Insulation Help

[MEMPHIS91]

Howdy guys I got a couple insulation questions/approval.
First is my question is about my chimney, using my seek geothermal I found that behind my brick in the center an entire brick wall is NOT insulated at all.
The chimney is behind the wall with a 12" gap. Left and right sides have insulation its just the center part that doesn't.


So my idea is to dump 16 cu ft of perlite in this hole , this sits right above the main fire and in between the main brick wall the the chimney so there is a good bit of heat here, then thermal/radiant barrier, then just blow in fiberglass above that. The perlite holds heat very well and has an R value of 3 per inch.
Has this ever been done? Or am I just crazy?

Next is the joist between my upstairs dog house bathroom and the laundry room. During the blower door test I found huge air leaks in that area, so I starting taking things apart found that there are HUGE gaps in the eve. Follow the pictures to see the area I am talking about.




I was thinking about jamming some fiber glass as deep as I can with my arm and then going crazy with some spray foam on top on it? There are 7 of the joist gaps.

Good news is that I got a lot of double bubble radiant barrier up on the knee wall of my vaulted ceiling, and some single layer on the rafters.