| Conceptual Reference Database for Building Envelope Research |
Prev Next |
| Essay:Moisture from outdoorExterior moisture, most often from direct rain penetration, was a contributing factor in 91% of problems. Most problems occurred at window and door perimeters and decks. [http://www.repp.org/discussion/strawbale/200101/msg00083.html]
A brief survey [by G. Desmarais, 2000 ]] The sources for outdoor moisture are rain and the humidity present in the air. Rainwater penetrating the exterior wall assembly usually causes more damage than the humidity in the air. If not controlled, rain penetration can cause damages in the building envelope. The potential severity of these problems depends on the exposure, which in turn depends on the climate and orientation of the wall. The humidity in outdoor air is not as likely to cause damages as that in indoor air. However, it may reduce the drying potential for the moisture accumulated through exfiltration and rain penetration, which extends conditions favorable to mold and fungi growth. Precipitation and rain penetration The amount of precipitation reaching the wall surfaces depends on the geographic locations, facades orientation and geometry, prevailing wind, and microclimatic conditions (Zhu et al, 1995; Fazio et al., 1995; and, El-Shimi et al., 1982). This amount can be minimized by diversions such as sloping surfaces and gutters, screens such as projections (overhangs, balconies and porch projections) and baffles such as batten strip on vertical wood siding (Beall, 1999). Measures can also be taken to prevent water accumulation by providing drainage, drying, ventilation, so that the water that penetrates a building enclosure may not necessarily cause damages. Water penetration of building envelope is the process in which water enters the envelope system through an exposed surface, joint, or openings (Beall, 1999). Three conditions are essential for this process: (1) the presence of water, (2) an opening to provide a path for the water movement, and (3) a force to drive the water (such as gravity, momentum, surface tension, capillary suction, air currents, air pressure, and hydrostatic pressure) through the opening. No water penetration will occur if any of these conditions is absent. The design against rain penetration has been aimed at mitigating one or more of these three contributing conditions. Solution to rain penetration The face-seal approach is based on elimination of the paths for water by sealing the cladding to form an impermeable barrier. It is widely used for systems with prefabricated panels and EIFSs (Exterior Insulation and Finish Systems). There has been a high rate of failure because this approach has some inherent disadvantages such as: requiring perfection in seals, difficult to maintain, changes in joint width due to thermal and moisture movement, improper selection or application of sealant, weather damage to sealant, etc. In addition, if water does get in through some crack, this system provides no means for drainage or ventilation. Therefore, it is recommended not to rely on this method alone to avoid rain penetration. The last mitigation measure focuses on neutralizing the physical forces that transport water by employing capillary breaks, drips, protected openings, and rainscreen. Among these techniques, there are extensive research efforts on the pressure-equalized rainscreen principle and its applications. This approach minimizes the forces that move water. The first element of this approach is a cavity between the cladding and the exterior sheathing. Vents are located on the cladding to provide venting and pressure moderation. By reducing the air pressure difference between the outside air and the cavity, the driving forces for water penetration is minimized. The second element is that the inner wall must be as airtight as possible. The 10:1 rule of thumb is usually employed, i.e. areas of the unintentional openings in the inner wall should be 10% or less than intentional openings on exterior layer. The last element prescribes drainage and ventilation through weep holes and drip flashing for the water and moisture that does penetrate beyond the cladding. Moisture in outdoor air Moisture presented in the outdoor air affects the moisture content of hygroscopic materials such as brick, sheathing and wood. The relative humidity of the outdoor air, though not controllable, should be considered when designing the exterior wall assembly and details. For example, the high levels found in some climates (Coastal British Columbia and Atlantic provinces) may require compositions providing increased drying potential through the use of more vapor permeable materials and lower moisture contents at the time of construction.
Moisture accumulation and dissipation in building envelope Moisture carried by exfiltration Moisture transport by Diffusion in buidling envelope
| |||||||