After all the issues I had back east with moisture in my fiberglass bats, it was on my mind. The basic concept (as I understand it) is to have enough insulation that the dewpoint is never at the interior side of the foam. You have to spray thick enough to insure that the heat is blocked out completely otherwise the cool air from the garage will condense the moisture in that warm air and cause wet ceilings, etc. My old fiberglass was a black, moldy mess after a few years and I had to tear it all out and throw it away.
These sprayfoam guys were pretty adamant that a 5.5" thick open-cell foam (R21) would be more than adequate to prevent moisture issues. I don't know the exact correction factor, but they kept saying that an R21 spray foam application had an "effective" R-value of more like R38 fiberglass bat application due to being airtight.
The real surprise was when they told me about all the extra work I would have to do to deal with my attic mounted furnace and hot water tanks... both of which are LP gas fired. They rely on fresh attic air for the combustion process, so you can't just sprayfoam and seal up the entire attic. There won't be any oxygen for the units, so you have to build "fresh air" boxes around each one and then bring in fresh air ducting from the roof in addition to the exhaust vents that are already there. What a pain.
-G
I think? there is a difference in where you want the vapor barrier from warm to cold climates. Up here we vapor barrier on the inside face of the studs, I've heard in warm climates you vapor barrier on the exterior side of the studs. Might check that.
I don't know about the "effective R-Value" up here it's R-7 per inch of closed cell. No effective blah blah.
Interesting that they say you need to duct the air to the gas appliance, we do a lot of gravity fed systems still (not a high efficiency piped boiler). We simply have to supply a certain size opening into the room based on the furnace/boiler/hot water heater size. No ducting just an opening to the exterior, sometimes it's one high and one low in the room so you get a bit of natural convection air flow. In your case we would install say a 12" square duct at one of the nearest gable ends and maybe another some distance away.






