Currently, my chimney is brick and has a 8" type B metal duct pipe coming out of it as a sleeve, which exhausts home heat and waterheater appliances.
The chimney was dormant for many years before I started using it after some remodeling and reengineering home infrastructure. However, I didn't fill the space between the masonry and the sleeve. And maybe some 18" into the chimney from the top of bricks starts the old sleeve, which was ceramic I believe or made of a similar material as roof tiles.
Should I fill the space on top of my chimney between the sleeve and the brick structure with something, as was done with my neighbor's chimney?