I've spoken to quite a few homeowners recently about insulating attics with spray foam insulation like Icynene. Many worried about keeping the attics properly ventilated. Here's another area where the building science has evolved. Particularly here in the South, ventilating attics is not necessarily the right thing to do, as with crawl spaces. And, attic fans are not helping either.
The better thing to do is seal the attic and insulate the roof deck with spray foam. With new construction a radiant barrier like TechShield should also be used.
The basic thinking has been to cool the attic with ventilation. But what we now realize is that almost all of the heat transfer into the attic space is through radiation directly through the roof deck. Vents will do nothing to stop this. And when it's hot and humid outside all vents do is draw humidity into the building envelope.
For a good article on this see "Joe's Top Ten List of Dumb Things to do in the South", on the Building Science website. A great general resource. And take a look at Arnie Katz's article on this subject at the Advanced Energy site. Another great resource.
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