Not sure, but probably closed, since it won't absorb moisture. Maybe there's a clue here: http://www.rd.com/familyhandyman/content/17606/

Down here we use open cell above grade and closed cell below Building Science articles do mention both cc and oc being used in attics, but they explicitly recommend cc. See "Unvented Roofs for All Climates" and "Understanding Attic Ventilation" (See Link below.) As I mentioned above, cc is excellent in attics as it acts as a secondary liquid water barrier. At the same time, it is permeable, allowing vapour to move both ways depending on the season. As for "hiding" leaks, I have seen commentary from foam appliers who handle all kinds of foam say that in case of leaks cc focuses the water, so it's easier to spot the source. Because oc foam is more permeable and holds water, the R value is degraded, which means the sheathing is more easily reaching dewpoint, which is not desirable. The same logic applies below grade. The one time I used oc foam, it was applied in cold weather and shrunk back considerably around the rim joists; this is recognized in the literature but brushed off as a minor quibble. When I've used cc foam in basements, rim joists, garage ceilings and attics (but not under the decking) I had none of those problems. Clint Eastwood's Dirty Harry had a famous comment about opinions--that I won't use here. That's why I try to reference mine to Building Science, whose principals are among the leading building scientists around; and who are also the source of much info at the US Energy Department --Energy Efficiency and Renewable Energy--Building Technologies Department. Here is a link that might be useful: Vented and Unvented Roof Recommendations

flex in a crawl...sigh the things we do... you don't get into specifics about the audit and I've learned not to expect much from a lot of 'audits'. but... ducts leak.flex less than hard pipe. where they connect to the plenum @ start collar where they terminate at supply box and where supply box connects to floor. just by removing a supply grill inside the house you can see the poorly sealed cut. before you spray foam..mastic seal these areas I listed above. paint on mastic or mastic tape like hardcast brand #1402 duct leakage is high in existing house. sealing the ducts with acca ul approved products should be done prior to foam don't let foam company sell you duct sealing with foam foam is not a duct sealant. not approved & over time fails quickly. if ducts are strapped in between floor joists..how will foam company get foam to floors? better take a look under there and see if ducts are strapped or loose or lying on the ground. (ducts shouldnt lie on the ground) do any duct sealing first...its a pita to dig out foam to work with ducts...been there done that. I'd use closed cell. if you use open cell then a vapor barrier has to be installed on the face of the open cell once install is complete. unless you have a perfect vapor barrier on the ground...that is still perfect after foam install. while closed cell will trap moisture it is the way that it is installed that will make the difference. install is 2-3" of closed cell in between floor joists. the joists themselves are not encapsulated in foam (directly opposite from unvented attics..different product different needs = different install) where the foam meets the floor joist is where the future leaks will exit. if you go under a house that has had closed cell installed you will see that in a couple of years the foam seperates a bit from the joists. closed cell is rigid when dried and doesn't expand and contract like open cell. this seperation helps to aid exit of water from leaks. did the audit cover duct leakage and how to address? what shape are the ducts in currently? are they strapped up or not? your system is in the house in a closet? shop your foam companies wisely. ask for customers names that have had similar installs and speak with them. as for as offgassing..all foams offgas. but it is outside of the living space and shoudn't be a problem as long as there are no gaps for the foam to enter the house or the ductwork. you may want to spend the night somewhere else if you worry that it will make you ill. any plumbing work in the future will have to be re-foamed once work is complete. usually cans of great stuff can be used to seal these smallish areas. if not the foam company has a bigger can of foam that will cover most of these larger areas. best of luck

Thanks for the replys. Forgot to add that the firm that wants to do closed cell + fiberglass said it was a code requirement to cover the foam with what he called a code approved ignition barrier. I thought that any foam rated E-84 was OK by itself. Comments? One question I need to ask is why not just spray 4 inches of closed cell and be done it with. That should provide at least R-25. Compared to the inefective R-11 that was in there it would be a big improvement. One reason this may not be practical is it may require two passes as I'm not sure you can apply closed cell at a 4 inch depth in one pass due to excess heat generation and risk of self ignition. Two passes would add to the cost and most likely push it well above the open cell. As for converting the crawl space into a mini basement, I think the jury is still out on that one. If not done correctly I think the consequences are worse than leaving it the way it is now. I also think the cost would be much higher since you still have to insulate the foundation walls and completely seal the volume from the ground.