Duravent chimney kit for woodburning stove. This pieces center fits perfectly around the triple wall chimney pipe for the roof and the square outer part is slightly larger than the ceiling box that penetrates through ceiling and roof. This is referenced nowhere in the super detailed instructions. It is not the flashing or the storm collar already have both those.
Solved* Attic insulation shield, not necessary in my application
It’s called ‘flashing’ and is meant to keep the weather out.