I have a potential project coming up where there will be a small addition, 500 square feet,  added to a house in climate zone 6 Washington State. The Methow valley  can get a large amount of snow in the winter and stays pretty dry the rest of the year.  
 The plans call for floor framing off a stem wall which will be 20″ above grade. The crawl space will be vented ( This is matching the detail with the current floor assembly in the house ). My plan was to fill the joist bays with R30 Rockwool batts as this will match, and be to code, ( I don’t think it would be worth the money to improve upon the floor R value if the rest of the house will not be able to match it? There is not money in the budget to retrofit the whole house.  ) 
  To my main question:
       My understanding is that it can be a good idea to add a layer of foam board to the underside of the joists in a vented crawlspace to keep the framing warm and limit water vapor into the floor assembly. If you have a vapor barrier under the joists, and a laminate or vinyl flooring above, could there be trapped moisture in the floor assembly. Even from lumber that is a bit moisture laden. Is this detail worth it in a dry climate? 
  Side note: 
      The thermal layer of the addition is designed to match with the existing structure, which is all built to code. I would like to add more and increase the R value of the roof, wall, and floor assemblies, but it doesn’t make sense to me if the rest of the house is not at the same value. Would it make a substantial benefit over cost?
  Your thoughts and experience would be much appreciated.