Masonry Magazine August 2016 Page. 45
condensation and vapor drive. To protect against the heavy moisture flows caused by precipitation, exterior finishes should be backed by a drainage plane, which redirects water that breaches the outside ofthe wall away from the building.
Ventilating airflow -which enables dampness behind the exterior cladding to dry more quickly-can be facilitated by the use of rainscreen systems that create a capillary break, a space that disrupts the surface tension that holds water in place on a wall surface and allows it to drain out of the assembly.A 1-rnm gap will facilitate drainage; biggergaps maybe called for to cre�ate ventilating airflow to promote drying in wetter climates.
It's also important to plan a moisture escape path for condensation and water vapor on the inside of the wall. Cold air contains less water than hot air, and diffusion will carry moisture from a warm place to a cold place. In order to allow the inside to dry, poly sheeting, vinyl wall coverings and low-perm paint should be replaced with more advanced vapor retarders.
Choosing the Optimum Vapor Retarder
IN "MIXED-CLIMATE" REGIONS like most of North Amer�ica, moisture travels into and through walls in different di�rections from winter to summer. Typical polyethylene vapor barriers do well at keeping moisture out of the wall cavity in the cold season, but can trap it there when summer heat reverses the direction ofvapor drive -a problem that can
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be exacerbated when airtight construction techniques and moisture-retaining reservoir cladding products are used.
An ideal approach is to incorporate a material that can vary its vapor permeance. "S.mart" vapor retarders do just that. These materials are specially engineered to accom�modate the effects of seasonal weather changes on mois�ture flow by altering their physical structure in response to changes in relative humidity (RH). During winter, when RH is low, smart vapor retarders provide high resistance to vapor movement from the interior into the wall cavity. But when RH increases to 60 percent or higher, the mate�rial opens up, allowing water molecules to pass through and preventing moisture from condensing inside the wall.
Conclusion
MOISTURE ISSUES REQUIRE appropriate products and solutions based on climates, environmental conditions and building codes.
Smart vapor retarders can be a particularly valuable component of wall systems in areas with mixed climates. Incorporating ventilation drying in a wall cavity would be another tactic to combat the damaging effects of moisture. The key is to make sure that moisture that gets into the wall system has a path to get out. IMAS
Ted Winslow is product manager, Building Science, Systems & Technical Marketing for CertainTeed Insulation.
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August 2016 � MASONRY I 43