Found this gray matted stuff under my 1910 apartment floorboards, is it insulation?

That dense, matted gray stuff is almost certainly **steel wool**. In a house from 1910, it’s the go-to solution for plugging gaps where mice or rats might be getting in. Over many decades, it collects dust, pet hair, and general floor grime, wich makes it look like a piece of heavy felt or a massive lint ball. But if you try to pull it apart, you’ll feel those sharp metallic fibers.

Rodents absolutely hate the stuff becasue they can’t chew through it without it shredding thier mouths. It’s a very common sight behind baseboards or around radiator pipes in older apartments. Since your cat was digging it out, there’s a good chance there’s a draft or a specific scent coming from that gap that’s getting the cat’s attention. Your cat basicly found a "door" that was locked to the mice. You definitely don't want the cat eating it or playing with it. Aside from being incredibly dirty, steel wool is essentially a bundle of tiny metal blades. If a cat swallows those fibers, it can cause serious internal damage. It isn't traditional insulation meant for heat retention, but it is an intentional barrier used for pest control. The best move is to get some fresh **coarse steel wool** or, even better, some copper mesh since copper won't rust over time. Stuff it back into that crack and use a screwdriver to pack it in deep so the cat can't get its claws into it again. After that, you should probably seal the gap with a bit of wood filler or some caulk to keep the cat out and the pests on the other side of the wall. If you just toss it and leave the hole empty, you're basically leaving the front door open for mice.

I acutally wondered about that hair-based stuff too because my uncle’s place has horsehair insulation in the walls. It looks almost identical when it's ancient and covered in grime. Maybe try testing it with a magnet? If it's the steel wool mentioned above, a magnet shold pick it right up. If it doesn't stick, you might be dealing with that old-school animal hair or even rock wool. Either way, defintely keep it away from the cat—the dust alone in these 100-year-old gaps makes me sneeze just thinking about it!

If the magnet test fails, try a "burn test" on a tiny snippet outside. If it smells like a singed dog or burnt hair, you're definitly looking at horse or hog hair. They used to stuff animal hair into those transitions as a cheap draft stop back then. If it doesn't really burn but just glows, it's likely early slag wool. Either way, wear gloves and maybe a mask. That old stuff is a magnet for a century's worth of lead dust and coal soot. You really don't want the cat licking those fibers after they've been sitting in a 1910 floor cavity for a hundred years.