Thanks. Is the sheetrock necessary? When we originally insulated part of this we did the following:
1. Removed vinyl siding and OSB.
2. Nailed wood braces (1x4" if I remember correctly( to the back side of the 2x6" framing (between the 2x6 framing and the firebox). These braces were nailed horizontally and spaced every 3 or 4'.
3. Placed styrene insulation in between the 2x6s.
4. Reinstalled OSB and vinyl siding.
So basically the styrene insulation is sandwiched between the bracing and the OSB.
You don't NEED it, but it is RECOMMENDED. It's another barrier & it can be sealed. You may be OK, tho...