Moisture Management in Wall Assemblies: Air, Water, and Vapor Barriers

Selecting the appropriate protective barrier based on climate, codes, and design criteria
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Materials Requirements
The minimum requirement for water-resistant barriers is for the material to withstand a fixed hydrostatic pressure of 55 centimeters (cm) for five hours without leaking, as per the American Association of Textile Chemists and Colorists test, known as AATCC-127 or the Hydrostatic Head test. This standard is based on a No. 15 asphalt felt and is the minimum requirement. There are many water-resistant barriers that meet and exceed this requirement, such as fluid applied membranes, self-adhering membranes, and many non-perforated building wraps. Perforated wraps have virtually no hydro-head and do not qualify as water-resistant barriers.

Water Vapor Transport Through The Building Enclosure

Water vapor can be transported into the building enclosure in two different ways, via air transported moisture and vapor diffusion.

Air transported moisture is the main source of water vapor in the building enclosure. Air leakage occurs through porous materials and unintentional openings in building assemblies, such as cracks and channels, due to the difference in total air pressure across the assembly. The air flows from higher to lower pressure and it could transport significant amounts of water vapor into the building enclosure. The amount of water vapor contained in the air depends on temperature and relative humidity, with warm air being able to hold more moisture than cold air. Even though air leakage has consequences beyond moisture transport, including energy efficiency, indoor air quality (IAQ), and comfort, the air transported moisture is one of the most damaging consequences of uncontrolled airflow.

Air Barriers

Materials that resist airflow are called air barriers. Many building materials can resist airflow and therefore could function as air barrier components. However, for effective envelope airtightness, these materials must be joined together into airtight assemblies, which are further joined into a continuous air barrier system for an airtight building enclosure. Air barrier membranes are materials designed to resist air infiltration and provide a more practical way to achieve continuity. A continuous air barrier controls airflow, hence the moisture migration through air currents as well as other undesirable consequences of unplanned airflow.

The location of air barriers within the building envelope is not important from the standpoint of controlling air leakage, as it can be located anywhere in the wall assembly. However, the location is very important for durability and constructability. For durability, it is preferable to have the air barrier behind the exterior cladding, to protect it from direct weather exposure. For constructability, it is preferable to have the air barrier outward of the structural frame, for maintaining continuity at penetrations associated with structural elements. Air barriers are ideally located towards the exterior of the building envelope, because it is easier to achieve and maintain continuity.

The installation requirements for air barriers are similar to those four water-resistant barriers. In wall assemblies with insulation inside stud cavity, the air barrier is typically installed on the exterior face of the exterior sheathing. In exterior insulation walls, the air barrier can be either sandwiched between the exterior sheathing and the exterior rigid insulation or outside of the exterior rigid insulation. The former installation is most common; the latter should be used when thermal insulation performance can be affected by wind-washing. According to the Building Science Corporation, wind-washing is the phenomenon of air movement that occurs due to wind entering building enclosures, typically at the outside corners and roof eaves of buildings. Wind-washing can have significant impact on thermal and moisture movement, and hence thermal and moisture performance of exterior wall assemblies.

The location of both air and water-resistant barriers in the wall assembly depends on continuity, durability, and maintainability. A single membrane could perform as an air and water barrier, if proper materials are chosen. An air barrier membrane is often water-resistant; however, not all materials that meet the minimum requirement for water resistance, such as asphalt impregnated felts, and papers, are effective air barriers. Other materials, such as perforated wraps, are not effective as water or air barriers

Air Barrier Codes and Performance Requirements
Four main performance requirements for air barrier materials and systems include air infiltration resistance, continuity, structural integrity, and durability.

These criteria are described in Chapter 26 of the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) Fundamentals Handbook and summarized by the Air Barrier Association of America

Air Barrier Materials
There is no national standard for acceptable air infiltration rates of air barrier materials in the U.S. The National Building Code of Canada, which has required air barriers since 1995, specifies that air infiltration rates of air barrier materials must not exceed 0.004 cfm/sq. ft. (cubic feet per minute per square ft.) at 0.30 inch water pressure differential. This standard was adopted by Massachusetts in 2001, and is proposed for adoption by Minnesota in 2007.

Examples of materials that qualify as air barriers include self-adhered membranes, fluid applied membranes, non-perforated building wraps, and closed cell polyurethane foams. Examples of building materials which do not qualify as air barriers include asphalt impregnated papers and felts, perforated housewraps, expanded polystyrene (EPS), plain and asphalt impregnated fiberboard, uncoated concrete block, batt and semi-rigid fibrous insulation, and cellulose spray-on insulation.


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Originally published in Architectural Record.