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MMEL530 02-03-2012 10:11 PM

Mold on new sheetrock
10 months ago I remodeled a spare room into my daughters room. I removed the old sheetrock, insulation, and rewired. I replaced with new kraft faced insulation and then installed a 6mil plastic vapor barrier. I installed new sheetrock, painted and finished. About 2 months ago, I noticed mold starting in the corners on the sheetrock, and spreading to the base molding. I just now noticed there is another spot in the middle of the room doing the same thing. Before the remodel there was no mold on the sheetrock, but also there was no plastic vapor barrier. Im thinking the mold has something to do with installing the plastic vapor barrier??! Any ideas?

jklingel 02-03-2012 10:25 PM

Your location is very important. Where r u? What else have you done to alter the situation (specifically, anything to raise the humidity, or reduce/eliminate insulation in the corners)? Has it been colder than in past winters? Does the rest of the house have a vb? What is the rest of the wall made of, outside the insulation?

MMEL530 02-04-2012 11:27 AM

I am located in Northern New York. To best of my knowledge there is no vapor barrier in the rest of the house. It hasnt been any warmer this yr, but the temp has fluxuated quite a bit. The wall has plywood on the outside then 2" foam board and board and batten siding covering that.

jklingel 02-04-2012 12:10 PM

Ah. Fairly cold. If your insulation is well done (ie, complete, detailed well, and no excessive air leakage) then I suspect you have a humidity problem. The vb should not focus any water on the inside unless it is very cold and you have the humidity there to generate condensation. Does the foam touch in the corners, or is there a gap? Are the corners lots of studs and no, or little, insulation? Obviously, the corners are cold. The questions is, Why? On the outside, it is generally a good idea to have a rain screen and weather resistant barrier between the siding and the foam. I doubt that having or not having that has anything to do w/ your present problem, though, unless you have air leakage and the accompanying water is condensing out there, thawing when it gets warm, leaking back inside, and wicking everywhere. That sounds too improbable, but that is all I can imagine.

gregzoll 02-04-2012 01:08 PM

Placing the plastic barrier over kraft faced insulation, is only going to cause problems. How humid is this room, along with air temp? The bad thing is, you may end up having to tear everything down and redo it, minus the plastic vapor barrier, if you are keeping the kraft faced insulation.

jklingel 02-04-2012 01:20 PM

Greg: I am no expert, but I believe the foil and poly are not of real concern, IF they are right against each other, as most folks install them. It is not good, but this came up on greenbuildingadvisor, if I recall, and the consensus was that it was "OK, but not great". I think that is how it settled out, anyway. Yes, digging in may be the only option, and doing so when it is cold. That way, one can see if/where any ice is forming in the wall. It's a can of worms to solve from afar.

MMEL530 02-04-2012 01:40 PM

Thanks. Ive been reading up and starting to think the plastic vb is the problem. I never had this issue before, and now I do.

jklingel 02-04-2012 01:53 PM

NY probably does not need a vb, but local codes vary (scientific or not, local codes are codes). If your vb is not required, I'd opt for good air sealing, in and out, and no vb. has/had maps of where vb's are generally not recommended, etc.

Nailbags 02-06-2012 12:00 PM

Your problem is placing the plastic over the vapor barrier. you don't do it if the insulation already has a vapor retarder on it you just locked moisture between the sheet rock and insulation.
my question is who gave you that idea to do such a thing? Because they just cost you twice the cost and twice the labor.

Maintenance 6 02-07-2012 04:05 PM

You can blame it on the plastic or the vapor barrier or whatever, but you've got moisture in the drywall on the warm side of the wall that would otherwise have penetrated into the insulation layer. In order to form moisture, the drywall and vapor barrier had to drop below dew point and/or moisture levels had to exceed 60%. Look for the cause of the excess moisture. No exhaust in bathroom? No air turnover? Room temperatures low enough to allow condensation? This is something more than a vapor barrier issue.

Gary in WA 02-08-2012 09:28 PM

I agree with Maint. above. Also, you don't need an interior vapor retarder facing or the vapor barrier plastic IF the 2" foam board outside is 2# closed-cell:


builttolast 02-09-2012 12:10 AM

Plastic "vapor barrier" became a vapor LOCK. You didnt need that barrier my friend, and as others have said - it just cost you more in material and labor. Plus as maint. said, its more than just the plastic. I cant add anything to what he said.

zhuzhuxiao 02-09-2012 01:11 AM

NY probably does not need a vb

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