How large is the distance from the dirt to the bottom of the floor joists?

I went out and measured and it's about 6 inches.

Pics would help. For instance, I don't understand how there could be any plumbing in the space you describe. I've seen some low crawls--but not six inches!

I don't want to hijack this thread, but I have essentially the same problem with a couple of added rooms on my house. In my case there's also no foundation and minimal space below. The siding comes down to soil level. The floors are COLD and it seems like a rot/insect problem waiting to happen. I'm interested to hear what folks would recommend.

So this would be a house on piers. If there's any access at all, foam board or spray foam under the flooring would be effective. But the foam should be at lest six inches above the ground, to prevent it from serving as a hidden route for termites.

This post was edited by worthy on Wed, Jul 9, 14 at 16:00

In my case there is nothing down to the ground. The ground is dug out a bit beneath the rooms to allow space for the plumbing. However that dug out part isn't even deep enough to call it a crawl space.

Insulating below the floor is a good idea. But if the space is open (as norar_il has, or if we were to remove the siding that's in contact with the ground), then animals could get in, too. I was hoping someone would have suggestions on how to enclose it with brick or blocks or something. Any advice?

You need to first put a strong vapor retarder over the earth and then close the gap at the perimeter. There are many ways to do that but the material should resist rotting (Azek, etc.) Insulation would help but it is not as important as air sealing. Perhaps 1" styrofoam glued to the back of the closure panel. Perhaps the bottom edge of the closure panel should be buried an inch or two in the ground. It might not last forever but the only permanent solution is to jack the house up and add a foundation.