I have seen this often in older homes. I thought it was about time to positively identify it. My web search brought up a balsom / redwood bark product…Possibly an r-value of 1. It is a reddish brown fiberous material, very fine and dusty. I wish I had taken a better pic, but this is an after thought scraped from my boot. It is flamable so any flame resistance that may have been applied is no longer working. Can somebody confirm or deny what this is?