By Mark J. Donovan
If you have cathedral drywall ceilings and notice dark scallop patterns near the edge where the roof line hits the interior wall, chances are you have mold growing on the ceiling. This situation is due to the combination of poor insulation in the roof rafters and high humidity in the home.
Specifically what is happening is the cold outside area is transferring to the interior ceiling surface, and as a result the ceiling surface is becoming cold. When the warm, moist air inside the home comes in contact with the cold ceiling surface, condensation occurs in the form of small water droplets. These water droplets not only enable dust particles in the air to adhere to the ceiling, but also lead to the formation of mold. It is this dust and mold that shows up on the ceiling edges as dark spots. The reason it is a scalloped pattern is due to the fact that the cold transfer is less at the roof rafters. Thus wherever there is a roof rafter behind the drywall ceiling the less of a tendency for condensation to occur.
To combat this situation you can use a dehumidifier in the home. Secondarily you can add more insulation to the ceiling, albeit it is difficult to do in a retrofit situation. Blown insulation may be the best approach as small holes can be used to funnel the insulation into the ceiling. The holes can then be plugged and patched, and the ceiling repainted.
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