Originally, I though you were saying your rear wall looked like this (cross section):

XXXXXX
XXXXXX
.XXXXX.
.XXXXX.
..XXXX..
..XXXX..
..XXXX..
..XXXX..
..XXXX..
..XXXX..

But now it sounds like you're saying it looks like this:

XXXXXX..
..XXXXXX
XXXXXX..
..XXXXXX
XXXXXX..
..XXXXXX
XXXXXX..
..XXXXXX

Is that right?