Understanding Attic Ventilation

Ventilation in southern climates can be a major cause of humidity problems. The problem of condensation in attics, especially in hot-humid climates, is caused by humid outdoor air coming in contact with cold surfaces in the attic. Most offending cold surfaces are usually supply ducts, but can also be ceiling drywall and metallic penetrations through the ceiling if low interior set points are maintained. The attic air dew point can be higher than the outdoor air dew point because moisture stored in the wood roof framing structures at night is released during the day.

The greatest problem with attic condensation will occur during the daytime when the air conditioning system operates for long periods, causing supply ducts, supply diffusers, and ceiling areas near supply diffusers to remain cold.

For more information on the effect of Attics insulated with spray foam, read our page on venting attics and humidity.