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This paper explores the influence and role of both drainage and ventilation drying on the ability of enclosure assemblies to control moisture. Drainage is often the most direct method of removing water from within a wall (i.e., from exfiltration condensation or rain penetration), but it is often not sufficient to provide moisture control. Design approaches that rely solely on drainage to remove moisture from behind the outer layers or cladding ignore the significant quantities of moisture that can be stored in the outer layers of most enclosure walls. Most cladding systems have relatively low vapor permeability and therefore tend to restrict diffusive drying. Moisture trapped in or behind the cladding can be transported into the enclosure by solar-driven diffusion, especially in air- conditioned buildings. Rather than control vapor diffusion, a 6 mille vapor retarder close to the interior may, in many instances, exacerbate wetting and greatly retard drying. The role of ventilation within walls, especially for North American conditions, has not been well researched and there does not appear to be any consensus with respect to the effect of ventilation on drying. We have found that airflow behind the cladding (ventilation) can be an important means of removing moisture stored within and behind vapor impermeable cladding. Calculations, lab experiments, field monitoring, and anecdotal evidence all show that ventilation can not only improve the drying capacity of wall assemblies, it is sometimes necessary for proper performance. Several years of temperature, humidity, and moisture data collected from full-scale wall assemblies installed in a natural exposure and test facility are used to demonstrate these points.

AUTHOR: John F. Straube, Ph.D., Eric F. P. Burnett, Ph.D.
CITATION: Thermal Performance of the Exterior Envelopes of Buildings VII
KEYWORDS: December, Florida, 1998
YEAR: 1998