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.
