For what it's worth....
Most of the time when I see fungus growth on the joists, and move the insulation....the part of the joists that were not visible due to the insulation, do not have any fungus on them. Obviously, that's not the case on all houses, but for a bunch of them it is.

So spraying just the visible parts might just be a correct treatment plan. Keep in mind that I am NOT a termite inspector, nor a mold inspector, nor do I have any experience with the application of the crap they spray on these houses.
JF