Not sure what you mean by floor joist in attic? Not picking at you, but so we are all on the same page,
If you mean what you are standing on while in the attic, those are the ceiling joists for the house. Rafters are the boards that go at an angle to make the roof. If there are W shapes between the rafters and ceiling joists you actually have engineered trusses. Your house's floor joists are what is under the floor of your house, if you have them. A slab home like mine does not have floor joists.
Now to answer, covering with insulation is ok because you can easily move it and put it back, it is still considered accessable. Not accessable would be if you put drywall or plywood over the box without a cutout.