I am renovating a 1930 Seattle Bungalow, the attic was converted to a finish loft and I'm currently gutting the main floor bath. In the ceiling of the bath (the floor of the loft), I found what looks like mineral fiber insulation (dark red). Is this something I should keep? It might be a good noise barrier, but is no longer needed for insulation purposes.
Thanks for any info what this might be.

 

Hard to say. Could be rockwool of some sort.

 

Apparently it caused no problems in the last 70 years or so, so I would leave it in place. You may get a benefit from a sound transmission standpoint, bu I don't know who would benefit the most- bathroom or the loft.

The only possible problem could come if you did not have a vapor barrier (paint, poly, etc) between a high humidity room and any cooler areas. Usually the older paints and the number of coats did this adequately.

Dick