Should I suck out this old 70s insulation or just blow a fresh layer over the top?

Sucking everythign out is a massive pain, but if you've already found shoddy wiring and see pest tunnels, leaving it as-is is just asking for a headache down the road. Cellulose acts like a big sponge for rodent urine and smells, and those tunnels mean your R-value is compromised becuase the material has been moved around and packed down. If you just blow new stuff on top, you're burying those electrical fire hazards and the **pest waste** under a fresh layer, making it even harder to fix when somethig eventually shorts out or the smell gets bad. Since your back is shot, don't even think about doing the removal yourself. Renting a vacuum is a two-person job at minimum and it's brutal work. If you have the budget, hire a crew to suck it out so you can acutally see the ceiling joists and top plates. This is the only way you're going to be able to **air seal** properly. Most of your heat loss in a 70s build isn't just a lack of insulation; it's air leaking through light fixtures, plumbing stacks, and the tops of interior walls. You can't seal those gaps with the old gray fluff in the way.

Once it's empty, you can walk around easily, fix any sketchy wiring, and spray foam all those bypasses. Then you can blow in fresh fiberglass or cellulose to the modern R-49 or R-60 standard. It'll make a huge difference in the utility bills for your tenants. And check with your local utility company before you start. Many providers offer massive rebates or even free air sealing services for rental properties, wich coud take the sting out of the cost. If you absolutely can't afford the cleanout, at least rake back the areas around your light fixtures and the fan you already fixed to check for more hidden junction boxes. But honestly, for a long-term rental, doing it right the first time prevents you from having to crawl back up there in five years when a tenant complains about a wierd smell or a flickering light.

Listen, I've spent more time in cramped attics than I care to admit. Back when I started, we’d just throw some kraft-faced fiberglass over whatever was there—even that old balsam wool—and call it good. We didn't know a lick about air sealing then. But with a 70s build, you relly gotta watch for that aluminum wiring if it hasn't been pigtailed. If those critters chewed thru the jacket on aluminum, burying it under fresh blow-in is just asking for a house fire. I once pulled out a nest that was literally charred on one side from a short. Get it out of there. Your peace of mind is worth more than the cost of a vacuum crew.

Since you're worried about the back and want a clean slate, go with Owens Corning EcoTouch PINK Fiberglas batts once that mess is cleared out. I switched to them becasue they're way less itchy than the old school stuff and don't settle like cellulose does. If you’re hiring a crew anyway, have them install the R-49 batts. It makes it so much easier for any future electrical work because you can just lift a batt, check the wires, and drop it back. No more digging through gray dust to find a junction box.

I’m with the others on the full cleanout—it’s the only way to be safe with that old wiring. One detail people forget is checking the soffit vents once the old cellulose is gone. In those 70s homes, the insulation is ussually packed tight against the roof deck, wich kills your airflow.

While it’s empty, pop some plastic baffles in those rafter bays. It’ll stop your new insulation from clogging the vents and prevent moisture buildup that leads to mold. It's a quick job when the floor is clear but impossible to do right once you've topped it off.

I'm not totally sold on the full suck-out yet. Have you ruled out a simpler spot-check first? Everyone’s jumping to a $3,000 cleanout, but if the smell isn't there now, you might just be chasing ghosts. Are you sure the 'tunnels' aren't just old settling from ten years ago? I'd hate to see you gut the place and realize the wiring is actually fine. Maybe just pull back a few sections where the fan was and see if it's truly a disaster before committing your wallet.

Quick update—I decided to take the advice on the full cleanout. You guys were right about the smell; it was pretty gross under the surface. Getting it sucked out saved my back, but now that the floorboards are bare, the wiring looks even worse than I feared. shoud I get the electrician in before I start looking at those soffit vents or wait?