I would first make sure the ductwork is sealed and well insulated. Also the floor boots should be sealed around their perimeter up through the finished flooring

I see no reason why you cannot seal the bottom of your floors and do nothing to the attic. In this case, they are two separate systems. Since your floor is basically one layer of wood, then sealing the underside will not affect its ability to dry to the inside, should the need arise. As acwizard has said, make sure that the ductwork and boots are sealed well when they spray the floors. This will increase your ac efficiency a bunch.

Thanks for the replies. What exactly are the floor boots? Is that where it meets the sill?

'Boots' are the transition pieces that are attached to the end of the HVAC ducts and come through the holes in your floor. Basically, oddly shaped pieces of sheet metal. The finish floor registers fit into these 'boots'. It has nothing to do with the carpentry of your house.

What wes said about crawlspaces and attics having no particular bearing on one another.

And you're right that open cell would put the flooring at higher risk of rot from humidity drives from the exterior, but an inch of closed cell on the wood would be enough, and the rest could be filled out with open cell at a more favorable R/$, but only relevant if you're going higher-R than R12. And with ducts in the crawlspace it may be better/cheaper to put down a ground vapor barrier, seal off the vents to the outdoors, and use the closed cell foam (or termite-resistant rigid blue-styrofoam) to insulate the crawlspace walls, using smaller amounts of $pray foam to seal & insulate at the foundation sill & band joist.

See: http://www.buildingscience.com/documents/information-sheets/5-thermal-control/crawlspace-insulation/files/bscinfo_512_crawlspace_edit.pdf

With a sealed conditioned crawlspace duct leakage stays inside the pressure boundary of the house, and the risk/amount of condensation on the ducts drops by an order of magnitude.

Moving back up to the attic, even though it's a separate system, using foam to air-seal the attic has the advantage of blocking infiltration losses from whole-house "stack effect". The air conditioning loads on the gulf coast are primarily latent heat (humidity), and even modestly warm high-humidity air leaking in makes the place less comfortable and raises the mold potential. Ventilated attics are a mis-applied solution to problems that occur in much colder climates, and only ADD humidity to attics in warm humid climates, and does remarkably little in the way of sensible (temperature only) cooling of the house as a whole. Air-sealing the attic and making the roof deck the pressure boundary (insulated at the roof deck or not) lowers the humidity of the attic. By sealing both the crawlspace and the attic you're blocking both the entrance and the exit paths for air infiltration, and your conditioned-space stays drier & more comfortable for less compressor power.

The mean January outdoor temp in New Orleans is ~ 50F, which means the even in winter the interior of an unvented attic will have an average temp well above the ~37-40F dew point of the conditioned space air. In colder climates some measures would otherwise have to be taken to protect the rafters and roof deck from wintertime condensation in an unvented attic, but in your case, not so much. If your summertime attic temps run mcuh higher after closing up the venting, adding 6" of blown cellulose over the existing attic floor insulation would be a much cheaper & more effective fix than a full roof-deck of foam.

Unfortunately I dont have any way to seal off the crawlspace from the outside. There are no walls on 3 sides of the crawlspace, and building them would be quie expensive.

Sprayed under the floor it is then, (for no earth-coupling benefit.)

With the ducts outside of the pressure boundary of the house, sealing the ducts at every seam with duct mastic becomse a primary factor for system efficiency- ten of greater impact than insulating the duct ( but insulating them is also still important, AFTER you've sealed them.) Any duct leakage, whether on the returns or the supplies will either pressurize or de-pressurize the house, driving air infiltration through every possible pathway into the house with the air handler, at rates much higher than mere natural convective forces would deliver.

Dana1,

Would wrapping the ducts in sheet vinyl and then spraying them with closed cell polyurethane spray be a good way to seal the ducts in all climates?  I ask this because I would think the vinyl would seal the ducts and keep any spray foam from entering plus the vinyl would facilitate removing some of the spray foam if repairs are necessary.

Alton- sounds to me like that would work, in theory anyway. Full access to the ducts for spraying (particularly on the side of the ducts facing the floor) might be tough to get a good spraying angle on though.

Aerosol-glue type duct sealing techiques are probably easier for really awkward retrofits. (AeroSeal is the only one I'm familiar with and they don't have a dealer anywhere near N. Orleans, or Auburn AL) It's pretty slick though: http://www.youtube.com/watch?v=Hcoz4kjCMXk

Dana1,

Thanks.