I installed an ASHT+ double insulated outside chimney from my basement in 2000. This is a product of Security Chimneys. I just replaced the insulation blanket that goes around the chimney pipe inside of the thimble that goes through the wall. I used a mineral wool type called Thermafiber Safing Insulation and have checked this out with the chimney manufacturer today to see if this was suitable. The original was mineral wool and was 2" thick which is what the Thermafiber is as well. In the installation manual it also calls for putting R-20 insulation in the wooden box frame that the thimble is attached to inside of the wall. R-20 seems to be rather hard to find as Home Depot and Lowes don't even carry it. I did a search for it online and the Canadian Home Depots do carry it. The chimney is a Canadian product so I believe this has something to do with it. I also asked the manufacturer if it was ok to use an insulation with at least a higher R value and she said it was, just not lower. This is what I have done in the past but I believe I had the insulation fibers facing the inside and outside of the house instead of facing the thimble. The R-30 I just put in gave me lots of trouble trying to squash it small enough to fit in the thinest spaces of the box between the thimble and the wooden frame. Does anyone know why that insulation needs to be there, is it just to insulate the cold air from coming into the house or is it another layer of protection from the thimble heat for the wooden frame in the wall? I should have asked that question of the manufacturer today while I had them on the line. I think it's there to protect the wooden frame but doesn't the resin in regular fiberglass insulation melt around 250 degrees or so??? I did ask her if it was ok to use more of the mineral wool in the frame cavity and she said no it should be regular insulation. Any ideas? John