I will be using a radiant barrier on the ceiling, floor, and all walls of my JayBee--to stop radiant heat flow by both reflectivity (97% reflective) and emissivity (only 3% emissive). For this, I will be using two different products. The one I will use under both the subfloor and the ceiling boards has a double layer of enclosed air bubbles with a reflective, moisture-barrier surface on both sides. (I will use what is left on the roll after I've installed it on the house to make insulated window curtains.)
The product I will use on the walls is a perforated, kraft-paper product. The perforations make it breathable so it can be used under the siding as a house wrap. (I will be installing a separate moisture barrier on the inside of the walls.)
These products should add R14 to the house's envelope, which should especially help with the walls, since the walls will only be 3.5 inches thick.