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While removing/cleaning the bathroom fan hood, I noticed that the edges of the hole in the drywall ceiling have a very fibrous-looking layer/sheet on each side of the gypsum.

This house is very old (going on over 100 years) and I’m not sure when this bathroom’s ceiling was last renovated.

I will take a sample to the lab tomorrow, but wondering if anyone has seen the sheeting look so fibrous like this?