In the last decade, studies have shown air leakage to be a significant potential source of condensation and moisture accumulation in building envelope assemblies (see CMHC,Commissioning and Monitoring the Building Envelope for Air Leakage, by David J. Odom, III; andPreventing Indoor Air Quality Problems in Educational Facilities: Guidelines for Hot, Humid Climates).

Figure 1a: Fluid applied air/moisture barrier applied to sheathing by roller.

By constructing an airtight building envelope, the risk of moisture problems-decay, corrosion, loss of insulation value, mold growth and indoor air quality (IAQ) problems-which can occur because of air leakage and condensation, are minimized. At the same time, airtight construction is likely to be less capable of drying than "air-porous" construction, in the event of water leakage or other unforeseen circumstances that cause water to enter into a wall assembly. The designer then must strive to prevent rain water penetration into the wall assembly, to construct an airtight building envelope assembly of compatible air barrier materials, and to enhance the drying potential of the wall assembly in his/her overall design strategy.

Figure 1b: Fluid applied air/moisture barrier applied to sheathing by spray application.

When incorporating fluid applied air/moisture barriers in wall assemblies, the following considerations are important to effectively control condensation and prevent moisture penetration:

Design Considerations

• Air permeability
• Continuity with other air barrier materials
• Structural integrity
• Durability
• Water penetration resistance
• Water vapor permeability
• Mechanical ventilation
• Construction details and sequencing
• Code compliance
• Climate

Air Permeability

The layers of material that make up a wall assembly have different air permeability. Figure 2 provides a comparison of typical materials used in wall assemblies and their air permeability values.

Energy codes in the United States have begun to require air tightness of the building envelope, but they are not specific about levels of air permeability for air barrier materials. The generally accepted level based on National Building Code of Canada requirements is 0.02 L/(s·m2) at 75 Pa pressure (0.004 cfm/ft2 at 1.57 psf). While many common building materials like plywood and gypsum wallboard meet this standard, a sheathed wall assembly will not perform well as an air barrier unless the joints are treated with an air barrier material. The sheathed wall assembly with treated joints then becomes an air barrier sub-system of the total building envelope air barrier system. The total building envelope air barrier system consists of all the interconnected air barrier materials-for example, treated wall sheathing, roof membrane, foundation waterproofing, windows and doors, and the air barrier connection materials between them.