On a pipe-in-pipe direct vented, natural gas, fireplace, can you insulate the outside of the combustion inlet with foil-back fiberglass? Is there a different recommended insulation that can be used?

I haven't seen any mention of yes/no in the install manuals, so I don't know. Or how's about boxing it with firepanel? I find the outside of the combustion inlet gets quite warm after running for a while.

Although the clearances are still outside the install recommendation, I don't want to chance heating up the wood mantle and bookcase from the inside. I guess I could tack fire panel to the mantle and bottom of the bookcase instead, but that wouldn't help the walls and floor. I'm just afraid it may overheat the hollow area under the mantle and bookcase... This is a corner unit as well so the pipes are a little longer than they would be if it was a flat-wall mount.