fungi need liquid water. Molds and mildews typically need a humidity higher than 80% RH. If plumbing and roof leaks aren’t enough to worry about, condensation can also provide the liquid water necessary to cause problems. Even without liquid water, high relative humidity can lead to mold growth.
In buildings, cold surfaces exposed to warm, humid air can result in condensation and high RH. In the winter, inside warm air can leak outward and contact cold exterior materials and condense. In the summer, warm, humid outside air can leak in and condense on or raise the RH near cold air conditioned surfaces.
In South Carolina, the dew point or condensation temperature of outside summer air ranges from about 72F in the Greenville area to near 75F along the coast. If this air leaks into a building cooled by air conditioning below its dew point, condensation, mold and decay are possible. To deal with this possibility, air flow needs to be stopped as much as possible, surfaces need to be kept warm and objects that do get wet need to be able to dry.
Buildings and building materials will get wet. To prevent fungal problems, they must dry quickly. Fireplaces, lack of air conditioning, leaky walls and windows, and a lack of insulation actually helped historic buildings dry relatively quickly. With the advent of tighter buildings, indoor plumbing, air conditioning, and insulation, buildings were exposed to more moisture and to slower drying conditions. Controlling moisture is now more important than ever.