I have a flat roof on the back addition of my house. The AC tubing for these two rooms is under the house and vented through the floor. Is it less energy efficient? I could see where the shade and openness of under the house would be better than the sweltering heat in your typical Texas attic. Of greater concern is the lack of protection from animals & rodents and water since I am on the bay and could have flooding. Is there some heavy duty tubing or some protective tubing available for these conditions? Thanks

If the space under your home is not blocked in with cinderblocks or other material it's a good idea to completely close in under your home. The openness under your home CAN lose the cold air from the ducts. Also in the winter months your heat will be more efficient not losing the heat to the outside cold air. Best to block in the entire area under your home and not just the a/c vents. Also you don't want little critters making their homes under your new addition do you? As little rodents like mice can gnaw holes in the a/c tubing and enter your home that way.

You mention "ducts" and then "tubing". They are very different. Is this the air supply ductwork or the freon tubing? Is the air handler under the house?