I have a question about my fireplace. I took down all the wallboard around my fireplce because it had gotten wet many times before the leaking roof was repaired. When I started to remove the wallboard directly above the firebox I noticed that there is no insulation whatsoever. My question is this, can I put insulation in this very large cavity? In the winter I could feel cold air entering the house from this area. The fireplace is ducted and I don't believe it is the double duct kind- exhausts and intakes with the same pipe- Any help will be appreciated.