One object can inhibit the other object from performing its purpose.

It would probably be best to properly remove the old insulation before installing the new insulation to insure the wall is properly insulated.

It's probably Diatomite (diatomaceous earth) used 50 years ago for insulation and control of fire, sound, pests, etc. It's made from a siliceous sedimentary rock composed mainly of the fossilised skeletal remains of diatoms, which are single-celled organisms related to algae.

It shouldn't contain asbestos but you should test it anyway. It's abrasive so prolonged exposure will irritate your lungs, nasal passages, skin and eyes.

Post a photo of where its located.

"When cleaning up Diatomaceous Earth do NOT use a regular, filtered vacuum or one that has a bag – these vacuum cleaners will get clogged and the powder may destroy the motor. Instead, vacuum up Diatomaceous Earth with a shop vac or a vacuum that has a high-quality HEPA filter." https://www.saferbrand.com/articles/clean-up-diatomaceous-earth

I would take that as hire someone else to do the work and do not come back into the house for a couple of days. But that's just me.

Thank you everyone! I was trying to find a shortcut to avoid tearing it all down, but I see it is going to be best not take a shortcut. It is something I can do myself, but the trouble/money ratio is too high. My local disaster restoration (conveniently located around the block) gave me an estimate for $1,450 including tax. The price will come down a little if the plaster tests negative for asbestos. That price seems reasonable to me. Since I already demolished some of the inside walls, what remains is 146 ft^2 insulation/lath/plaster, 52 ft^2 plaster/lath, and 90 ft^2 lath. Does that price seem reasonable to you? Thanks!

The powder is highly unlikely to contain asbestos; testing it is just a precaution..

The existing conditons are not clear. Is the power at the bottom of the wall cavity and you want to spray insulation into the cavity without removing the lath & plaster? That shouldn't be a problem as long as it doesn't blow the powder back into the room.

I agree the insulation is not asbestos. It's cellulose. I'm testing the plaster.

What happened to the powdery insulation that you thought contained asbestos? Your description of this issue makes no sense.

It's cellulose, which does not contain asbestos. I didn't know that when I started this thread.