In almost any insulation project, you are bound to encounter plumbing pipes in walls and ceilings. It's important to insulate around these obstacles properly, especially in exterior walls where an improper installation could lead to frozen pipes. Remember, in that particular application, the object is to block cold air from the pipes.
You should have no problem tucking JM insulation into position. Take your time to ensure that the insulation fills the width of the cavity, and do your best to eliminate gaps wherever possible.
Never stress pipes by inserting the insulation too forcefully or tightly.
In walls where plumbing fixtures are to be placed, insulation must be installed first. Where there is plumbing in exterior walls, install the insulation BEHIND the pipes, between the pipes and the exterior. To protect pipes from freezing, insulation should never be placed between the pipes and the warm side of the wall.
For vented crawl spaces in cold climates where water pipes run through the joists, install insulation below the pipes to protect them from freezing.