On Nov 9, 11:56*pm, Robert Neville wrote:
RicodJour wrote:

*It's more about permeability than R-value.

Well that was my thinking. The roof deck should be water tight already, and if
you spray the underside of a roof deck that isn't water tight, you don't want to
be trapping water there. Open cell at least allows the water to pass through and
not rot out the deck.

Granted, a leaking roof is always bad, and it is worse with a spray
foam undercoat - of either type. You'll just find out about the leak a
bit sooner with open cell. The trade off is that open cell is much
more of a problem with moisture from humidity condensing inside the
insulation in cold weather climates. As far as I know any code that
allows an unvented attic with spray foam, requires the foam to be
closed cell.

R