[Greenbuilding] Building Wrap over Exterior EPS

[conservationarchitect at rockbridge.net]

My standard detail for exterior wall framing is a stud wall at with structural sheathing to exterior covered with 6”+- eps foam with seams sealed with Pur Stick.  2x3 furring strips aligned with studs with 9” headlock screws with sealant on foam side of furring strip to seal the hole created by screw.  The 2x furring strips allow conventional nails to be used.  Using 3/4” plywood is an alternative that requires screw fastening of siding.  I like the wider space of the 2x.  

My question is with this assembly, is it necessary or beneficial to add a layer of building wrap above to the exterior of the eps foam.  I prefer eps to xps because it is vapor permeable and an air barrier.  This is the benefit of building wrap.  The thickness of foam makes the air warmer on the warm side, reducing the threat of condensation.  On my own home retrofit, over existing plywood siding with  typical framing and fiberglass batts, I installed 3” iso, building wrap, 3/4” osb furring strips and plywood siding supported with screws that did not penetrate the building wrap.  My thinking at the time was that it was a redundant layer block air flow from any flaws in the sealing of the seams.  

With new construction designed with uniform planer surfaces, achieving full seal should be doable.  I am able to order eps foam to be formed to the design dimension, eliminating waste.  With a rain gap of 3/4”-1 1/2” is there a need for the the building wrap to protect wall from liquid water penetrating the siding.  My thought is that the liquid will drain to weeps at bottom (customized galvanized Z strip flashing), and liquid that holds onto eps will evaporate as will incidental vapor the penetrates foam from interior.  I am making the rain drain cavity continuous with attic ventilation for good draft.  Therefore, I do not see the risk to the eps from liquid.  Only standing water will damage the eps.  Of course, eps is used for coffee cups that holds liquid, although likely a higher density than the type I eps I plan to use.  

I hope you all are still out there.  
Thanks
Eli 
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