Troubleshooting

Masonry Construction, Sept, 2004 by Norbert V. Krogstad

What caused the mold?

Q Our firm recently tuckpointed a hotel in the Chicago area that was constructed in the late 1960s. The walls were brick veneer with 4-inch concrete block backup. There was a 1 1/2-inch air space between the brick and concrete masonry. There were steel hat channels filled with batt insulation onto which gypsum wallboard was attached on the interior face of the block wall. Rooms throughout the complex were similarly constructed.

In conjunction with tuckpointing the building, interiors of the guest rooms were renovated. There was considerable mold growth in the rooms two years following the work. The mold appeared to be growing on the paste of the vinyl wall covering. The problem was by far the worst on the south and west elevations. The interior walls of the rooms were painted prior to the renovation.

Do you have any ideas about what may be causing the problem? Is the vinyl wall covering the culprit?

A There are a number of sources for moisture problems in exterior building walls. Therefore, I would recommend that you contact an investigative engineering firm to conduct a water leakage/moisture investigation.

There are, however, several factors in the description of the problem that lead me to suspect that the source of the moisture may be related to water stored in the masonry walls. Many brick walls can absorb and store considerable water following rains. The amount depends on the absorption rates of the masonry units and the condition of the wall. Water that penetrates the exterior surface of the brick masonry veneer is absorbed by the brick units and the mortar, and can remain in the walls for a long period of time.

Because the rooms are air conditioned and there is a vinyl wall covering on the interior side of the gypsum wallboard, moisture from the brick cavity and block backup could be condensing on the back of the vinyl wall covering. Paste for the wall covering provides food for mold.

Vinyl wall coverings have a very low vapor permeance, generally well below 1 English perm. Consequently, the vinyl wall covering greatly amplifies the problem because it does not allow moisture to evaporate into the interior. Experiencing the worst problems on the south and west sides of the building is common because these walls see the most sun and heat in the summer.

Vapor retarders are often used on the interior side of the metal furring in cooler climates such as Chicago to control interior moisture from moving into the wall system and condensing during the winter. If such a vapor barrier was installed in this building, I would expect that moisture would condense on the exterior face of the vapor barrier and be prevented from reaching the vinyl wall covering. Because mold is growing on the paste for this wall covering, a vapor barrier is either missing or contains enough voids to render it ineffective.

An investigation should be performed to determine the as-built construction of the wall system and to identify the sources of moisture penetration. I would strongly recommend working with a certified industrial hygienist to determine if containment is required during such an investigation in order to prevent the spread of mold throughout the building during investigation and repairs.

Pick the right mortar

Q Our company has been performing repairs on a sandstone colonnade. The repairs were to include repointing the sandstone cladding with a Type N portland cement lime mortar. Another part of the repair was to core holes in the colonnade to insert grouted pins.

When coring was being performed on the colonnade, water from the process began to leak out of the pointed mortar joints between the stone block on the columns. The contractor ground out the Type N mortar joints and "temporarily" repointed them with Type S mortar. This action halted the leakage. The contractor and the office of historic preservation wanted to leave the Type S mortar in place to avoid regrinding the joints a third time.

I want to make sure that we are not creating a condition for spalling of the stone block. The stone was tested in our lab and found to have a compressive strength of 9700 psi. The Brick Industry Association Technical Note 8 states that Type S mortar has a compressive strength of 1800 psi. It doesn't appear that there will be a problem leaving the Type S in place.

Am I missing something?

A My concern is not with the relationship between the compressive strength of the mortar and that of the stone, but rather the compressive strength of the new mortar compared to the compressive strength of the original behind the pointing mortar. Repointing mortar should never be stronger or "harder" than the original behind it to prevent stress concentrations from developing on the outer edge of the masonry.

These stress concentrations can cause spalling of the stone, even if the compressive strength of the mortar is less than that of the stone. Whether or not the stone spalls depends on the magnitude of stresses in the column and the difference between the compressive strength of the repointed mortar and the original mortar.