Moisture Under Attic Storage Sheeting - Insulation - DIY Chatroom Home Improvement Forum

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Old 01-06-2012, 10:37 AM   #1
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Moisture Under Attic Storage Sheeting

About 3 years ago I installed some 5/8" plywood in my attic to store some Christmas decs/etc. I recently had some electrical put in for ceiling fixtures in the rooms below the attic and had to pull up some sections. When I pulled them up I noticed some moisture and mold starting to form on the under side of the sheeting. When I installed I did use furring strips to raise the sheeting about on inch over the existing cellulose, but must not be adequate ventilation to get any moisture out. House was built in 1980 in Pittsburgh, PA and there may not be a vapor barrier between the cellulose and drywall below?

My question is would there be benefit in drilling some ventilation holes in the storage sheeting that wouldn't compromise it's integrity to let some of that moisture out that is obviously building in the space between the sheeting and the ceiling drywall below? If so any recommendations on how many holes, spacing and the diameter of holes that would be sufficient? It wouldn't affect my ability to store if there are holes, so thought it may be helpful.



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Old 01-06-2012, 10:56 AM   #2
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replace your plywood with shipping pallets


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Old 01-06-2012, 11:03 AM   #3
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Air leaks around lights, wiring where it comes through the top plates ect. may be causing some of this.
Also lack or soffit vents, blocked vents, no ridge vents.
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Old 01-06-2012, 01:05 PM   #4
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There were no electrical boxes or penetrations prior to me getting them installed at the time I found the moisture The roof has a full ridge vent and I have soffit vents and two gable vents. There is no sign of moisture anywhere else, but where I created this semi-sealed area under the sheeting.

So anyone think holes will help? I can't get pallets up there without cutting them in half and that isn't likely to happen. Looking for some other options.

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Old 01-06-2012, 01:30 PM   #5
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Yeah. I've seen that happen before. The glue in the plywood acts like a vapor barrier on the wrong side of the insulation system. Drill the holes. All it takes is a little airflow to alleviate the problem. I've even seen the bottoms of plastic storage tubs sweat in the same manner when they were stored in a cold attic.
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Old 01-06-2012, 03:11 PM   #6
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It is important that you get some ventilation in there, if you can't do this or if it doesn't solve the problem, you may need to replace a few things. The sooner your solve the issue, the less complications you are going to face. Unfortunately, this actually happens often but it can be a quick fix in many cases.
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Old 01-07-2012, 06:02 PM   #7
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Thanks. I will drill some holes and hopefully it will dry out some.
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Old 01-08-2012, 09:04 AM   #8
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Wiring chases and plumbing vents are the more obvious pathways, but not the only, or in some cases dominant pathways.

All the top plates need to be sealed as well.

Drilling holes in the plywood will not "ventilate" the space between it and the conditioned space properly.

You should remove the plywood and spray foam or seal all the top plates against bulk air transfer. I would recommend that your furr up the space to get better insulation depth prior to putting the plywood back.
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Old 01-08-2012, 06:13 PM   #9
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Cellulose insulation absorbs and wicks the moisture (condensation) from the living space air below. The moisture rises with the warm air through the ceiling material due to the “stack effect” from mechanical and natural drives;

Now you have blocked the attic air currents from removing the moisture at the top edges of the insulation (also the colder edges to create condensation), the moisture will condense on the colder plywood- exposed on the attic side- as that is the “first condensing surface” and add “night-sky radiation”:

Drilling holes will not help unless the holes remove most of the plywood area---- storage = mold, unless there is a proper air space above the insulation to let the moist air out to exhaust at the peak;

On a slanted (pitched) roof surface, 2” air-space is recommended minimum, under your newly-elevated, holed storage may be 4”- hard to guess because of the large surface area and more importantly --- no slope. A better solution is to use spaced boards as said already, to let the moisture out, better still--- no attic storage…..



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