Thoughts on sealing up old wood staining vs. treating it first?

I’ve been in that exact spot with my 1938 Cape Cod. When I pulled the old rock wool out of the kitchen walls, the sheathing looked like it had been thru a swamp, even though it was dry as a bone. My rule of thumb is that if the wall is open, you might as well spend the $30 and twenty minutes to hit it with a fungicide. I usually spray everything down with Concrobium instead of a bleach-based cleaner like RMR-86 if it's alredy dry. Bleach is mostly water and can actually feed mold spores deep in the wood fibers if you aren't careful. I just mist it on, let it dry, and then hit the heavy spots with a stiff wire brush. It gives alot of peace of mind before you tuck that Rockwool Comfortbatt in. Once that smart vapor barrier like Intello or MemBraine is up, you’re basicly sealing the assembly, but knowing those spores are neutralized makes sleeping a lot easier.

Honestly, I’d leave it alone. In these 1940s builds, that dark discoloration is usally just tannin bleed from the old-growth cedar or hemlock, or even just residue from the original asphalt-saturated felt. If you’ve confirmed the source of the leak is dead and the wood is testing at a low moisture content with a pin meter, you’re just chasing ghosts by spraying it.

Adding more liquid—even a fungicide—to old, thirsty wood is just introducing a new moisture load right before you seal it up. I’ve seen guys go overboard with the RMR-86 only to have the smell linger in the wall cavity for months beacuse the wood soaked it up like a sponge. If it isn't fuzzy or structural, it isn't an issue.

Since you're using Rockwool and a smart retarder like CertainTeed MemBrain, the wall is going to have plenty of drying potential anyway. Those products are designed to let the assembly breathe if things get humid. Save your money on the chemicals and put it toward a high-quality sealant for your top and bottom plates. Airtightness is going to do way more for your long-term air quality than scrubbing some 80-year-old stains.

While the previous assessments regarding tannin bleed are valid, you should specifically examine the surface for any signs of localized delamination of the wood fibers, as old-growth lumber often exhibits surface-level degradation that can harbor particulate matter regardless of current moisture levels. If the wood is structurally sound, I suggest a thorough HEPA-vacuuming to remove loose debris before installing the Rockwool, as the insulation's density can trap fine particulates against the sheathing, potentially creating an environment for secondary issues if any moisture ever reaches the dew point within the wall assembly despite your smart vapor retarder's variable permeance.

I'd just hit it with a quick sand or wire brush and see if it's just surface level. If it doesn't flake off, it's likely just staining like the other guy said. But hey, if you're allready going the Rockwool and smart barrier route, you're doing it right. Those old 40s houses need to breathe. Just make sure you aren't seeing any actual 'fuzzy' growth. If it's flat and dry, I'd prolly just move on and get those batts in. Life's too short to scrub every inch of a wall cavity that's been dry since the Truman administration.

This sounds like such an awesome project! I'm just starting my first DIY insulation job on a tiny bungalow and I'm obsessed with how much better Rockwool feels than the old itchy stuff. Since you're already in the walls, have you noticed if the old balsam wool had a weird smell? I found some old stuff in my attic that smelled like campfire and I can't figure out if that's normal or just decades of dust. Also, are you guys using specific masks for the old dust or just standard N95s?

Quick update — I tried the wire brush suggestion on a few of the darker spots. It didn't realy flake off, wich makes me think the tannin bleed theory is spot on. But I did notice some of those 'fuzzy' wood fibers mentioned. If I just spray it with a mold killer to be safe, will that mess with the smart vapor barrier’s ability to seal against the studs later?