Climate Zone 5 assembly advice for renovation – GreenBuildingAdvisor

I am working on an existing lake cottage in Massachusetts, Climate Zone 5.  The project needs to meet the stretch energy code and the client is interested in high-performance or passive house strategies for reducing energy consumption as much as possible.  The existing is an uninsulated, 2×4 wall construction (old enough to be a true 4″ deep stud) and the new addition would be 2×6 wall construction.

My initial thought was the following:

1/2″ Interior gypsum wallboard

2×4 with unfaced fiberglass batt (R-13)

1/2″ Zip Sheathing

3″ of Foamular 150 (2 layers of 1.5″, joints staggered)

2×4 pressure treated furring (16″ oc)

Diamond Kote or LP Smartside lap siding (Owner still reviewing)

For the 2×6 wall assemblies I would increase the insulation to faced R-19 batts with the same basic assembly but I am concerned about the dew point with R-19 interior and R-15 ci.

The contractor is suggesting using Zip-R sheathing (he suggested the R-6 over R-21 batt insulation) but I’m concerned how that will work in the existing 2×4 walls where there is no interior vapor barrier.  The contractor is concerned about nailing through the 3″ of foam to get enough support for the siding.

What systems have people used that are cost-effective and provide good performance?