Chimney and Flue Chases
Chimney chases are very important to air seal properly, they are often major air bypasses and can allow air and moisture into the attic.
Fire caulk and metal flashing should be used to seal the chimney to the surrounding joists. You must also seal the bottom of the joists to the ceiling, and check for any large openings around the four sides of the chimney or flue that may need sealing as well.
The chimney was sealed to the joist, but the other side of the joist has been left open to the chimney chase.
The chimney was caulked to the joist, but the bottom of the joist was not sealed to the ceiling, these are air bypasses and must be sealed as well.
If there is flooring existing around the chimney, lift the flooring around all sides of the chimney and seal all around, including over large openings like this one. Make sure to maintain 3” clearance with any combustible materials like foam and rigid foam board.
The flashing around this flue chase was caulked but no air barrier was installed over the large opening in the front, metal flashing and fire rated caulk should be installed to complete the air boundary due to the proximity with the heat source.