See "Understanding Foundations" from Building Science Corp. "The most cost effective approach involves a combination of rigid insulation and a frame wall assembly .... The least risky interior insulation assemblies from an installation perspective and water sensitivity and drying perspective utilize spray foam--either closed cell or open cell." Either type allows drying to the basement. On my most recent project I used BASF closed cell insulation for the entire basement, rims, garage ceiling and other hard-to-reach areas. On others, I've used open cell insulation at the rim joists and XPS on the walls. I don't like open cell because it requires more to achieve the same R value. Also, when sprayed on cold walls, it shrinks noticeably and I've ended up trying to fill the gaps with hand held spray foam.

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

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

with foam the install is crucial. inexperienced untrained installers do not get the air sealing that makes for an air tight seal. we use open cell here, full rafter depth and faces of 2x rafters covered. this is a hot humid climate. ask to see installs of jobs they have done for others or to visit sites where jobs are ongoing. insulation should not dip in and out in rafter bays but be mostly even. 6" of foam is 6".. not 3" to 6". open cell has less R-value than closed cell. and all should be installed to specific R-value spec' best of luck