looks like a magnified version of Masonite siding back side, any I.D.
possibly a celotex, beaver board, buffalo board and a number of other names type product usually about 1/2" or thicker panels with some brands asphalted on the surface an attempt to resist moisture
many moons ago we used as underlayment for brick, stucco and concrete control/expansion joints
Wood fiber insulation is available as loose fill, flexible batts and rigid panels for all thermal and sound insulation uses. It can be used as internal insulation : between studs, joists or ceiling rafters, under timber floors to reduce sound transmittance, against masonry walls or externally : using a rain screen cladding or roofing, or directly plastered/rendered, over timber rafters or studs or masonry structures as external insulation to reduce thermal bridges. There are two manufacturing processes:
* a wet process similar to pulp mills in which the fibers are softened and under heat and pressure the ligin in the fibres is used to create boards. The boards are limited to approximately 25 mm thickness; thicker boards are made by gluing (with modified starch or PVA wood glue). Additives such as latex or bitumen are added to increase water resistance.
* a dry process where a synthetic binder such as pet (polyester melted bond), polyolefin or polyurethane is added and the boards/batts pressed to different densities to make flexible batts or rigid boards.