The North Beach branch of the San Francisco Public Library actually uses denim as insulating material within their walls.

http://sfpublicworks.org/project/north-beach-branch-library

"...and 11,350 square feet of insulation made from recycled denim, equating to approximately 4,500 pairs of jeans."

That's a lot of jeans!

It's kinda important to build with less flamable materials whenever possible.

A library, by definition, is full of flammable materials anyway :)

I remember this dude putting a bunch of materials to the test with a direct blowtorch flame. https://www.youtube.com/watch?v=8NC79e0oztM. I was hoping he had a demo of denim; it's mentioned in the video description, but skimming the video I couldn't locate it in a reasonable time. You can look at his demo on treated cellulose around 1:48

Denim Batt at 12:39

https://youtu.be/8NC79e0oztM?t=12m39s

bummer ><, I was hoping it would perform a lot better. The brief coverage of the poor performers explains why I couldn't skim it and land on it. Only the strong contenders had long enough coverage to skim with a mouse hover over the timeline and spot them in the thumbnail. Sounds like it was trading on low allergic response vs flammability according to his commentary.

I imagine it was sprayed with a flame retardant?

Fabrics used for insulation are treated with flame retardant.

People also insulate with cellulose -- ground up newspaper. It is treated with boron to make it flame retardant.

Old clothes make excellent raw material for the production of shop rags. They can also be used to make high quality paper.

plus all those sketchy stuffed animals, when they run out of oily rags and newspaper!

(regarding the denim sound insulation and fire)

Maybe, but there's a pretty good chance it's treated with flame retardants before use, and cotton fabric generally doesn't really burn that well - or at least not fast.

There's a reason people working with hot glass wear long cotton shirts/pants for it.

I had a chemistry professor mention to the class she always travels with natural fabrics, cotton and the like, whenever she's onboard a flight. She mentioned something along the lines of it increases your chances of survival if ever there's a fire because synthetics melt and stick to your skin and burn like crazy.

I took some glassblowing classes and the first day safety lecture included a bit about this. Eye protection, closed toe shoes, and cotton clothes, precisely for this reason.

Especially when we were first learning, it was easier than you might think to spill or splash 2000F liquid glass on the ground when gathering it out of the crucible, and that'd turn synthetic clothes into napalm.

That's an ... interesting failure of statistical thinking. Fire in the cabin is by far less risky than many other potential harms he encounters en route to the airport.

Assuming your clothes fit properly, your choice of clothing will have little effect on your chances of surviving your journey to the airport. However, clothing that might help you survive a plane crash will likely have the same benefit in a car crash.

I used the term fire loosely, it was basically a plane crash and subsequent fire, an inferno in other words not a small camp fire around the center aisle seat.