I recently replaced a K&T circuit in the second floor of my house (c1920), pulling a trunk line to the attic and then making drops from there. In the process I had to move a lot of the attic insulation out of the way and noticed that it seemed to be disentigrating in a lot of places. I’m not sure but I think it is what I have heard called rock wool or mineral wool or something like that. There also seems to be some celulose type as well. My question is along with sealing as many openings as I can with Great Stuff or something simaler I want to upgrade the existing insulation. I’m thinking blown in cellulose. If I go this route should I remove the existing insulation or just add to it?
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No need to remove it. A house old enough to have K&T wiring most likely has plaster instead of drywall. The plaster and lath will support an additional layer of blown in cellulose insulation, most likely. If you have any reservations about the amount of weight you are adding to the ceiling, then use unfaced fiberglass batts. Just run it across the joist instead of in the joist bays. Achieves a minimum of loading on the ceiling that way.
Dave
I wasn't concerned about weight. As you noted the ceilings are lathe and plaster. I just wasn't sure if there were any performance issues with the old insulation being left in place. Thanks for your reply.
I do not think there is any very compelling performance reason to pull the old stuff out. You could blow a layer of cells right over it.
The strongest argument for removing it is to examine the structure below for air leaks. If breaches in the attic plane are sealed, the old mineral wool will work fine.
OTOH, the mineral wool dust is brutal. Wear a respirator when disturbing it. Some had asbestos, but most did not. Sometimes it's just preferable to remove it if substantial amounts are being displaced anyways. Moving around dusty piles filled with rodent debris (and who knows what else) is never fun.