(from forum member jtp)
CONSTRUCTING A CHASE
Typically if you have a chase (a framed box) on an outside wall of the house, a bump out, this is what we would recommend. Treat the walls of the chase on the main floor as an outside wall of the house. 2x6 walls, sheeting, insulation, poly, drywall. Tape or caulk the seams. At the ceiling height (or at 10ft, whichever comes first) build a draft stop and insulate that like an attic (R38 I think or something). Where this cavity is, DONT insulate the wall of the room, just hang drywall. This helps the chase area remain part of the room and breathe with the house. This makes the chase are part of the envelope of the room it is attached to. Above that it is recommended to insulate the chase to keep it warm. If you dont want to then ignore the chase and insulate and seal the rooms walls as normal.
If you do not insulate the chase area around the fireplace, you essentially have your fireplace outside, with some rain protection. Fireplaces dont make good windows, no matter what kind they are. Trust me, direct vent gas fireplaces dont make good windows either. We tore apart countless walls (from the outside) to repair bad or lacking insulation jobs due to cold air complaints last winter. I hope to have substantially decreased this for this winter through employee and builder training. After the installers tear into a wall to fix it they remember what was screwed up on the next job they are insulating.
Air cooled chimneys can dump air onto a fireplace, thereby cooling it.....
About the air cooled chimneys, we combat this with a CAK (Chimney Air Kit). It a metal housing that goes over the giant holes at the bottom of the air cooled section and is ducted to the outside. Its sealed up with hi-temp red silicone. This keeps the cold outside cooling air outside.
We have pretty much mastered this art at our store through customer feedback and customer complaint.;Chase_Construction