The plastic is there to keep moisture from the ground from entering the crawl space.

If you insulate the floor really well and don't insulate the foundation walls of the crawl space the walls may get below freezing. I am not inclined to think that is a good thing.

I suggest if you feel the crawl needs more insulation you might want to insulate the block walls and leave the r 13 alone. If the plastic does not cover all the ground I would cover the rest of it.

Having a semi- warm crawl space seems like a good idea to me.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.