Someone said earilier, the proof was in the pudding....I don't know as much as everyone else about draft and what not, all I can tell you is that with the same wood, and the same burning practices, my house is about 5 degrees (2 floor 1700sq feet) warmer since we put that insulation in. It seems like we were heating the whole masonry chimney, and the insulation helped the heat stay in the firebox. Am I simplifying it too much? Please don't detect sarcasm, I don't actually know, just learning. All I can tell you is my problem is a whole lot better now and that I will be insulating the rest of the chimney as soon as I can figure out what to put in there, so as not to creat a creosote problem. Seems hard to believe that something so simple could have made such a huge difference, but it did! I wasn't even confident it was going to work, but my thermometer in this room says it is 82.2 degrees!!