Insulation under vinyl siding

My husband and I are currently getting bids to place vinly siding over the existing masonite siding on our home. We are, however, concerned about the insulation the contractors use between the "old" siding and the new vinyl siding. The contractor we want to go with uses Celotex insulation, which is foam sandwitched between a thin layer of foil and a thin layer of a black paper-like material (not sure exactly what it is). We are concerned moisture will be trapped either in the foam or between the foil-side of the Celotex and the actual house, which will cause problems later. Presumably, there is a vapor barrier on the interoir of the house - between the drywall and the home insulation. Will this prevent moisture from reaching the Celotex insulation on the outside of the home ? If not, will moisture get trapped between the celotext and the "old" siding, damaging the "old" siding and the interior insulation??? Thanks for your asistance!