Water vapor/moisture migration

Water exists in the atmosphere in three forms: as an invisible, gaseous vapor, as liquid droplets, and as solid ice crystals. One important and relatively unique characteristic of water is its almost universal exis­tence in air in the form of vapor, its gaseous form. Vapor is not to be confused with steam. Steam is water that has been elevated in tem­perature above its boiling point (212°F or 100°C), whereas water vapor, also in a gaseous state, is below the boiling point.

Water is present as vapor in indoor and outdoor air and as absorbed moisture in many building materials. Water moisture is continuously being generated by the activities and bodies of a home’s occupants. A family of three generates 16 lb per day. Additional activities such as cleaning, cooking, and showering, and even the presence of plants, produce 1 lb of moisture per day.[1] The amount of moisture in a home is also especially high immediately after construction is completed. For example, significant amounts of water are used in the mixing of concrete, mortar, and even plaster when applicable. A new house of medium size may require 300 gal of water to be mixed in the concrete alone.[2]

Moisture-related problems may arise from changes in moisture con­tent, from the presence of excessive moisture, or from the effects of changes of state, such as freezing, within walls or deterioration of materials due to rotting or corrosion.

Moisture moves in four ways: bulk moisture, capillary action, air leakage, and vapor diffusion. The primary source of bulk moisture is rainwater. Improper flashing and caulking permit bulk moisture to enter a building envelope. Capillary action occurs when moisture moves through porous materials such as concrete or through cracks in materials. Proper detailing and waterproofing help prevent capillary action. Both air leakage and vapor diffusion transport vapor in its gaseous state. This is more difficult to eliminate because the average person respires up to 1 gal of water vapor per day.