Is it okay to blow new insulation over my old 70s stuff?

The grey-brown crumbly stuff is likely old rockwool or a very early cellulose mix, and combined with those 70s pink batts, it's doing almost nothing for your R-value. While you technically *can* blow new cellulose over the top, you're missing the most critical step for energy efficiency: **air sealing**. Insulation is like a wool sweater; it keeps you warm, but if the wind blows right thru it, it doesn't matter how thick it is. Those gaps around your junction boxes and joists are chimneys letting conditioned air escape straight into the attic. I've done this both ways and I will never blow over old material again. When you pull the old junk out, you'll find every hidden bypass, wire penetration, and top plate gap that needs canned foam or caulk. If you just bury it, you're trapping fifty years of dust, rodent droppings, and allergens under the new layer. That 'dusty mess' you mentioned will eventually migrate down into your living space through the light fixtures and wall plates, expecially once the house is pressurized by the HVAC.

Removing it yourself is a brutal weekend of work but it saves a fortune. You can rent a vacuum specifically for attic insulation, but if the old batts are too heavy or the loose fill is wet/clumpy, you might have to bag it by hand. It's miserable, itchy work, so wear a P100 respirator and a disposable Tyvek suit. Once it's bare, you can inspect the wiring for any scorched insulation or open splices that need to be addressed before they're buried forever.

After you've sealed the cracks and installed baffles at the eaves to keep your soffit vents clear, then blow in the new cellulose to about R-49 or R-60. You'll notice a much bigger drop in your utility bills than if you just added thickness on top of the old debris. Plus, if you ever need a plumber or electrician to go up there, they won't be wading through a century of mystery filth to find a wire.

Listen to the air sealing advice. If you just dump 15 inches of GreenFiber on that mess, you're burying potential fire hazards. Those 1970s Romex runs and old junction boxes need to be visible. I’d grab a few cans of Great Stuff Pro and a pro-grade foam gun to hit every top plate and wire penetration once the old junk is out. If you find any charred wires or sketchy splices while you're at it, you'll be glad you didn't just bury them under R-60. Plus, old rockwool is heavy as lead; removing it prevents ceiling sag before you add the new weight.

I remember back in the 80s we'd just blow more rock wool over whatever was there and call it a day. But that old 'grey-brown' stuff might be holding more moisture than you think. If you bury it under fresh cellulose, you're basicly creating a sponge that’ll rot your joists. I’ve seen houses where people just kept adding layers, and the bottom was a soggy, moldy mess when we finally hauled it out. Also, watch those old recessed lights. If they aren't 'IC' rated, you'll start a fire if you bury them. We used to build little boxes out of scrap drywall to keep the heat away from the cans before the modern covers existed.

Spot on. If you decide to pull it all out, keep an eye out for any recessed 'can' lights. Since it’s a 70s build, those fixtures almost certainly aren't IC-rated, meaning they can't handle direct contact with insulation. If you just blow over them, they’ll overheat and potentially start a fire.

While the attic is empty, you can pop some fire-rated covers over them or swap them out for modern LED wafers. It’s way easier to fix that safety hazard when you aren't digging thru a foot of fresh cellulose to find where the heat is coming from.

If you go the removal route and decide to DIY the air sealing, don't waste time with cheap cans. I swear by the Froth-Pak 210 kit for hitting the big gaps and top plates once the old dust is gone. It covers way more ground and seals tighter than hand-cans ever will. Once that's cured, just rent the blower from a big box store and throw down some Owens Corning EcoTouch pink fiberglass batts over the joists. It’s way cleaner than blowing in loose fill and you won't be breathing in cellulose dust for the next ten years.

Update: After reading about the air sealing, I poked around and found some old recessed lights that definatly aren't IC-rated. If I hadn't checked, I woud've buried a huge fire hazard. Now I'm leaning toward a full removal so I can actualy see the top plates and use that foam gun everywhere. Is it a nightmare to DIY the vacuuming?