Pulled the drywall off my north wall and the old insulation surprised me

Same thing happened to me two winters back, except my finding went the other way. Pulled the drywall off a bedroom on the north side of our place expecting to add a layer, and the batts in there were half slid down into the wall cavity, leaving the top two feet basically empty. That explained why the top of the room always felt like a fridge.

The surprise was that the spots where the insulation was still sitting tight held heat fine. The problem areas were all about air moving through the wall, not the R-value. I foamed the gap between the sill plate and the subfloor and around a couple of electrical boxes, and the room changed more from that than it ever would have from extra batts. One thing I'd suggest while your wall is open, since you mentioned the window trim. Check behind the trim itself if you get a chance. I found a finger-wide gap under my window stool that the caulk on the trim face was hiding, and that was the worst leak in the whole room. A bead of low-expansion foam in there before the trim went back on made a bigger difference than anything else I did.

Enjoy the free insulation win. Not often a demo weekend turns into money saved.

In my house, I ran into a different kind of issue when I opened up a wall for a renovation in the laundry room last spring. My existing insulation looked clean and dry like yours, but the whole thing was framed out of 2x4s, which meant the cavity was only about 4 inches deep. I realized the original installation used only a standard 3-inch batt, which, while decent, was nowhere near what the modern building codes expect for that area of the house. I didn't feel the need to remove the old material since it wasn't visibly damaged, but I spent time carefully filling the gaps between the batts and the framing itself. Instead of tearing out the entire structure, I just used a low-expansion foam to seal the interstitial spaces around the insulation inside the studs. It made a huge difference in the thermal transfer compared to the air pockets that were left behind. It wasn't about the state of the fiberglass, but the overall depth and how well the cavity was packed.

I ran into a diferent situation when I tore into my garage walls last winter. The batts looked fine at first glance, but when I tapped them, I heard a faint vibration. I assumed it was just settling, but it was actually movement inside the cavity. I ended up grabbing some specialized fiberglass tape and just layered it over every single seam before putting the drywall back on. It completely stopped that annoying rattle and made the room feel significantly quieter and more stable. It seemed like that little piece of tape really helped lock the insulation in place. Have you guys had any luck figuring out if the vibration was just the batts shifting or actual movement of the framing?

I’m skeptical that just sealing all the air leaks and keeping old batts is a complete fix. While air movement is definately a big factor, I’ve seen situations where the fiberglass itself is compromised even if it looks clean and dry. If the framing around those batts is dense, you are basically creating major thermal bridges right through the insulation. Even if the air is shut off, the R-value performance can be heavily undermined by those structural connections. You might think you've solved the problem, but you’re just insulating a cold frame.

I had something similar happen when I looked at some older walls in the garage. Everything seemed clean and dry at first, but when I checked the insulation on the north side, I realized the kraft paper facing was warped. It looked okay, but it had soaked up moisture from the concrete foundation below over years. It's not about the air leaks, though. If the insulation itself is wet, it's basically done its job already and you won't save any energy.