Is there a warranty issue if DripX is used next to existing fanfold insulation?

If replacing only part of your roof, you should not mix fanfold insulation and DripX. These products are not the same and serve different purposes. Fanfold is a type of foam insulation often used under siding. There should not be any issues installing metal panels over fanfold, however you may need an additional vapor barrier.  DripX is used to control condensation over open purlin or at the eaves and ridges. It’s a felt material applied directly to the backside of our metal panels and it’s purpose is to trap moisture and then release it back into the air in the form of normal humidity. If you use DripX, you do not need to install a vapor barrier. You should check with the installer if you are concerned about condensation.