AVS Special Member
Join Date: Dec 2008
Location: L.A. - Lower Alabama
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Here's how I understand it, but the advice is worth what you're paying. In areas where a vapor barrier is required, you want to prevent the warm moist air from coming in contact with the concrete as it will cause condensation. In areas further south where no vapor barrier is required, the thinking is that the concrete will not get cold enough to cause condensation. That being the case, the air flow doesn't matter, and as Tom pointed out, may be helpful as the vapor drive in this part of the country will be from outside in and the air movement may help to dry any moisture that is there.
All that assumes that you don't have a moisture intrusion issue to begin with, but a vapor barrier wouldn't make a difference there anyway.
Another point to consider, and maybe this makes the decision easier, a faced insulation would have to have drywall against it to meet fire code. Since there is a gap, it needs to be unfaced (again, this is just what I've read on the ever-correct internet). You also need to consider fire blocking for those walls, which can get to be a pain. There needs to be blocking between the wall/ceiling as well as blocking every ten feet horizontally.