insulating a closed "crawlspace"?

[carwashguy]

I am getting ready to install hardwood flooring in my kitchen and hallway.  There is one section of the kitchen floor that bridges what used to be a flagstone patio/walkway that was between the original part of the house and the garage.  Let me describe it a little more to be clear....The house was originally built with a covered 6' breezeway between the house and the attached garage.  At some point someone enclosed the breezeway and removed a section of wall in the kitchen to make a mudroom/entry.  The floor is 12" above the flagstone in the mudroom and steps up to 18" above the flagstone in the entry.  I am only installing hardwood over the 18" section at this time and will cover the 12" high section with tile sometime down the road.

I want to insulate the floor as this will be the only time I have access to it.   The area has no venting to the outside and has no penetrations into either the house or the garage.  Do I need to put down a vapor barrier and tape around the perimeter?  Should I somehow attach pink Polystyrene (XPS) to the underside of the joists and then fill the joist with fiberglass insulation?  

Crawl spaces are somewhat rare here and I have no idea how to proceed...

[Snipe Hunter]

This should tell you all you need to know.

Owens Corning Insulating a Crawlspace

[Robert Adams]

With no venting you need to put a plastic barrier on the ground sealed to the walls and insulate the walls. You only insulate the floor itself on a vented crawlspace. 

     Sealed crawlspaces are much cheaper to insulate because you only do the walls. Remember it's just a short basement. 


      Lots of good info at the building science website.

[frule]

Poly sheeting vapor barrier is necessary on the ground. Take it up the walls. Leave a 3" band at the top for termite inspection.
Insulate walls only with 1-2" foamboard....again, leave 3" gap at top. Remove any wood, cardboard, etc. before putting down the vapor barrier. There is zero reason to insulate the floor....and lots of reasons not to insulate the floors....id the walls are well insulated.

If it gets little traffic, 6-mil poly is fine. Most put the foamboard up first, then cover with the vapor barrier. If you are concerned about mold, you can add a dehumidifier, or connect the area into your HVAC system to condition it. There is tons of
how-to-do on the topic of sealed crawl spaces online.