To insulate a crawl space can I use styrofoam pink insulation instead of fiber glass, if so what thickness should I use. Do I need to put plastic to the joist or only on the floor.
Thank you Bernard

Best insulation for a crawl space is spray foam hands down. It solves the problem of ANY air space for mold and air infiltration.

The old standard way is to fiberglass the floor joist with the vapor barrier towards the warm side of the house which is right under the floor.
This exposes the glass to the elements which may be damp. Hence the spray foam

I use to rip a 3/4 to 1" wide strips and nail them flush to the bottom of the joist. The cut and inlay a 1" foam/blue insulated board inside the floor joist bay. Now we have a ridged barrier You can essentially roll out the fiberglass in the floors bays and staple the vapor barrier to the tops of the floor joist then the floors TAG ply.

You mentioned plastic for a vapor barrier. That is only used if you are using unfaced rolls/batts. In Canada it's a code. And other northern parts of the U.S.

If this is already built the spray foam is really the only best option here to obtain the R value needed and have the vapor barrier hassle of stapling it up inside/below the floors plywood.

Energy Savers Tips: Insulation (http://www1.eere.energy.gov/consumer/tips/insulation.html)
What is insulation R value? | Insulation | Insulation Info (http://insulationinfo.org/insulation/what-is-insulation-r-value/)

I agree and rigid does not work very well in this application. The foam board is a good insulator but it allows too much air migration past the edges and thus defeats it's value. Do not use unless you plan to tape or foam seal all edges.