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.

That will be a good bit of cellulose if you fill it up.

That would theoretically be best but you need to, at minimum, close up the top of rigid foam.

What is the purpose of this cavity? Or builder had standart shower pan 36x36, and didnt bother to build 36x48 shower? Plus tile saving:laughing:?
What shoul I use to cover rigid foam?

I would open up that wall from the closet and use the extra space, or maybe a small cedar closet or a safe room.

we going to use this space to make bigger shower in the future. Right now its 36x36, additional 12" in depth will be good "addition" for shower area.

If that is the case, rigid board the top and seal all the seams.

Once you are done, overblow it but I would also put some 2x4s across so that nobody steps on that foam board and takes a 1 story fall. :eek: