Vapor Barrior on floor?

Hey Ya'll,
I've started in on my addition which has a crawlspace that is not heated/conditioned (i.e. with a skirt board but otherwise open to outside), do I need a vapor barrior?2x8 joists and planning to put 1/4" Ply screwed from underneith with fiberglass batts between the joists and 3/4" ply on top. Vapor Barrior needed? Where at? Should be close to R-25, no?

For the walls, tyvek outside and 6mil visqueen on the inside w/ fiberglass batting between?

Thanks,
Jason

I'm not a contractor or a pro, but down here in NC, we usually put down a 3/4" plywood subfloor level directly on top of the joists with a 5/8" plywood flooring level on top of that.
If the floor is over an unheated crawlspace or basement, then a layer of roofing felt (slightly overlapped) is put between the two layers of plywood as a vapor barrier.
Insulation is placed between the floor joists and held in place with "tiger claw" wires. If the insulation has a vapor barrier backing, the vapor barrier is placed on the up side toward the bottom of the subflooring. (Insulation vapor barriers always go toward the heated space.)
Good luck!
Mike

Found this today ... seems to be consistent

http://en.wikipedia.org/wiki/Vapor_barrier#Vapor_barrier_in_basement_or_crawlspace

Thanks,
Jason

We used a 6 mil vapor barrier over pea gravel in our crawlspace to eliminate a mold problem.

Well, I done it! Vapor barrior over ground and below the subfloor.

Dirt, 6mil visqueen, air space, 1/4in ply, 2x8 joists, w/ fiberglass batting between, 6mil visqueen, 23/32 ply...and that's where I am.

Got my first wall up last night so don't tell me I shouldn't have put the plastic under the subfloor! Two more walls to go up (and then two to come down inside).

Thx