I think of lot of the problem is that if you take a small tuft of fiberglass insulation and hold a lighter to it, it "burns" and "melts" away like a piece of wet cotton candy, complete with glowing red embers as the fibers "burn". I think we've all seen this- leads to a complete distrust of fiberglass as a fireblocking material. I've heard many a fireman lecture about how fiberglass is no firestop, based completely on this simple try-it-at-home demostration!

But if you take a larger sheet of fiberglass and repeat the experiment, you get no such "burning" or "melting"- just a small black scorch mark, as the fiberglass does its insulating job and traps the gas of the flame, slowing the flame's spread.

I haven't been motivated yet to build an actual wall cavity to simulate this for real for myself (would make for great youtube, ah?) but I've been thinking about it- not like I have a shortage of construction scraps to use to build a model!

Last edited by SteveFehr; 05/08/07 12:07 PM.