Q: What causes the dark vertical lines that occur on gypsum wallboard installed directly over the furring strips in brick veneer/concrete masonry walls?

The construction is 4-in. brick masonry walls with 4-in. concrete masonry backup. The interior has light gauge steel hat channels attached to the concrete masonry with rigid insulation between the channels. There is a vapor barrier over the insulation and hat channels. The gypsum wallboard is anchored to the hat channels on the interior face of the vapor barrier.

We were originally thinking that the problem was associated with leakage through the masonry. However, the building has large overhangs and during most rains the water doesn't even get to the brick masonry. Also, the vapor barrier would stop any moisture that did get in the masonry from reaching the interior wall board anyway.

How can this problem be avoided?

A: The darkening of the gypsum wallboard over the hat channels is sometimes referred to as “ghosting.” This phenomenon is typically caused by condensation from interior air during cold weather.

Because the insulation was placed on the interior side of the concrete masonry, both the concrete masonry and brick masonry are cold. The hat channels interrupt the insulation and are in direct contact with the cold masonry walls. As a result, the metal hat channels themselves are cold and cool the gypsum wall-board attached to these channels.

During cold exterior temperatures, a film of condensation can form on the interior face of the gypsum wallboard over the hat channels, depending on the humidity of the interior air. Over time, this thin film of water collects dust from the air and can create staining.

To correct the problem, either eliminate the cold furring strips or reduce the humidity in the interior air during cold weather, which is the simplest solution. However, if the humidity cannot be reduced s sufficiently, the interior s side of the exterior walls must be rebuilt.

One solution is to ins stall rigid insulation directly against the inter face of the masonry and install an air and vapor retarder on the interior face of the rigid insulation that is fully sealed to the window frames and floor system. A separate light gauge steel stud wall using 15/8-in. or 2½-in. metal studs could then be installed on the interior side of the vapor retarder.

The electrical conduit should be installed within this interior stud wall to avoid penetrating the air/vapor retarder, as shown in the illustration.