air movement?

[John Dirks Jr]

See the staining in the pic. I suspect this is air leaking through from the soffit area.

I'll hit the area with bleach and then caulk the lines between sheathing and studs.

What else might I do to address this? I'm sure there are other areas but this is the wall that's open now.

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[Jim Katen]

The stains are the result of moisture the moved there from the inside, not the outside.

I'd skip the bleach and the caulk.

[kurt]

I don't think I've ever opened up a wall or ceiling in a bathroom when I didn't see something like this.

It's what happens when there's a vapor retarder (plywood) on the exterior in northern climes.

[John Dirks Jr]

The insulation in the exterior wall was fiberglass batt about 2 inches thick and completely encased in paper. I've not seen this kind before. It was not fixed to the studs in any way. I suppose that has something to do with the staining.

[Tom Raymond]

If you can stomach the cost is a great place for foam, but a buck a board foot is pretty steep for DIY insulation.

Skip cleaning it unless it's moldy. Insulate it with glass, it's cheap and it will outperform the rest of the envelope from the sounds of things. Enclose the exterior walls before you place the tub, it's the plenum space under the tub open to the stud cavity that caused that stain.

[David Meiland]

Agree with the above. Skip the bleach and don't bother caulking the studs to the sheathing, although it is worth finding air leakage in the room and caulking or foaming it, after all mechanicals and before insulation. Use a large box fan to suck air out of the house, then crawl around the remodel room and find all the places it's coming in. After drywall, crawl around again and foam all the holes for plumbing stubs, electrical boxes, etc. If you can keep air from blowing out of the room through those cracks, it won't get in your stud bays and cause that staining.

[kurt]

Yes, the open wall under the tub is always the culprit. It's a good idea to put 1/2" foam on the inside of the wall to cover up that opening.

[Jim Katen]

The insulation in the exterior wall was fiberglass batt about 2 inches thick and completely encased in paper. I've not seen this kind before. It was not fixed to the studs in any way. I suppose that has something to do with the staining.

That stuff was very common in my area in the '60s & early '70s. The paper on the inner side was coated with pitch or resin to make it a better vapor retarder. The paper on the outer side had no coating and wasn't as retarded.

Here's a pic from a rehab project. The old rockwool batts at the upper walls were wrapped with paper on both sides. The new fiberglass on the lower walls is unfaced.

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[John Dirks Jr]

Nice pic Jim. I like that diagonal stud bracing all notched in like that.

[Jim Katen]

Nice pic Jim. I like that diagonal stud bracing all notched in like that.

That's how we built them when I was a kid working with a framing crew of exceedingly grumpy, exceedingly old Italian gentlemen. They thought that plywood was the Devil's sheathing and refused to build anything without let-in bracing.