May I suggest that you first wrap the sheathing, or SIPs, with GRACE ice and water shield. Secondly, install your weather resistant purlins on 36 inch horizontal centers. Be sure to anchor the purlins securely to the structure with silicon sealant and pole barn spikes driven into the frame and deck screws placed in between. Make sure to use pressure treated lumber for your purlins or some other weather resistant lumber, larch, hemlock, etc... Thirdly, install a z-bend counter flashing, made from aluminum or galvanized steel, along the topside of each coarse of purlins. This will keep water from running down behind the purlins. Finally, seal the top seam of the counter-flashing with a 3 inch strip of GRACE ice shield or SIP tape. This will keep water from sneaking behind the counter-flashing. And added benefit of using GRACE ice shield is that it's black in color and will not show through the siding as bad as white Tyvec.

Install your board and batten siding with galvanized or stainless steel nails. Two nails on each side of the board in each purlin intersection and one nail in the middle of each batten directly over the underlying seam in the siding boards. Ths will allow the battens to "move" independently of the underlying siding boards thus minimizing the checking and cracking of each.

I realize that this is a pricey solution, but I'd bet that it's a lot cheaper than replacing the SIPs after a few years of Tyvec leaks.

I hope this helps, good luck with the transformation.

D.Ball