Putting in a Class A chimney where a ZC fireplace used to be. It's enclosed in a OSB board wood chase exterior of the house that runs two stories up and has a fire stop between the floors. The house wall has been insulated the whole way up except where the ZC fireplace was installed. The rest of the 3 wall of the chase are not insulated. The chase faces North. The house is about 13 years old. I live in the mountains of western South Carolina. Temps do dip into the low 20's. The problem: Once I gained entrance to the chase I notice I had a what I would call mold. Not much below the fire stop and a little more above the fire stop. Mostly the effected area is on some of the side walls of the chase. It's only in a few spots at this point. Let me also say that there is no air circulation from the fire stop down. ( I guess that's why they call it a fire stop. ) From the fire stop up there doesn't seem to be any opening either as the old air cooled chimney looks to be sealed on it's outer shell going though the top of the chase cap. I plan on getting rid of the mold that is there by spraying bleach on it and than applying "KILLS" lacquer to the effected area. Did the ZC fireplace contribute to this situation being that the insert was in "the wall" and had no insulation around it. (stove wasn't used very much) The wood stove will sit outside the chase. Now my question is do I try to vent this chase somehow? Do I insulated the whole thing? Do I just leave it alone? Advice please.