Solar driven moisture in brick veneer

What is the nature of the problem?

In some homes located in col 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.

Why does this problem occur with brick and not with vinyl siding?

The key difference between brick and vinyl is that brick is a moisture reservoir and vinyl is not. Brick absorbs rainwater and holds it within its internal pore structure. Vinyl siding is not porous and does not absorb rainwater. When a moisture reservoir is located on the exterior of a wall assembly it can act as a source of water that can migrate by a process called vapor diffusion. Water vapor will move by vapor diffusion from a high concentration of water vapor to a lower concentration of water vapor. It will also move from a warm surface to a cold surface.

When brick gets wet after a rainstorm and then is warmed by the sun, the water vapor within the brick is at a higher concentration than its surroundings. The water vapor within the brick is also at a warmer temperature than its surroundings. Some of the moisture will move out of the brick into the outside air because the outside air is colder than the brick and because the outside moisture concentration (absolute humidity) is lower than the moisture concentration in the brick. Unfortunately, some of the moisture in the brick will move out of the backside of the brick into the exterior wall cavity through the exterior sheathing. This happens because the brick is also warmer than the wall cavity and because the moisture concentration in the brick is higher than the moisture concentration within the wall cavity.

Think of a brick veneer wall as moisture "capacitor" that is "charged" with moisture during a rainstorm that subsequently "discharges" in the direction of a temperature gradient and concentration gradient. Moisture flow by vapor diffusion occurs from warm to cold and from more to less. When the wall cavity is colder than the brick, and drier than the brick, water vapor will migrate into the cavity by the process of vapor diffusion. This condition only happens during the summer after a rainstorm.

If a reservoir is not present, this mechanism does not occur. Vinyl is not a moisture reservoir; therefore this moisture transport mechanism cannot occur with vinyl siding. If the brick wall never gets wet, this mechanism cannot occur. If the interior wall is never colder than the brick, this mechanism cannot occur. Of course, brick walls will get wet and during the summer, when the interior is air conditioned, the interior wall will be colder than the brick.

"Less common but frequently recommended ˇ°screen assembliesˇ± include a secondary line of defense. Consider the case of brick veneer outside a framed wall, or wood siding attached to furring strips over exterior sheathing. Any water absorbed by the brick or wood can be driven inward by wind and solar heat. But the air space behind the exterior finish promotes drainage and ventilation, and a drainage plane applied over the exterior sheathing provides final protection against water movement.: -- http://www.icfhomes.com/DYKpages/dykARTICLEwtrmgmt.htm

Building Science .com

"rovides objective, high-quality information about buildings. This resource combines building physics, systems design concepts, and an awareness of sustainability to promote the design and construction of buildings that are more durable, healthier, more sustainable and more economical than most buildings built today."