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Wet sheathing: Is this a problem?

Tony Bouchard | Posted in General Questions on

Hi Guys,

I’m not sure if I have a real problem, or just a side effect of the timeline of construction. Basically, I’m looking for reassurance that this nothing to worry about, or an alarm that I need to react to this in a special way.

Location is Maine, zip 04240. I’m finishing up a room over a garage. The room is 2×6 walls with Roxul. Ceiling is Roxul, and 1 inch of foam board. Floor is 16 inches of blown in cellulose just installed last week. Heat is from a heat pump, I have it set for 62 in the day, and turn it off at night. Walls are Vinyl siding, house wrap, osb, then the Roxul and soon to be MemBrain and Drywall.

Started installing the MemBrain barrier and drywall last weekend, have almost 1 of 3 walls complete. Tonight, in preparing for this weekends work, I noticed a small wet spot on the floor in one corner,
it is about the size of a credit card. No idea how that got there, so I started poking around and
found… The exterior sheeting behind the insulation was really wet. Well, this is new, and I
suppose not a good thing. I checked around the room, on the same wall I have a half insulated stud
bay. The open half is dry, but under the insulation the sheeting is all wet.

My first thought is… Hmm, so this is why we use a vapor barrier. And it is just because I have no
real air barrier installed, physics happens. Hopefully once I get the MemBrain and drywall installed
and finished, there is no problem.

My second thought was… Crap, I’ve got a real problem that needs to be addressed before I can
continue on with the project.

I figure that the first thing someone is going to say is that I have a leak somewhere. Well, the
garage has been up for more than 10 years, and no leaks, the exterior shell is sound. I’m pretty
sure that installing the Roxul insulation didn’t create a bunch of leaks and problems with the structure. No issues until I started completing the project, I suspect the heating and outside weather is the cause, and this isn’t a big deal.

So, problem, or no problem?


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  1. Steve Knapp CZ 3A Georgia | | #1


    I think your suspicion is right (warm air condensing on cold sheathing), but let’s see what the experts have to say. Out of curiosity, how much insulation do you have in the attic and where is the foam located?

  2. GBA Editor
    Martin Holladay | | #2

    We get reports like yours regularly from GBA readers. Here is a link to a previous discussion: Wet basement OSB in new build.

    To avoid the problem, the key is to get the drywall up fast, before the humid indoor air has a chance to make the sheathing so wet. This happens when you build in the winter. Going back and forth between heating the space and cooling the space doesn't help.

    Without drywall, the humid indoor air has unrestricted access to the cold sheathing.

    Installing exterior rigid foam would have been a good idea, which is why we often recommend that practice here at GBA. But you didn't do that.

    There are two schools of thought on what to do about the situation now. One school of thought says, "Keep on working. Everything will dry out in May and June."

    Another school of thought says, "Pull out the insulation and let the sheathing dry out before proceeding."

    It's a judgment call. What to do depends in part about how wet the sheathing is.

    Whatever you decide, hang and tape the drywall quickly after you insulate the stud bays. Or pray for a spell of hot weather.

  3. Tony Bouchard | | #3

    I have R46 of Roxul in the trusses, and 1inch of thermax under that. I have air sealed the foam boards very well, and don't think I have any problems in that area.

    Like Martin, I believe that it will dry out eventually, that is what would happen normally I suppose. And I can't believe that this is totally abnormal for winter building. Too bad there is no hot weather in the forecast for another 5 months.

    I will be contacting some local contractors to see if just moving forward is advisable or if some different action is required first. I think that I will pull a few bays to see just how much moisture is in there. Can't believe that stopping building until spring is really the only solution.

    Unfortunately, this all started because I had one section of drywall professionally finished (difficult stair well), and heat was needed for that work. Before that, I had no problems.

  4. Andy CD Zone 5 - NW Ohio | | #4

    Tony, there's your moisture: " section of drywall professionally finished... Before that, I had no problems." You're working in a garage that's ten years old; all the usual moisture sources (new concrete, bathrooms, people, plants, etc.) don't apply. Except for the several gallons of water in each pail of drywall mud, which dried out nicely from the stairwell but ended up on the first cold surface it found. I'd just carry on with the build--but maybe leave a few windows cracked open as you heat, which allows humid air to escape through there instead of condense on your walls. Your heater will have to work more, but it's worth it to minimize dampness from drywall and paint.

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