I need to insulate under a house. I know how I would need to install the insulation. But I'm curious in what is recommended or what is the best level of insulation. R-?

Depends a lot on where you live. In Key West, not much if any. In Minnesota, R-30

Thanks for your reply ma0641, It made me think I forgot to give location. I'm in Washington state.. I know that each state makes a difference.

If you Google "Crawl space insulation", it will give you recommended amounts. You also need to consider the dampness of the climate there.