Regardless of project location or building type, the goal of a successful building design is to keep water out and provide thermal control within the interior spaces. Understanding the basics of moisture control and the role of air, water, and vapor barriers in design of wall assembly systems is important in order to avert potential problems that can arise from air and water infiltration.

The building envelope, or building enclosure, separates indoor and outdoor environments. A building enclosure controls heat flow, airflow, water vapor flow, rain, groundwater, light and solar radiation, noise and vibrations, contaminants, environmental hazards, odors, insects, and fire.

When moisture infiltrates the building envelope, several undesirable conditions can occur, including mold and mildew, structural steel corrosion, and rotting wood. These conditions can result in high energy costs, ongoing maintenance problems, compromised indoor air quality, and failure of one or more architectural and engineering building systems. If they are not adequately addressed, the problems caused by moisture infiltration can potentially increase risk and liability concerns for architects, design professionals, building owners, and building occupants.

Moisture is the general term used for water in its different physical states, and includes solid (ice), liquid (water), and gas (water vapor). For all practical purposes, moisture moves through the building enclosure as liquid water and as water vapor. Moisture problems are generally the result of liquid water accumulation, which could be from liquid water infiltration or condensation of excess water vapor transported by air currents or by vapor diffusion. Condensation of excess water vapor occurs on surfaces with temperatures below the dew point temperature, which is the onset of condensation.

Uncontrolled air leakage occurs from higher to lower pressure and could transport significant amounts of water vapor. Photograph © 2006 David S. Allee

Some moisture problems cannot be avoided. Proper design can help reduce risk and make a building more tolerant to moisture. This article will describe moisture control strategies, and the use of protective barriers, including water-resistant barriers, air barriers, and vapor barriers. Requirements related to the position of each barrier in the building envelope for moisture management and condensation control will also be addressed.