I'm an insulation contractor and I also try to close up the "big" air sealing holes before we blow in additional loosefill insulation. I want to close off the enormous cavity the builder left for the fireplace vent stack. My concern is how does a fireplace draw fresh air from combustion? Do I need to leave this cavity open to prevent back drafting?
Thank you for your responses.

http://www.atticadventures.com/wp-content/uploads/2012/12/Fireplace-Vent...

http://www.atticadventures.com/wp-content/uploads/2012/12/Fireplace-Vent...