Attic Condensation Problem

Old 01-23-06, 09:35 AM
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Question Attic Condensation Problem


I live in Northern Ohio and last year at this time I had a substaintial condensation problem in my attic. I had multiple issues going on bath vents dircted into the attic, lack of roof and sofit vents. Since then I have had a new roof installed with one large ridge vent and added soffit vents. I also added insulation to the attic not obstructing the soffit vents. To my surprise I went in the attic last weekend and noticed I had some condensation building up. It was not dripping from the roof sheating but obviously is a concern anytime you have moisture in your attic. I have two gable vents on this part of the house on each end of the attic so it creates a tunnel affect. I was told that this might be my problem. That this time of year the cold air from the gables meets with warm and creates the condensation. Do I have to much venting? Should I close the gable vents and let the soffits and ridge do their job? Maybe I am crazy but the attic seems to be colder with the gable vents closed I have another lower level section of the house with an attic(one gable vent) that I do NOT have this problem with. My crawlspace seems to be fairly dry but it gets very wet in are part of the state this time of year. I close the crawl vents up for the winter but maybe I should leave these open? I have turned my humidifier off on my furnace and the humidity in my house is at 38% - 43% throughout the year. My house was built in 1968 but I suspect this problem has been going on for sometime as the roof sheating had some mildew on it which I had treated with micro-ban last year. Help???

Thank You,

Old 01-23-06, 12:04 PM
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First, having gable vents with ridge vents and soffit vents ends up short-circuiting the soffit vents. As warm air rises in the attic, it will exit by the ridge vents, creating a partial vacuum which is filled by air being drawn in from the soffits. However, the gable vents short-circuit this. So, some air goes out from the ridge vents, and is replaced by air from the gable vents. The lower parts of the roof never get ventilated. The general rule is, either gable vents, or ridge vents at the outlet, but not both.

The fact that it is colder in the attic when the gable vents are blocked just proves the above statement, since it shows you that cold air from the outside is entering through the soffits and ventilating your WHOLE attic, not just the top.

When dealing with condensation, the first step is to ensure that your vapour barrier between the warm, moist interior air, and the (supposedly) dry cold attic air is effective. If not, as the warm air escapes from the ridges, the partial vacuum draws some air from the interior of the house. Any source of moisture (bathroom vent, kitchen vent, dryer vent) must be vented directly to the outside (in fact, it is required by code in all jurisdictions that I know about), and if you don't have this, then this would be the absolutely first thing that I recommend you correct. Next, seal off the gable vents. Thirdly, ensure that your vapour barrier is complete. Fourth, check that the soffit cavity has at least a 4 inch high channel into the attic so that the soffit air is not obstructed.

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