Reason for stains

Masonry Construction, July-August, 2004 by Norbert V. Krogstad

Q We built a large wood frame residence with a brick veneer just outside of Philadelphia. The exterior wall is brick veneer with generally less than a 1-inch cavity, building wrap, OSB sheathing, 2 x 4 studs with batt insulation, 6-mil poly vapor barrier, and an interior drywall.

One year after completion of the house, the owner noted that the wood base around the first floor was warped in many locations. When we removed the warped base, we noticed considerable mold growth on the surface of the gypsum wallboard. We opened up the wall in an attempt to determine the source of this moisture.

We expected to find stains on the OSB sheathing. Instead, we discovered only stains on the top surface of the 2 x 4 plate and the bottom of the studs. Based on the pattern of moisture staining, it appeared water was flowing off the 6-mil poly vapor barrier.

The problem is primarily occurring on the first floor, but there are also some areas on the second level. There are no reported leaks in the basement or on the ceiling in areas below the staining on the second floor. In all cases, the problem is occurring where the exterior surface of the wall is brick masonry.

The problem is generally much less severe below windows than it is in blank areas of the wall. The windows are vinyl with integral flanges that were integrated with the building wrap. It does not appear that the windows area contributor.

The worst damage occurred on the half of the south elevation where the lawn extends to the face of the building. The other half of the south elevation, where there is a patio up to the edge of the house, does not seem to have the same magnitude of problems. The concrete foundation rises approximately 4-inches above the top of the grass, so groundwater should not be a problem.

What is causing this problem?

A From your description, the staining may be from condensation on the exterior side of the vapor retarder. Moisture within the brick masonry can enter the cavity air and condense on the exterior face of the polyethylene membrane. Water condensing on the membrane flows down to the sill plate and results in staining.

It is possible that the problem is worse at the walls adjacent to the grass areas, especially at the first floor, due to watering. New lawns need to be watered frequently until they are well established. This watering often takes place overnight. It is likely that the walls are getting wet as well as the grass.

If the masonry is saturated every evening, the air within the cavity becomes very humid when the sunlight strikes the wall during the day. By keeping the masonry walls in this area wet, there is considerable moisture available to evaporate into the cavities.

The problem can probably be solved by better controlling the sprinkler spray patterns. Vents can also be installed in the masonry to help the walls dry out.