Air Barrier Continuity

The overall design concept of air barriers in building construction is the creation of a continuous airtight membrane around the building envelope. Therefore, air barrier materials in wall assemblies, to be effective, must be continuous. Breaks in air barrier continuity cause air leaks. In cold climates the breaks can allow significant amounts of warm moisture-laden air to escape from the interior environment and condense on a cold surface in the wall assembly. Conversely, in hot, humid climates, breaks in the air barrier permit moisture-laden air from the exterior environment to infiltrate the building envelope and potentially condense on a cold surface in the wall assembly. Any penetration through the wall assembly or termination of the wall assembly must therefore be detailed to maintain the continuity of the air barrier materials to effectively create an air barrier system. Without continuity of the air barrier materials in the wall assembly, air barrier system performance is less effective. The design/construction professional must take material compatibility and construction sequencing into account when designing an airtight assembly to ensure continuity. A number of connecting air barrier materials exist that are compatible with fluid applied air/moisture barriers to make transitions from one material to the next, for example, rubberized asphalt membrane tapes to connect from wall sheathing to foundation, or low-expanding urethane foam sprays for use between windows and rough openings.

Air Barrier Structural Integrity

Structural integrity of air barriers is important because wind loads are transferred to the most airtight components in a wall assembly-the air barrier materials-and in turn, are transferred to the structure. Negative and positive wind loads stress air barrier materials. If the materials tear or displace with loading, they lose their effectiveness as air barriers. Some building wraps have low air permeability, but they do not perform well when commonly installed because they have many seams that reduce their effectiveness against air leakage, and they are non-structural. If the seams in building wraps are not taped, they do not perform well as air barrier materials. Because building wraps are non−structural, they are susceptible to displacement and tearing from negative wind gusts in cavity wall construction. This compromises their performance in service.1

Fluid applied air/moisture barriers are fully adhered. Adhesion to sheathing exceeds the strength of the sheathing. Tensile adhesion tests show that the paper or glass mat facing fails in gypsum based sheathings, while unfaced sheathings like plywood show adhesive failure at loads in excess of 344 kPa (50 psi, could equate to more than a 2560 km/hr [1600 mph] wind speed). The structural strength of the fluid applied air/moisture barrier in effect equates to that of the sheathing. Deformation while in service is limited to the deformation of the sheathing. This means no tears and no compromise in performance caused by structural loading, provided the sheathing and supporting frame are adequate to resist loads.

Air Barrier Durability

While capable of resisting wind loads without compromise in performance, air barrier materials must also demonstrate durability in a number of other ways, particularly if the air barrier is concealed and inaccessible for maintenance. Durability criteria include:

• Resistance to puncture
• Resistance to pests-rodents, termites, carpenter ants, and other insects
• Resistance to low but sustained negative pressures from building stack effect and HVAC fan effect
• Ability to withstand stress from thermal and moisture movement of building materials, and stress from building creep
• Resistance to UV degradation (during the construction period)
• Resistance to mold growth
• Resistance to abrasion

Fluid applied air/moisture barriers generally do not provide a food source for insects or other pests. By virtue of their excellent adhesion to sheathing and prepared concrete or masonry substrates, they are resistant to puncture and they resist loads imposed by stack effect and fan effect, as well as wind loads. Their resistance to stresses imposed by thermal and moisture movement, and building creep, is mainly dependent on the ability of the joint treatment material to span gaps in sheathing without cracking. This performance, in turn, is dependent on the physical properties of the specific joint treatment material. Similarly, the UV resistance, resistance to mold growth, and abrasion resistance are dependent on the physical properties of the joint treatment and waterproof coating materials.