I think that you have it figured out. The only question is then low or high density spray foam. The difference is water and water vapor permeability. For that choice you have to take into account the local climate and what kind of flooring material you have. You don't want a vapor barrier sandwich.

Thanks for your response. Please elaborate on what you mean by a vapor sandwich?

I have oak tong and groove traditional floors over a combination of old fir subfloor and plywood (were repairs were required). I assume that I want to use a high density foam to prevent water vapor from rising into the subfloor. And that the vents in the walls of the crawl space would provide ventalation.

You do not want to enclose anything with a vapor barrier on both sides. Your traditional floor might count as a vapor barrier depending on the finish. If that is the case, you probably need to go with low-density foam so you don't seal the back side as well.

I can't be very much help in advising you on whether you want a vapor barrier and if so what kind of vapor barrier you should have and where to place it. That is very climate-specific. Just as an example, where I live, it is hot and humid so if you install glass batts that have a vapor barrier, you either slash the backing or install it with the backing facing the exterior. Sealed crawlspaces are recommended here as well.

Thank you, ionized. I do not think the floor will act as a vapor barrier because I've finished it with a penetrating wax oil rather than a sealing finish. Beyond that, I will need to do some local research about vapor barrier standards and requirements.

In addition to the vapor issue, my other questions about the foam are: will there long term off-gas? How difficult will it be if I need to get through the foam for any future and unexpected reapirs, is an ignition barrier necessary (even though there no heat source in the crawlspace), will it be effective against rodents, will wood destroying insects become as issue?

Fiberglass will only provide insulation value of the air is NOT moving.

If the air is moving you need a foam product.

Sprayed on foam also has the advantage of proving protection against infiltration, the single largest cause of heat loss (or gain) in most houses.

It does not take much ambient air leaking in to move a lot of heat.

Try www.buildingscience.com/ for information about vapor barriers. Another good source for local building advice might be your state land grant university. I do not know anything about your floor finish. I guess you will cringe when I mention pre-finished flooring, but keep reading ;-) The spaces between the boards lets moisture through. With finished in place floors (poly finishes) you have a good vapor barrier. I guess that you understand the "sandwich" prohibition now.

If you have a vented crawl space, off gassing will primarily be through the crawl. Ignition is a good question. I am afraid that I do not know much about fire and foam. I know that it burns and that crawl spaces raise fire insurance so it is a good thing to look into.

Any kind of covering over the underside of your floor is going to increase the risk undiscovered or delayed discovery of rodent and insect damage. That includes batts, foam or boxing-in with rigid insulation. I do not know if there is any hard information about the relative risks of each method.

The rigid insulation alternative is not one that has been addressed in this thread yet. It would be less expensive than foam, if DIY, but it is a lot of work. It has to be taped very well. You could take a hybrid approach and cover selected areas with foam board -- under the most troublesome plumbing areas for example. When you get done with that, you foam the rest.

Low density foam pulls out pretty easily. I do not know about high density. You would just have to spray some more in after you make repairs. The problem would be finding the stuff that you need to fix. I would considerer tagging stuff before foaming if it will be covered by foam. You could use durable plastic tags in some color code. If you have a leak, water will run right through low-density foam. I do not think that is so with high-density foam.

One important feature of spray foam is the air-sealing capacity. Batts will not do that. That is the problem with it where I live in the humid Gulf South. Air gets around batts even when freshly-installed. It is even worse when it sags. Humid air hits the cool floor, water condenses and does not dry well. Floor rot happens. Even with the best batt installation, the joists become the coolest thing under the house. Condensation happens there and then rot happens. These things worked great before AC existed, but not anymore.

I forgot that brickeyee already mentioned the air-sealing capability of foam. It can' be overemphasized, though. Homes built over crawl spaces often leak a LOT of air from that side. That is one huge advantage to concrete slabs.