Should I replace this old fiberglass insulation while the ceiling is open?

Those dark spots along the edges and wiring are usally a sign of **air bypass**, where the insulation acts like a giant air filter catching dust as house air leaks toward the roof. If the drywall was sagging, you almost certainly had a moisture problem too. Warm, humid air from inside the house hits the cold roof deck and condenses, which eventually rots the drywall and leads to mold growth in the fiberglass. Since you’ve alredy got the ceiling open, rip that old stuff out. It's the only way to see if you have water damage on the underside of your roof decking. If the batts are compressed and discolored, their R-value is shot anyway. You want to check for signs of leaks from the shingles, but even if the roof is tight, the lack of a **vapor barrier** or proper ventilation is probably what killed the old setup. You need to make sure there's an air gap between the top of the insulation and the plywood decking. Look into installing **rafter vents** (foam baffles) that run from the soffit up to a ridge vent. This keeps the roof deck cool and prevents ice dams. If you just stuff thick batts in there against the wood, you're begging for rot.

For the new install, go with faced batts and make sure the paper flange is stapled to the face of the rafters to create a decent vapor retarder. Or better yet, use rockwool if you have the budget; it handles moisture better than fiberglass and has a higher R-value per inch. Since you have electrical runs, just make sure your fixtures are **IC-rated** (insulation contact) so you don't have to worry about heat buildup. It’s a bit of extra work now, but redoing drywall beacuse of mold or sagging in five years is way worse.

That IC-rated thing has me a bit nervous now. How do you acutally check for that on older fixtures? I’ve got some recessed lights up there and I relly don't want to cause a fire by stuffing more insulation around them. Also, for the rafter vents, do they make the room way colder in the winter since you're letting outside air flow right against the baffles? I’m trying to balance keeping the roof dry with actually staying warm.

Check the inside of those light cans for a UL sticker; if it’s not marked 'IC,' it’s a fire hazard to bury them. If they're ancient, I'd just swap them for modern LED wafer lights. They’re airtight and you can pile insulation right on top. As for the baffles making it cold—the air stays above the baffle. The real secret is air sealing. While the ceiling is open, grab some spray foam and seal every wire hole and top plate gap. If you stop the air from leaking out of the room, you won't feel a draft regardless of the airflow on the roof deck.

Seriously, just get a good respirator and rip it out. Old fiberglass is a nightmare to handle, but you really want to see the wood underneath. I found a small leak near a vent pipe that was totally hidden by the old batts until I pulled them. If I'd just drywalled over it, I whould've been back at square one in two years. Also, those old batts are magnets for mice and bugs over decades. Starting fresh with clean material makes the whole house feel less 'musty' once you seal it back up. It’s a messy Saturday, but it’s better than wondering what’s rotting above your head.

Those black spots near the wires are often caused by 'ghosting' where thermal bridging makes the area cooler, attracting dust. Since it's a vaulted ceiling, double-check your rafter depth before buying replacements. If you have 2x8 rafters, you've only got 7.25 inches of space.

Once you install a 1-inch foam baffle, there's very little room left. If you compress a standard R-30 batt into that remaining 6-inch gap, you'll lose a huge chunk of the R-value you're paying for. Look into High-Density (HD) batts; they're specifically designed for shallow cavities so you don't end up blocking the airflow above the baffle.