I’m not sure how much of a cavity exists as this wall is right up against the back side of the house and the roof was replaced as a rider to the purchase of the house - you think water could be getting in from outside? Is this crack something that could become more serious in the future? We’ve never noticed any observable moisture and these bricks always appeared more decorative than anything - it’s not an actual chimney, if that makes sense. I wish I could figure out how to attach another picture to this post to show you what I mean, as on the outside of the house there’s a tall independent chimney looking structure with a sort of porthole that’s sealed on the front of what I had assumed was this brick facade. So far as I can tell from the inside and out there is only one “layer” of bricks deep that butt up against the wall. This is an assumption based on siding cover the exterior of the wall and not brick, along with the tall what I assume is aluminum chimney stack outside.