SIERRA CONSTRUCTION GROUP, INC
There are several reasons why condensation forms in the attic on the roof sheathing.
Research at BRC and elsewhere clearly indicates that moisture is carried into attics from below on small currents of air. Moisture laden air leaking from the living spaces is one major source. Surprisingly, crawlspaces can supply much of the damaging moisture to attics. Moist air from crawlspaces rises through openings around plumbing, ducts and wires. Once in the attic, the air cools allowing its water vapor to condense on roof sheathing.
"Don't rely on ventilation alone to take care of moisture in the attic," says Rose. "The best protection against condensation and mildew in the attic is a dry basement or crawlspace. Also important is an airtight ceiling."
Other factors that contribute to excess moisture in the attic and home are: