Attic Ventilation

Old 12-28-05, 03:02 PM
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Attic Ventilation

When I bought my house over the summer, the inspector informed me that my attic ventilation was inadequate. The base attic area is 1200 square feet. The house has only three flat one foot square vents near the ridqe with no soffit ventilation. Would adding an appropriate amount of soffit ventilation be sufficient? We will be replacing the roof this spring, so changes can be made if need be. Our local climate is hot and humid in the summer and cold and dry in the winter. Thanks.
Old 12-29-05, 10:31 AM
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There is a specific amount of square inches of ventilation per square foot of attic space and I can't recall what it is right now, but it is almost always less than the amount of ventilation acheived by installing a continuous ridge vent and a continuous soffit vent. In short a continuous ridge and soffit vent will be plenty. The only time that this might not be enough is if the ridge is very short and the rafters are very long. Note that the ridge and soffit vents only work together. One without the other will do almost nothing. Also, for these vents to work you must ensure that there is no insulation blocking the air path from the soffit to the ridge. If the roof is vented adding soffit vents may help now, but IMO a ridge vent should be installed during any future roofing job.
Hope this helps.
Good Luck.
Old 12-29-05, 12:45 PM
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we sell to contractors all day. ridge vent is it, and if it's time for a new roof anyway it's a no - brainer
Old 12-29-05, 01:33 PM
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I'm no attic ventilation expert, but I do know that the ratio is 1 sq ft of ventilation to 300 sq ft of attic footprint. This would mean you need a minimum of 4 sq feet (576 sq in) of venting for your 1200 sq ft attic. (1 ft can vents actually have a round hole inside that is less than 8" around, so they only provide roughly 58 sq in of ventilation each) Ideally, your 576 sq in of needed ventilation should be split 40/60 between exhaust (near the peak) and intake (near the soffit). Using this formula, you would need to add 1 or 2 additional can vents (to bring you up to 230.4 sq in. exhaust) and then you would need 7 or 8 soffit vents that measure roughly 6x12, with a free opening of roughly 5x10 (to give you 345.6 sq in of intake). It would be best if these soffit vents would be spaced evenly around the eves so as to create an even draw from all parts of the roof. It's also highly important that there be an allow way between your rafters nearest the soffit venting so that air can flow freely from intake to exhaust. This is usually accomplished by using styrofoam proper vents with insulation stuffed below them to insulate the top plate. A less energy efficient way is just to leave those cavities open- less efficient becasue the top plate is uninsulated.

If you eliminated the can vents entirely and went with a continuous ridge vent as flopshot wisely suggests (more efficient than cans), you would need a minimum of 10 ft of ridge vent, provided you could open up 2 1/2" of sheathing at the peak to provide clear ventilation. Of course, having more ridge vent (the entire length of the ridge) is what usually is done, and more venting is seldom a problem, provided you have adequate insulation in your attic.

If for some reason you can't vent from your soffits due to blocking or some other reason, you might need to look in to a product like SmartVent, which goes on the edge of the roof where your first row of shingles goes. A continuous vent is cut in the roof sheathing, and the SmartVent provides the intake, and then you just shingle over the stuff.

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