I have seen several threads on the forums about insulating between the original steel firebox and a newer insert. Well, I have another level of questions on this. My original firebox has the "Heatalator" vents in the front, so there is a thin sheet metal sleeve around the steel firebox to allow convection flow from the bottom vents to the top vents. I made up a little diagram to help out with some visuals. The dark grey is the existing masonry work, the yellow is the sheet metal (about 1/16" thick), and the red is the 1/4" thick steel plate for the fire box. I was thinking about using Roxul in between the thick steel plate (red line) and thin sheet metal (yellow line). Would this do me any good, or would I be better off insulating directly between my insert and the red line (the thick steel)? I have read through the manual, and have not found anything that says I can't put the Roxul between the main fire box and the insert. I am hoping to get some advice on this.