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:
1. Bathroom fans that are not vented to the exterior 2. Plumbing and light penetrations that are not properly sealed 3. Un-insulated attic doors 4. Humidifiers 5. Wet basements and crawl spaces are among the biggest problems with indoor moisture
So what can you do to get rid of the condensation?
Eliminate the source of excess moisture. This can be done using de-humidifiers, sump pumps, installing vapor barriers in wet crawl spaces or damp basements. You should consult with a Certified roofer, insulation or water proofing contractor to help determine an effective solution in removing excess moisture from the home.
Make sure your attic is properly insulated. This will help reduce the amount of moist warm air escaping from the home into the attic.
Ventilate the attic. Having proper ventilation will help vent moist air out of the attic. Using vented soffits and a high quality ridge vent is the best way to achieve this. See our page on ventilation for more information.