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One more cold sheathing question

Vlad O | Posted in General Questions on

I see that cold sheathing issue has been thoroughly discussed, including an excellent Martin’s article. The takeaway I got (right or wrong) is “eliminate air infiltration, allow wall to dry at least in one direction and don’t worry”. This is happening in Northern Virginia, ASHRAE Zone 4, 111 days with freezing temperature a year on average. House was built in 1986.

Since I happened to have a room with drywall already removed, I decided to add insulation on the inside. First Idea was to add rigid foam, but I was concerned that sheathing is not permeable, and went with mineral wool. Here’s how the wall assembly looks now, from outside in:
– vinyl siding
– no gap
– 1/8″ fan-fold XPS insulation (
– 1/8″ hardboard with foil on one side and some sort of metallic paint on another
– 2×4 16″OC wall with Roxul batts
– a second 2×4 16″OC wall with about half of Roxul batts, half reused fiberglass batts with craft paper facing inside. Studs are staggered.

I put acoustic caulk on every join, except vertically along studs, so air infiltration is reduced but not eliminated. I was getting ready to put drywall on, but then reached behind batts to the sheathing and found it WET. Droplets of condensate are all over. Kind of makes me think that I’ve done something stupid.

Not all stud bays are equally wet. Ones that sit behind two layers the longest (I only work on it a couple of hours a day) are the wettest, but not equally.

What can I do now? Two extreme paths that I see (but would rather avoid) are
– wait until the weather is warm, get a two-part spray foam, fill the first row of 2×4 bays with it, rest with Roxul.
– Remove the second row of studs, and make the wall exactly like it was before. I’m not sure it will eliminate the issue, though. Wood looks like it saw some moisture in its life – again, varies bay by bay. No rot or mold, though.

I appreciate any and all advice or suggestions!

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  1. GBA Editor
    Martin Holladay | | #1

    There are two salient facts in your account. One is that you have an unusual type of sheathing which you describe as "1/8-inch hardboard with foil on one side and some sort of metallic paint on another."
    I've never seen a product like that, but if you have a layer of foil on the outside of your walls, that means that you have an exterior vapor barrier, which is undesirable in winter and which goes a long way towards explaining why moisture would condense on your sheathing.

    The other salient fact is that you have removed your drywall. Once you remove the drywall, the warm, moist interior air has unimpeded access to your stud bays. That increases the chance that you'll get condensation on the interior side of your sheathing in cold weather.

    If your sheathing really is a vapor barrier, I think that your best solution is to remove the Roxul and fiberglass insulation temporarily from your stud bays, and then call up a spray-foam contractor who is familiar with the "flash-and-batt" method.

  2. Vlad O | | #2

    Thank you Martin. I'll check out contractors that you described.

    On a related note though.... In this room, as I now understand, I made matters worse by removing drywall in the winter. But in the rest of the house, the problem must still exist, even if to a lesser extent, right? Air still gets into wall cavities, much slower thanks to drywall and facing on fiberglass. Dirt on the edges of fiberglass batts, and stinkbugs nesting there kind of point to that. So moisture must be condensing on the sheathing.

    Following "where are the bodies" logic from your article, things must not be too bad - walls are not crumbling. This is not a known issue in our neighborhood, where all houses are built the same, except probably for the fan-fold insulation that I bet was added by the previous owner who replaced siding.

    Should I be worried and is there a way to check without too much destruction?

  3. GBA Editor
    Martin Holladay | | #3

    If you want to make some inspection holes in exterior walls, you can start with a closet.

  4. dankolbert | | #4

    I can't think of the name off the top of my head, but it sounds like the fiberboard sheathing that was frequently used in production housing in the 1980's. Probably more like 1/4".

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