Principles of Ventilation

As warm air within a home rises it carries with it moisture generated within the home. (Warm air holds more moisture than cool air.) This warm, moisture laden air eventually reaches the attic. Proper ventilation will allow the warm air to migrate outside.

As a general rule, adequate ventilation requires 1 square inch of Net Free Ventilation per 1 square foot of attic space. Homes with steep pitched roofs will require more ventilation because of the increased air volume in the attic.

The ventilation should be spread evenly between intake and exhaust vents. Ideal ventilation consists of continuous soffit vents and continuous ridge vents. The dynamics of air movement will pull cool air into the attic through the soffit vents and exhaust the warm air through the ridge vent.

Without adequate ventilation warm, moist air cannot easily escape from wall cavities and the attic. The moisture will ultimately migrate through siding, fascia boards, soffits, etc. At the surface of the substrate hydrostatic pressure will compromise the adhesion of the paint. The result will be cracked or bubbling paint, and eventually premature failure of the paint film.

Unfortunately, many homes (particularly older homes) are not adequately ventilated. While additional ventilation can be added, the design of some homes can pose considerable challenges.

When evaluating your ventilation system, it is important to accurately identify the size of your soffit vents. For example, a home with 2,000 square feet of attic requires twelve to nineteen 16” x 8” soffit vents, and twenty-four to thirty-eight 16” x 4” soffit vents.

Equally important is the spacing of these vents. Ideally the vents should be evenly spaced around the home to allow for maximum air flow. Uneven spacing may result in “dead air”, that is, areas with little or no air movement.

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