A few years ago I was part of a couple of projects where we used Thermacal (Cornell Corp.) for exterior insulation on stick-framed retrofits (see image). The product comes in standard sheetgood sizes, with your choice of insulation layer and sheathing product. The image shows a 7-1/4″ polyurethane foam—3/4″ plywood panel that was used for the roof. The walls were clad with 4-1/4″ polyurethane foam—3/4″ plywood panels. Before, all overhangs were cut off and an air-tight layer was established on the former exterior sheathing layer of the building. Installation of the product is more difficult than installing just sheathing—fasteners (screws) are very long and need to be put into the framing members beyond. Joints need to be sealed. All-in-all this is not a cheap and easy task, but it provides a thermal-bridge free exterior insulation package with great R-value.