Ask the Builder

Black and Gray Lines on Walls

By Tim Carter
©1993-2008 Tim Carter

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Summary: Black and gray lines on the inside walls of a house confound people. These lines form at regular vertical intervals on the wall surfaces on outside walls, especially those that face north in the northern hemisphere. People who burn candles are especially prone to this problem.

DEAR TIM: I have gray and black lines on the exterior walls of my home. Some friends say the lines are caused by back drafting. Is this the real cause and what is the solution? Deborah D., Carlton, MI

DEAR DEBORAH: I think your friends watch too many movies, especially ones about the exploits of firefighters. The dark-gray lines are not back drafting at all. Back drafting happens when exhaust gases from any fuel-burning appliance are drawn backwards down a chimney. This happens when a house does not have enough combustion air entering through approved combustion air intake vents.

The lines on the walls are perfectly aligned with the exterior wall studs. The gray and black lines are created when airborne dust and dirt stick to the walls in these locations.

The dust and dirt stick to these distinct areas because of condensation. The condensation forms because the studs conduct cold through the wood and then transfer this cold to the wall surface. Water vapor in the air in your home then turns to tiny liquid droplets as the water vapor condenses on these cold strips.

You can stop the problem by lowering the humidity inside your home. Better indoor air filtration may also help the situation.

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