Empty Wall Cavity Question
I was working on the attic and saw open space near the soffit vent, so I took a picture of it and it turned out to be space behind the shower in the master bathroom. If you look at the picture, on your right is the bathtub, on the left is a closet from a spare bedroom, and where you see the insulation is the outside wall. My question is to how to properly insulate it from the attic. Should I fill the cavity with loose cellulose insulation, and how do I insulate this opening in the attic? I thought of putting a piece of rigid foam insulation and spray foam around it. Thank you in advance for any recommendations.