I'm replacing an old paneled wall that surrounds the original masonry fireplace. Took down the paneling, found 1950's fiberglass batts falling apart behind the paneling, and removed it all. Now need to re-insulate before drywalling. I'd rather use polyurethane foam board, but am wondering if can be placed above the hearth area. The bays are made of 2x4 stud sides, and at the back, are made of some type of hardboard (not plywood), and behind that must be the masonry wall of the chimney. The chimney is an exterior one, and I'm assuming since the hardboard was placed there originally, that area does not get hot. Would it then make sense that polyurethane is okay to go in there? The old paneling had been there 60 years, and did not look discolored above the hearth, though I don't know how often wood was burned in the fireplace. At present, there's a gas insert there, with a stainless steel liner, so the heat is much less. I'm just worried that if someday someone converted it back to wood, and fired it up, would the polyurethane melt/smell etc. Does this sound right to anyone? Thanks.