Here's a pic of a fellow who insulated the entire interior of a big Sprinter with cans of Great Stuff... 
https://docs.google.com/presentatio...HACfoTShUd9ewmooILZ0E4ocxT4/edit#slide=id.p11
Despite claims to the contrary, Great Stuff is in fact closed cell foam, not open cell. Meaning that is does not absorb or transmit moisture. Seems to be the making of an urban myth of sorts.
The pictures of rust are real though, degree of this being anecdotal is unknown. Having used more spray foam in all its flavors than I would ever wish on anybody, I can posit a theory about what's happening that causes problems.
When I spray from the commercial rig, heated parts A&B combine at last point while leaving the gun and coat surface in liquid form for a very brief second before reacting and expanding into foam. I would say that as a result, surface has darn near 100% product coverage and contact. The small DIY tanks are basically the same thing...lower operating pressure probably results in less than 100% contact, but not by much.
Great Stuff on the other hand, exits the can already in foam form, it just expands. It expands quickly and non-uniformly, creating little folds as it goes. I would suspect that actual surface contact/coverage is probably much less than 100%. I think its reasonable to speculate that what is causing rust is Not moisture penetrating thru the foam, but instead is condensation on van skin that becomes trapped by the foam.
If this is accurate, then two things are probably true: 1) an effective moisture barrier over foamed areas would prevent issues, and 2) install methods such as GaryBIS outline are fine because foam is immediately compressed into surface, maximizing both contact and creating a perimeter barrier behind the rigid foam boards. I think I would be comfortable with this method.
https://docs.google.com/presentatio...HACfoTShUd9ewmooILZ0E4ocxT4/edit#slide=id.p11
Despite claims to the contrary, Great Stuff is in fact closed cell foam, not open cell. Meaning that is does not absorb or transmit moisture. Seems to be the making of an urban myth of sorts.
The pictures of rust are real though, degree of this being anecdotal is unknown. Having used more spray foam in all its flavors than I would ever wish on anybody, I can posit a theory about what's happening that causes problems.
When I spray from the commercial rig, heated parts A&B combine at last point while leaving the gun and coat surface in liquid form for a very brief second before reacting and expanding into foam. I would say that as a result, surface has darn near 100% product coverage and contact. The small DIY tanks are basically the same thing...lower operating pressure probably results in less than 100% contact, but not by much.
Great Stuff on the other hand, exits the can already in foam form, it just expands. It expands quickly and non-uniformly, creating little folds as it goes. I would suspect that actual surface contact/coverage is probably much less than 100%. I think its reasonable to speculate that what is causing rust is Not moisture penetrating thru the foam, but instead is condensation on van skin that becomes trapped by the foam.
If this is accurate, then two things are probably true: 1) an effective moisture barrier over foamed areas would prevent issues, and 2) install methods such as GaryBIS outline are fine because foam is immediately compressed into surface, maximizing both contact and creating a perimeter barrier behind the rigid foam boards. I think I would be comfortable with this method.
