Joe Lstiburek was at the EnergyStar Summit this week, and was ranting about the wall system we should be using in this climate. Bottom line - for less money than than the historical R-13 2x4 wall system, you can build a styrofoam sheathed R30 2x6 wall. Truly a no brainer.
However, there are two significant barriers to implementation:
1. Your framers, who have such a difficult time coping with wall systems that use LESS wood.
2. Your structural engineer, who wants OSB sheathing used everywhere.
One slight potential problem within the assembly, vapor condensation on the inside surface of the styrofoam, has been bothering me for a few years. Joe explained that this has been addressed in IRC 2009 with a climate zone prescriptive solution. It specifies the minimum R value for the external insulation to avoid condensation. This value is also a function of the stud cavity R value, but Building Science Corp. typically recommends 1.5 inches of taped external foam.