Masonry Magazine November 2012 Page. 21
As warm, moist air on one side contacts the cool or cold, dry air on the other side, vapor condenses - right inside the wall cavity.
Diffusion versus condensation
TEMPERATURE AND MOISTURE GRADIENTS also are the key factors driving condensation. As moist, warm air moves across a falling temperature gradient, it cools. When the air reaches its dew point the temperature at which it can no longer hold water-condensation occurs on the surface bordering the temperature gradients.
In most wall structures, the temperature gradient is greatest across the insulation layer. As warm, moist air on one side contacts the cool or cold, dry air on the other side, vapor condenses - right inside the wall cavity.
On the other hand, under certain circumstances, some degree of vapor diffusion actually may help keep wall cavities dry, by allowing any trapped moisture to escape the same way it got in.
So, while an air barrier is intended to prevent air leakage through a wall, care must be taken to ensure it also helps minimize the chances for condensation in the wall cavity.
Modeling vapor drive
DETERMINING whether to use a vapor permeable or vapor impermeable air barrier - and how to use it depends on a couple of key factors:
* Climate: Where the wall is located
* Wall Design: Where the air barrier and the insulation layer are located in relation to each other in the wall; what type of cladding is used. When solar radiation heats the highly absorptive cladding, such as brick veneer and manufactured masonry veneer units, following rain, elevated vapor pressures occur and induce inward water vapor flow through the building wall.
To understand how climatic changes interact with specific wall structure designs, a good modeling tool like Grace's Perm-A-View exterior wall design modeling service can be invaluable. The Perm-A-View analysis is a dynamic simulation of heat and moisture transport through a multi-layer exterior wall exposed to natural weather. The Perm-A-View output is a complete report quantifying the potential for organic (mold) growth on moisture-sensitive www.masonrymagazine.com
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READER SERVICE #144
November 2012
MASONRY
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