Excess Moisture/condensation/frost In Attic - General DIY Discussions - DIY Chatroom Home Improvement Forum

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Old 12-29-2009, 11:57 AM   #1
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Excess moisture/condensation/frost in attic

Hoping to get your advice on this. Every year around wintertime, i've had excess humidity buildup in the attic. Last year, I went up into the attic and found that they never broke the 2x bathroom vents out into the soffit. I attributed the moisture buildup to this and had the builder remedy the problem. This year, I once again have excess moisture buildup in the attic, which frosts up against the sheeting, then melts during a thaw, and ends up wetting the insulation. Last year, I had the builder replace the insulation due to its performance characteristics being altered after becoming wet. This year, I will likely do the same, but the problem is -- the builder is STUMPED. They don't know why this is happening. None of the soffit vents are blocked, I have one of the really good high-rising Venmar vents, and about 600sq ft of attic space.

My own theory is that the dryer and the bathrooms vented into the soffit are causing the problem, primarily because the louvers are pointed back into the soffit. I had my spouse run the dryer while i was in the attic, and the entire thing smelt laundry-fresh when she did so. This leads me to suspect that with the natural roof convection, it's drawing the warm humid dryer air back into the attic.

The roof is a gable roof, on a mid-unit (connected on both sides) townhome, with a north-south exposure. Only the north side aspenite is wet, the south is fine. There are no leaks or penetrations in the roof, this is purely a condensation problem. I checked all of the bathroom vent boxes, and they were all well sealed and tuck taped. I also checked all of the rigid and flex vent lines in the attic, and all appear to be well sealed with no penetrations.

I need a professional opinion about this, since I am listing the home on the market, and do not want to pass on any problems to the purchaser, especially while the builder is willing to correct the problem at no cost outside of the warranty period.

In particular, I'd be curious to know if there's any section in the Ontario Building Code that stipulates blocking off the soffit ventilation within a certain distance of the exhaust vents

Thank you so much for your help.


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Old 12-29-2009, 04:17 PM   #2
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I am not an expert, but this does not sound right to me. You have both moisture and frost in the attic, not necessarily at the same time. Typically, condensation forms on the "warm side" of something, while frost would be on the "cold side" of it. Since you have both, you must have a warm moisture source going in to your cold attic. You seem to have verified this with your dryer-smell check.

Maybe you could re-route the dryer vent to a location far away, even if you just do it temporarily to test the theory that your dryer or bathroom vents are the source of the moisture.

I suggest getting an old box (will a shoe box fit?) and using duck tape to make yourself a flange and seal to attach this box to the dryer and bathroom vent outlets. "Plumb" from a hole cut in this box (you can use more duck tape to temporarily seal the joints) using 4 inch PVC wdv pipe. Put it a good 20 feet away (that's 30 feet of pipe with the drop to the ground).

Run your dryer a few times as well as take a lot of hot showers to help along the moisture and supply, then check in a week or two to see if the problem is better, worse, or no change.

If it does not eliminate the condensation, then you don't need to move the vents permanently, and you can continue looking for a different cause. If it does eliminate the condensation, then you know what you need to do, and can then figure out the best way to do it. In either case, you are only out the cost of some pvc pipes and fittings, and a little bit of duck tape.

Remember that I'm not an expert, but I like solving problems. First thing is to definitely identify the problem before you install a permanent remedy.


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Old 12-29-2009, 10:38 PM   #3
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You are correct. The soffit venting is returning into the attic space by the soffit vents. Especially since they are pointed towards the house, the area of positive wind pressure. This is why I recommend soffit intake vents located close to the fascia board, farthest away from the building, page 612: http://books.google.com/books?id=Z8a...lation&f=false

Be safe, Gary
If any ads are present below my answer or words underlined/colored, I do not condone/support/use the product or services listed/linked to, they are there without my consent.
17,000 dryer fires a year, when did you last clean the inside of the dryer near motor or the exhaust ducting?
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