I have a chimney that is located in the center of the house and is made of cinder block on three sides. The chimney provides flues for 2 fireplaces and a combined flue for the furnace and water heater. There is several inches between the cinder block and the wood studs. This appears to be to match nearby walls and in one spot to allow for a door frame. There is also an odd looking insulation. The house was built in the mid-fifties and has fiberglass everywhere else. My question is how close can you build to a cinder block chimney and what kind of insulation, if any, is appropriate.