www.epa.gov/iaq/moisture
Figure 1-5 Corrosion of structural steel in a ceiling cavity in a cold climate.
The steel extends into the exterior wall assembly. During cold weather, the
steel near the wall is chilled by cold outdoor air. The building is humidified,
and condensation from high indoor humidity provides the moisture that rusts
the cold steel.
Figure 1-6 Blistering paint on split face concrete block. Wind-driven rain
is the source of moisture contributing to the damage. Water wicks into
the concrete masonry unit (CMU) through pin holes in the paint. The sun
drives water vapor through the CMU. The assembly cannot dry to the interior
because low-vapor-permeability foam board, taped at the joints, insulates the
interior surface of the wall. The wall remains saturated throughout the spring,
summer and fall. The same paint on areas of the wall sheltered from sun and
rain shows no damage.
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