Am currently attempting a vinyl siding self install. In the process taking down the old trim and what I thought would be the fiberboard(?) I realized, there is no plywood. I've read that it did occur to use this type of sheathing on the other side of homes.

my questions are: Do I take it down and replace with all new plywood befor the house wrap and siding or take out just the rotted out pieces and replace? Was this something that was actually used or is this just another thing with my house? Do I need to worry about asbestos?