I am doing some work in our garage, rebuilding the walls, etc. I came across this yellow-brown insulation inside the walls. It is a rigid, slightly spongy board-like material. See the first two photos. This particular space between the studs is also a return for the heating system. Apparently it was common in old homes, to use the empty space between studs as air ducts. So I am slightly concerned that this material could be flowing through our HVAC system. The fuzzy gray stuff in the close-up is lint that has collected over the years. A few hours later I noticed that there is a shaggy brown sheet of insulation hanging off of one of the heating ducts in the basement. See the last two pics. Does anyone have any idea what these substances are, and if theyre dangerous? I was reading that it could be cellulose board, rock wool or slag wool, which are not dangerous. But could it possibly be Vermiculite, which contains asbestos? Any advice would be appreciated. Thanks!