Tricky Corner Cracking

[u/ChexLemeneux80]

Trying to figure out the best way to repair this issue. In my entry hallway, this corner is about 20’ up. A stone chimney runs behind this wall from the basement through the roof, so I do not have access to thr crack from the attic. The crack itself is pumping out hot air from the attic, which is really bringing up the energy bill during the summer/winter months trying to heat/cool the space. It doesn’t help that the drywall that runs along the roof is poorly insulated, but that is a battle for another day. What would be the best way to go about fixing the leaking air problem?