Re: What is this? Cardboard??
If the cardboard is thin, just a little thicker than a cereal box, it's a heavy paper that was fastened over wall boards (not wallboard, but actual wooden boards) in the old days to act as a smooth substrate for wallpaper.
If the cardboard is thick, like about 1/2", then it's a product called "Firtex" that was a precursor to sheetrock. It turned out to be a disastrous product for building walls, but it's still used for bulletin boards, cubicle partitions, and as an acoustic panel to absorb sound.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.