Dow has a line of rigid foam board insulation in a number of thicknesses from .5 inch to 3 inches, with various ways of being joined at the edges. There is a board with grooves to be used for wrapping neatly around curved surfaces. With 3.5 inches of Ownes Corning’s CoreBond spray in foam between 2 x 4 studs, then 1.5 inches of StryroFoam applied outside the exterior sheathing, you would have a 5.5 inch wall with an R-30+ insulation value. You will save a little money on lumber, and CoreBond, but you have to buy the StyroFoam. It still could be a better choice than 5.5 inches of CoreBond in some circumstances.