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.