To get the most out of the SilverBack™ vapor barrier and foundation insulation, be sure to install both correctly. This diagram shows how to install them in your crawl space.
If you want to learn more about crawl space insulation, check out this article.
By installing the crawl space vapor barrier over the insulation you will prevent holes in the vapor barrier from securing the rigid foam to the wall. It goes without saying that the holes would not be a good thing.
The rigid foam will not absorb moisture. However, please be aware that spray foam is not the same as rigid foam. The spray on version will absorb moisture and that moisture will lower its R value.
Learn about crawl space encapsulation and get familiar with everything you need. Crawl Space Repair has a number of different resources to help you. Every step of the process is important. Your DIY crawl space project is not as difficult as you think when you have the right resources.