On my walk around the neighborhood yesterday I saw the biggest roof mounted attive ventillator that I've ever seen - twice as big as the one pictured. So it's got me thinking about the issue of attic ventillation.
The sales positioning for these fans is that by ventillating your attic
you remove the heat and save money on your air conditioning. But I've read a respected view from Advanced Energy that refutes this claim. See the article on Advanced Energy's site here. They say that since almost all of the heat gain is through radiation that ventillation will do little or nothing to improve that.
I consider Advanced Energy to be an authority on matters like this.
They do real field research and testing. The houses they tested were
all being depressurised to some extent - that's what mainly causes the
risk of flame roll out. And you really want positive pressure in
your
house, at least here in the South. Positive pressure keeps the house
from sucking in impurities. The attic can also pull your cool air up through the
cieling and into the outdoors. So it's very important that a house with powered attic ventillation is
thoroughly sealed between the conditioned space and the attic space.
On the other hand, speaking to builders I hear the view that you've got to get rid of the heat that builds up in the attic somehow. Proper ventillation is the only way to do that. One builder tells me that in his own attic he has experienced cooler temperatures since installing a powered ventillator. Also I found a very detailed report here from the Florida Solar Energy Center that explores the issue well and comes down in favor of powered ventillation.
It's obviously a complex issue with no simple answers. If building from scratch or if you can afford it, I'd seriously consider spray-foam insulation and sealing the attic - especially if your ducts run in the attic space. For an existing house with ducts in the crawl space, I'd first make sure the attic floor was well sealed from the cieling below so that air isn't exchanged. Then I'd insulate well. Then I'd possibly consider a powered attic ventillation system last.