| Conceptual Reference Database for Building Envelope Research |
Prev Next |
| Essay:Moisture carried by exfiltrationAir pressure differentials across the building envelope are caused by wind, stack effect, and ventilation system. They can induce airflow through air leakage paths in the envelope. In the cold climate, the exfiltration of indoor air to the outside is the main moisture transfer vehicle into the envelope. When the indoor air, at a higher temperature and containing more water vapor, exfiltrates through cracks, the vapor moisture will condense on the interstitial surfaces near the cold regions (Desmarais et al., 1998 and Derome and Fazio, 1998).The most common paths for air leakage in walls are holes for electrical boxes and wiring, plumbing, and intersections between the exterior wall and the floor and between the exterior wall and the ceiling, door and window perimeters, porous materials, and paths left from construction or developed over time. The air leakage paths created by joints between materials and junctions between sections are often the weak points of the assembly and are especially prone to moisture accumulation that causes mold and fungi growth and material deterioration. To prevent this mode of moisture transport, an air barrier system should be included in the envelope to minimize air leakage. The required properties of an air barrier system are: low air permeability; structural rigidity; continuous around the entire building; durable; maintainable and buildable. The Canadian National Building Code 1995, which applies to small buildings, also has requirements for the air barrier system. Continuity in the air barrier system is critical. Even a small hole can allow moist indoor air to exfiltrate and may cause significant damages and render the air barrier useless. Maximum air leakage rates for the air barrier system (not including the joints) are recommended in the Canadian National Building Code (1995). Different strategies, using either interior or exterior approaches, may be employed to create the air barrier system. In the Sealed Polyethylene Technique, based on the interior approach, the polyethylene membrane commonly used in residential construction combines the functions of vapor retarder and air barrier. The Airtight Drywall Approach, an another example of interior approach, uses the interior finish as the main air barrier material and all joints with the floor, the ceiling, and other walls are sealed. The success and life span of this approach is linked to the occupant behaviors (make holes for pictures, install new electrical outlets). Moreover, the cracks developed on the air barrier can reduce or void its efficiency if they are not sealed. Exterior air barrier approaches include the Exterior Membrane Approach, in which the weather barrier is sealed to perform as the air barrier and the Exterior Sheathing Approach, in which the sheathing is sealed to provide the airtightness requirements for air barriers.
Moisture accumulation and dissipation in building envelope Moisture carried by exfiltration Moisture transport by Diffusion in buidling envelope [Guylaine Desmarais, 1999] | ||||||||||