How I finally figured out the gray loose-fill in my old attic - steps that actually worked

We went through this in our old place, a 1961 ranch with the same gray fluffy stuff everywhere in the attic. My first guess was rock wool too, but the texture looked a bit too soft when I parted it, so I mailed off a sample anyway. Cost me about what you said and the report came back as processed cellulose, prolly recycled paper with the old fire treatment. So the gray stuff is not always rock wool like people assume, it really can go either way and you can't tell by eye alone unless you see those pebbly chips.

One thing I did before sending the sample was measure the depth in a few spots with a tape and a note of where it settled thinner. Cellulose that's been sitting since the early 60s was around half its original height in the middle of the attic, thicker near the edges. That helped me figure out how much new loose fill I'd need when I topped it up. I ended up blowing cellulose right over the old layer after air sealing the ceiling penetrations first, that part made a bigger difference than I expected with the upstairs rooms. Rented the machine from the big box store and it was a half day job with my brother-in-law feeding the hopper. No regrets with the covering approach, just get the sample result back before you commit to anything.

One thing that helped me sort rock wool from cellulose in my own attic was a simple weight test, since a small handful of rock wool feels noticeably heavier and grittier than a similar pinch of cellulose, which tends to spring back a little when you let go. Also, those shiny specks the original poster mentioned are usually bits of slag from the rock wool process, so seeing alot of them is a decent clue on its own. I'd still get the lab result before topping up, but between the texture, the specks, and how it settles, you can usually narrow it down to one likely answer before the report arrives.

Honestly, I think relying solely on a weight test isn't super reliable. Once that loose fill has settled and been compressed for decades, the density variation between the two types becomes much less noticible. I've found that looking at the particle shape actualy tells me more. If the fragments are very uniform and somewhat jagged, it points toward rock wool. Papery cellulose tends to have a more varied, slightly fibrous appearance, even when settled down low. Has anyone here actually tried separating them chemically, or is the visual/tactile method the only way to go?