Insulation Question

Hi Everyone,
Birrrrrrrr!!, it's been a cold one.
I have a question on placing unfaced, fiberglass batt type insulation between an existing prefabed fireplace firebox and a propane fireplace insert firebox, wheter the insert box is totally metal or is ceramic firebrick,(like a Heat & Glow FB Grand), with the sheet metal blower shroud that surrounds the ceramic fire box.
My question for this is that most existing fireplace fire boxes are quite drafty and I believe that the insulation between the two might cut down on the draftyness of the existing fireplace firebox and also help raise the temperature between the two, to so the insert sheetmetal firebox shroud shroud would not bang when the blower is activated and circulates the air between the ceramic firebox and the sheet metal shroud that surrounds it, thus preventing metal exspansion/contraction to the shroud and preventing the bang noise that is associated with it.
Thanks,
John

It doesn't say to do it in the manual, thus its not tested and probably not safe. My guess is it would cause the insert to overheat (especially the FB-Grand because that thing gets damned hot already). Any components that are plastic or electronic (remotes, etc..) would have a high chance of failing.

The drafty problem is not from the old wood fireplace box, its the house. Fix the wall behind the fireplace and the draft goes away. I would guess there is either no insulation what-so-ever behind the old fireplace or just insulation with no air seal (drywall). Insulation by itself does not stop air movement, only thermal transfer. Insulation batting by itself is pretty useless if its not sealed up on all sides, unless its compacted and jammed into a damper. When compressed it looses a lot of R value though.

Sound logical to me, JTP. This unit is a great heat producer and does get quite hot in the front but I would imagine the sheet metal shroud that surrounds the ceramic firebrick firebox of the FB Grand deflects some of that heat that is radiated by the ceramic firebox.
It was just a passing thought. Your advice is always welcome in my corner!
Thanks,
John