I think the ribs are a good place to interupt the flow of heat (or cold) into or out of the trailer. Aluminum readily transmits heat (or cold) and years ago the aluminum window industry started adding a thermal break between the inside and the outside of the window frame, greatly improving the thermal efficiency of the windows. In most cases, this thermal break was a plastic connector which continuously connected the inside aluminum extrusions to the outside aluminum extrusions. It worked great.
Since there is no way to do the same to the ribs on our trailers, the next best thing is to add a thermal break between the inside skin and the ribs. I think the best product for this would be strips of EVA foam. EVA foam comes in many thicknesses, densities, and toughnesses. Most people know it as the colored foam used in most cheap flip-flop sandals. It is available from this supplier:
Ethylene Vinyl Acetate - Canal Rubber Supply Company - Sitemap
If I ever need to remove the inner skin of my trailer, I plan to add strips of 2mm thick EVA foam to the inside rib faces before reinstalling the skin panels. The plan is to use double stick "Super Seamstick Tape" (see
Seamstick 1" Super Seam BastingTape by 3M (60 Yds) ) to apply strips of EVA foam which are a little wider than the rib face then connect the inner panels to the ribs with rivets like the factory.
The weight is negligable and it could reduce the heating (or cooling) load by as much as 20% (my own guesstimate(unscientific))
Here's a pic of a thermal break used in an aluminum window frame.