What Size Block-Off Plate Do I Really Need?

[Big_meech]

I’m installing a block-off plate in my chimney and wondering about sizing. If the plate is just a bit larger than the throat or damper opening—rather than fully sealing the whole area—do you still get most of the efficiency benefits, or does it really need to extend all the way to the masonry walls?

 

[stoveliker]

The goal is to stop air flow. If there are no gaps beyond the throat, and you (silicone) seal it there, it's fine to keep it smaller imo.
On the other hand, if the rest of the masonry is really cold (outside chimney) it may pay to extend the plate with rockwool on top of it.