Pete,

Why use SIP's? If you were to use 4 inch ridgid High-R you could apply 1 by furring strips over the insulation. This would give you the air space for the shiplap and allow you to keep your R-value around 22ish. It may be more in labor costs, but the materials should be less expensive. Besides, in my opinion High-R breaths more than SIP's.

Good Luck,

Dan