chimney cap; chimney cowl
A cover or hood fitted on top of a chimney to prevent rain, downdrafts, or debris from entering.
The chimney cap broke, so rain came in.
Compound of 煙突 (chimney) + の (possessive particle) + 笠 (conical hat, cover). The word 笠 originally refers to a traditional Asian hat, and by extension any cover or hood that sits on top of something.