Insulate Crawl Space Floor

I'm in the process of insulating my crawl space and putting down a drainage mat and vapor barrier. The other thing I believe I should do is to put some insulation between the drainage mat and the vapor barrier. My question is: What should I use? I can buy styrofoam that will give me about 3R value, and I can line the floor of the crawl space for about $300.



The second question is whether or not I should use more insulation on the walls or not. My understanding is that R3 - R6 isn't very much. Should I double up my insulation?



Finally, I understand that air sealing is a big deal, but is air sealing a big deal when I'm putting it on the ground? I'm going to be sealing it with the vapor barrier, so do I need to tape the boards together or something? If I don't tape them together I might get little gaps where there is no insulation, but does that drastically impact their effectiveness?



One last thing, I'm not really sure if I need to "air seal" the top of the floor joists. Light does not get into the crawl space from the outside in any space except for the one vent, which I intend to seal. So Do I need to do anything else? How do I test for this?



Thanks guys!

Insulate Crawl Space Floor