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.


During the past few years, many homeowners have become increasingly aware of the energy savings afforded by properly venting their attics. On hot summer days, the temperature inside an attic can reach well over 120 degrees, putting additional strain on the cooling system.

The use of soffit vents and ridge vents to ventilate the attic has been well advertised. In addition to the energy savings, a properly ventilated attic can also help protect your paint.

Without proper ventilation, hot moist air can be trapped inside an attic. Moisture build up in fascia boards, soffits, and siding can contribute to mildew growth. In addition, the moisture accumulation can force paint off of a wood surface as it attempts to escape.

Exterior sources of moisture (e.g. rain or sprinklers) can be controlled by caulking, painting or otherwise sealing exposed areas (it is important to regularly inspect all caulk for signs of failure).

Controlling exterior sources of moisture is crucial for preventing wood rot, premature paint failure and other damage. Interior sources are much more difficult to control and are potentially much more damaging. Moisture generated within the home-from cooking, bathing, washing, etc.-must be provided a path to escape.

Such moisture will seek the path of least resistance. If venting is insufficient, the moisture will migrate through walls and ceilings. As the moisture reaches the exterior of the house, it can become trapped between the paint film and the substrate, resulting in premature paint failure.

As the moisture reaches the surface of the wood, it can force the paint film away from its substrate, resulting in bubbling, cracking and peeling. This is most apparent on soffits and fascia.

Passive systems, such as those utilizing soffit and ridge vents, are the most common and energy-efficient. Such systems rely on air pressure to draw cooler air through the soffit vent as warmer air rises and escapes through the ridge vent.

Older homes frequently have insufficient venting. A variety of soffit vents can be easily installed in most homes, and can reduce both energy consumption and maintenance expenses.

While adequate ventilation is most often discussed in relation to its beneficial effects on roofing materials and energy costs, proper ventilation can also help extend the life of paint.