I have a house that was originally built in Chicago in 1954 that had an addition put on in the 90's. The original roof and part of the original exterior was built over is now visible in the attic of the addition. This is a picture looking at the back of one of the original walls, it may have been an exterior wall. I'm not sure if this was sheathing or insulation. Notice there are white strings running through the material and it's uneven, almost like tree bark. The gray on the right of the photo is the exterior, which is brick.
Any idea what this material is?