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Interior Wall Moisture with Metal Siding

MCMaine | Posted in General Questions on

Hello, long time reader and have gathered a ton of information from the site, and ready many articles and forum posts. 
My question is in regards to a new construction 32×40 garage with a 2 bedroom apartment overhead located in climate zone 5a/5b in Maine. We built it this summer, and the down stairs is heated by radiant in the slab kept at 60 degrees, upstairs is dense pack cellulose in the 2×12 rafters and 2×6 stud walls heated with 1 15k btu heat pump.  The garage area is a 12′ 2×6 wall insulated with rockwool that I just completed last month. After our recent cold weather, single digits in the day, I pulled back some rockwool and found lots of moisture and frost. The exterior is zip sheathing taped and nail holes filled, with ribbed metal siding. We haven’t been parking snow covered vehicles in there, and there’s no moisture on any windows or the garage doors. 
I understand my moisture problems are more than likely just from there being no interior vapor barrier or sheetrock on the inside. Going forward will the assembly dry back through the insulation as the temperatures moderate, or will all the insulation need to be removed? Does the ribbed metal siding prohibit the walls ability to dry to the exterior? For a garage is installing intello membrane a good idea, and I’m assuming I’ll have to wait until the wall cavity is dry before I can install it? Thanks in advance!


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  1. Expert Member
    Michael Maines | | #1

    The channels in your siding will allow a small amount of drying, but not a lot. When your mini-split is running in air conditioning mode the walls will dry toward the interior, but if you didn't install any sort of interior vapor retarder, you will very likely end up with moisture in your walls for long-enough stretches of the year that you are at high risk of mold growth. Like me, you are probably in a part of Maine that was not required to follow the statewide building code before last July, but there is a reason why the code requires a vapor retarder. Warm air holds a lot more moisture than cold air, so no matter how dry you try to keep the interior, you will always have water vapor trying to get to the cold, dry, great outdoors in the winter.

    1. MCMaine | | #2

      That's unfortunately what I thought. I'm still able to access the downstairs garage portion and will install a vapor retarder prior to sheetrocking. At this point I'll have to wait until warm weather to make sure all is dry. On the second floor with the apartment, how helpful is the sheetrock and trying to make it as air tight as possible to minimize the potential mold/moisture problems without having the vapor retarder?

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