Insert in Masonry. Wrap Fireplace with reflective foil?

[Pertzbro]

Just an idea I thought up. I've read of people insulation around the insert inside the masonry firebox to keep the cold from passing through the masonry. Do you think this would reflect too much heat back at the insert? I'm thinking wrap the sides and back of the original masonry firebox before i install my insert. It's an outside chimney facing the north where we get our cold winds from in the winter. There would still be 3-6 inches of open space between insert and the reflective foil.

Might be a bad idea, but thought I'd pose the question for discussion.

 

[begreen]

Welcome. It sounds like there is plenty of room to line the fireplace with insulation board like micore, fiberfrax or roxul. This would be more effective than foil.