Why Is My Attic Roof Frost Covered?

Frost Formation

• Frost appears in attics in cold weather when warm, moist air moves from the lower level of the house to the attic, according to Reuben Saltzman of Structure Tech Home Inspections. Once the warm, moist air hits the cold roof surface, it freezes and forms frost. A homeowner's goal is to prevent the air from getting to the attic. One step is to not use whole-house humidifiers, Saltzman advises. Using a continuous exhaust fan greatly reduces moist air.

Control Home Humidity

• Warm, moist air always rises. Homeowners should turn on a kitchen exhaust fan when cooking, because gas ovens and heavy boiling of liquids add a lot of warm moisture to air. To limit the amount of warm, moist air in a home, the University of Georgia Cooperative Extension Service also recommends washing only full loads of clothes, using floor coverings that do not require wet mopping and opening windows to allow moisture to escape.

  Bathroom fans should also remain on during showering and at least 15 minutes afterward, according to Cornell University's Department of Design and Environmental Analysis. Homeowners should verify that exhaust vents from fans or dryers do not empty into attic space, because they pump out a lot of water vapor. These must be vented directly to the home's exterior but not near soffit intake vents, which will draw the air back into the attic.

Attic Bypasses

• Search for gaps around vents and wires coming through the attic, including furnace and plumbing vents, electrical boxes and wires. To find bypasses, homeowners might have to lift attic insulation and floorboards. Signs of moisture are wet or dirty insulation or dust buildup under the insulation. Seal plumbing and electrical penetrations with expanding foam sealant where they exit and enter the attic.

Ventilation

• Although the most important step is to prevent moist air from getting in the attic at all, the attic should also have adequate natural ventilation. If a vapor retarder is not installed in the home's ceiling, an attic generally requires 1 square foot of free ventilation for each 150 square feet of attic area, according to the Michigan State University Extension. If a vapor retarder is installed, attics need 1 square foot of free vent area for every 300 square feet of attic area. Vent types include eave, gable, turbine, roof and continuous ridge.

Crawl Spaces

• If you have a crawl space, install a vapor retarder -- treated papers, plastic sheets, and metallic foils -- on the crawl space floor, but first clean up any water or mold you find. Crawl spaces should also be vented to the outdoors. Standard vents that are 8 by 16 inches and are located near each corner in the top 8 inches of the foundation will provide good air movement, according to the University of Georgia Cooperative Extension Service. Each 8-by-16-inch vent provides adequate ventilation for about 75 feet of crawl space area.

Insulation

• After taking steps to control home humidity and to provide adequate attic ventilation, check the insulation in the attic or crawl space to make sure there are no openings that allow warm, moist air to enter.