DuPont and Owens-Corning advocate using a wall the companies are calling the "Extended Plate and Beam" (EPB) wall: Essentially, the top and bottom plates, and the window and door rough openings, are framed with 2x6s, while the wall studs are framed with 2x4s. The 2-inch difference is filled with rigid foam before the wall is sheathed with OSB, and the 2x4 cavities filled with fiberglass. As Matt and Mark explain, it's an effort to use conventional materials to frame-up an R-23 wall. The wall can be framed on the deck and sheathed, much like a standard wall. But in this case you are including the foam, which provides a thermal break for a majority of the framing, before the wall is lifted into place, saving time on the installation.