Air Leak Around Cathedral Ceiling Support Box

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New Member
Jul 16, 2023
Petawawa, ON
Hi all,
Pulled the pine trim off from around our chimney box. Pics below.
We have significant air leakage around our cathedral ceiling support box.
When the dryer and/or bathrooms exhaust fans are running it pulls a crazy amount of stale attic air into our living room.
Confirmed by doing a smoke test around the perimeter.
Even if just the furnace or AC are running it’s pulling attic air into the living space.
How can I safely seal this up?
Leaning towards great stuff fire block spray foam between the exterior of support box and the t&g pine boards.
All advice welcome.
This is where the air leaks in around the perimeter of the unsealed ceiling box and is drawn into the living space when the dryer or exhaust fans are running.

Silicone should do. Are there any drafting issues in the winter when fans or dryer are running?
I was thinking silicone too. I have imagine this is normal, especially if rest of house pretty tight.
Silicone or closed cell spray foam, or even some draft blocking tape would probably work.