solar driven diffusion

Solar Driven Moisture in Brick Veneer-What is the nature of the problem?

In some homes located in cold and mixed climates, with brick veneer cladding systems, mold contamination has occurred within exterior wall cavities. In some homes, decay at bottom plates has also occurred.

Why has this problem occurred?

Mold and wood decay require moisture. So in the simplest sense this is a moisture problem. Sufficient moisture has accumulated within the exterior wall cavities of some homes to lead to mold growth and wood decay. The source for the moisture is rain.

Rainwater is absorbed into the brick veneer after a rainstorm. Water vapor is then transported from the rain-wetted brick veneer through the exterior sheathing into the exterior wall cavities by a process called vapor diffusion. This water vapor condenses on the cavity side of the interior polyethylene vapor barrier. When sufficient moisture condenses on the surface of the polyethylene vapor barrier, it runs down the surface of the polyethylene under the action of gravity where it accumulates on the bottom plate of the wall. If sufficient water accumulates at the bottom plate, mold growth will occur ultimately followed by wood decay.

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Drainage, ventilation drying and enclosure performance, by Straube, J. F. and Burnett, E. F. P., 1998

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EIFS hygrothermal performance due to initial construction moisture as a function of air leakage, interior cavity insulation, and climate conditions, by Salonvaara, M. H. and A. N. Karagiozis, 1998

cause condensation on museum roof

The role of absorbent materials in moderating changes of relative humidity, by Padfield, T., 1998