I was going to insulate my exterior walls with a foam insulation. However, at least 1/5 of each exterior wall (all four) have a dead space.

For example, the space between my first floor bathroom and my second floor bathroom have a 3 feet dead space between the interior wall and the exterior wall. This is because the roof is at varying levels, making it impossible to bump the interior room out (think knee walls, but only a few feet tall).

I know this because I can look down from the attic into these spaces.

How do I insulate the wall in these areas without removing the interior walls?