Q: I need help making a decision. We own a 30-year-old house with a low-pitch roof. We replaced the roof about four years ago, added a ridge vent and put an extra soffit vent between each original soffit vent. Other than the ridge vent and the gable vents, there is not another venting system.

My question is (and I hope that I can explain it easily) — would more air be drawn up through the soffit vents to the ridge vent if the gable vents were closed off? It seems that the majority of the air being pulled through the ridge vent would be from the gable vents, since the ridge vent is higher. I hope this makes sense.

I realize that air from the soffit vents could be pulled through the gable vents and ridge vents, but just wondered if the gable vent actually draws air into the attic (since they are a bigger opening), to the ridge vents, thereby mostly cooling just the upper portion of the attic and not circulating air throughout.

I sincerely hope someone can help me with this question. We are having our attic de-squirreled and the company will either close off the gable vents or put up a heavier screen. If closing off the gable vents will help pull more air throughout the attic, then that is what we'll have them do. Thank you for your suggestions.

A: In light of contemporary research, your question is an interesting one. First, a little information about attic ventilation:

Attic ventilation serves different purposes in different climates. In cold climates the ventilation maintains the attic at close to exterior temperatures. If the attic were warm, typically due to heat loss from the interior of the house, the snow on the roof would begin to melt. The flow of water from the melting snow tends to dam up against snow and ice and can back up under the shingles resulting in leaking. This phenomenon is called ice damming.

In warmer climates like ours in Georgia, the purpose of attic ventilation is to release the air in the attic that is heated by the sun shining on the roof. Theoretically this process cools the attic, lowers air conditioning bills, and lengthens the life of asphalt based roofing shingles. I say theoretically because some contemporary research indicates that the positive effects of attic ventilation in warm climates are minimal.

Since warm air is less dense than cold air the warmer air in an attic will tend to rise to the top. This is called convection. Any vents near the upper portion of the attic will allow this warmer air to escape. The higher the vents are in the attic the better. When the warmer air escapes, it needs to be replaced, preferably from vents as low in the attic as possible. Typically these vents are in the bottom of the eave or "soffit," and are called soffit vents. It logically follows then that the best place for vents is the ridge of the roof (high) and the soffits (low).

In fact, research indicates that this is, indeed, the case.

The very best way to ventilate an attic is with continuous ridge and soffit vents. As for the gable vents your logic is correct. The gable vents and the ridge vents would, in some cases work against each other. As you said, the ridge vent would tend to get its air from the gable vents, the gable vents would also compete with the ridge vent for exhausting of the attic air, keeping some of the air from getting all the way to the ridge. Finally, due to their location, gable vents can allow wind to enter the attic. The turbulence from the wind tends to disrupt the convective flow of air through the attic.

Before you close off the gable vents, however, you should make sure that you still have adequate ventilation without them. With a good balance between your ridge and soffit vents that contemporary codes require, one square foot of ventilation area for every 300 square feet of attic area. (Length x Width of the attic). This means that for a 1,500 square foot home, you need five square feet of ventilation area. From what you have described to me, you should easily meet this minimum.

Do you have a home repair question? Send e-mail to Ask the Inspector at H&G@ajc.com. Our expert, Bill Garwood, spent 15 years as a building contractor before becoming a home inspector in 1990. He is part owner of a residential inspection firm and a company providing training in building inspection and codes.