Organic Growth - Fiber Board?

Has anyone seen this before? It is from a home built in 1967 and is some sort of fiber board, i do not believe it is an organic material, the back of the board looks similar to the spots on the front. Any insight would be greatly appreciated!!

looks like mold/mildew. Where was it? basement?

If it is what I think it is, we used to call it “Beaver Board” and I agree with Simon. It looks like it has mold/mildew on it but a lab would verify that.

1 Like