Moisture Problems Attic Ventilation

The Old House Web

Here aresome tips on diagnosing and correcting moisture problems. Parts of this story: ~~Introduction~~ Attic andmechanical ventilation ~~ Houseson crawl spaces and other moisture sources ~~ Caulkingand weather-stripping ~~ Basementwall condensation problems

Attic ventilation

Adequate natural ventilation is important in the attics of homes. If a vaporretarder is not present in the ceiling to slow migration of moisture from thehome's interior into the attic, attics require 1 square foot of free vent areafor each 150 square feet of attic area. An exception occurs when the attic ventsare located in a high/low configuration (i.e., half of the vent area in theeave/soffit area and the other half in the roof ridge area).

Attic ventilation
Types of attic vents

In that case, 1 square foot for each 300 square feet is adequate. If a vaporretarder is present in the ceiling, attics require 1 square foot of free ventarea for each 300 square feet of attic area.

A number of types of vents are available to provide attic ventilation. As withcrawl space vents, most include louvers or screens to prevent the entry ofinsects and rodents, and these coverings slow air circulation. Therefore, twicethe amount of ventilation is needed in most cases to compensate for thisreduction. With or without a vapor retarder, circulation of the air throughoutthe attic space would be more effective if the vent openings were distributedequally between low areas (eave and soffit) and high areas (roof ridge).

Mechanical ventilation

A final method to reduce interior humidity levels isto control in-home sources of moisture vapor generation.The kitchen, bath, laundry and utility room are primarymoisture generation locations. Considerinstalling exhaust fans or vents in both the kitchen andthe bathroom if they are not present. The vents should beducted directly to the exterior of the home rather thanto an attic or some interior space. Clothes dryers shouldalways be vented to the outside as well. Removingmoisture and depositing it outside is an effective way tocontrol condensation problems in both winter and summermonths.

Three additional sources of moisture periodicallycause problems for homeowners: humidifiers, newconstruction or remodeling, and malfunctioning combustionappliances.

The informationin this article comes from Michigan State UniversityExtension bulletin E-2109, Moisture Problems in the Home.

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