i'm about to start a rehab of a rental properity i own. the dwelling was built in the 70's. it has a std. 2x4 walls with black celotex and 5/8" ceder siding on the outside. the vapor barrior is plastic under the drywall. i want to cover the outsidewith 2" of stryofoam and add vinyl siding. i've read about plastic vapor barriors causing problems. should i remove the plastic if i add the stryofoam?