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Source: MASONRY CONSTRUCTION MAGAZINE
Publication date: November 1, 2008
By Norbert V. Krogstad
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?
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.
Click here to read full article from Masonry Construction