Housewraps and rainscreens – GreenBuildingAdvisor

Hello,

I am looking for opinions on the usage of housewraps and/or rain screens on walls and roofs with rigid poly-iso exterior insulation.  A little background first.  I am building a small home, off-grid, home in Eastern Washington.  We are climate zone 5b and very dry (only about 14″ of rain annually and very little humidity).  The structure is a 40’x40′ post-frame with 9′ side walls and attic-room trusses.  The trusses are 2′ OC and will set on top of 5.5″x12″ beams that span between 3-ply posts which are 10′ OC. The walls and roof will be sheathed in 7/16″ Zip to provide shear (per structural plans).  Roofing will be a clip style standing seam metal and siding will be exposed fastener metal panels. The attic room will be used for living space and is 20′ wide and I am going to run ductwork and utilities in the truss areas outside of the attic room, so I want to keep the entire area within the conditioned envelope, so the insulation will be at the roof line. The combination of 2′ OC trusses and the insulation at the roof line, makes installing a vapor barrier on the interior side extremely difficult.  This lead me to two options: 1) closed cell spray foam on the underside of the Zip sheathing 2) continuous insulation on top of the roof sheathing (min R-20 per IRC-806).  I am going with option 2 and installing 3.5″ of poly-iso (R-22) on top of the Zip with R-23 or R-30 mineral wool strapped to the underside between the trusses.  The poly-iso will go down in two layers.  A continuous 2″ layer followed by 2×4 furring strips spaced for standing seam roofing clips (~3′) with 1.5″ poly iso between furring strips. I am going with a similar assembly for the walls, except only 1.5″ of poly-iso (I can get a good vapor barrier on the inside of the walls).  This brings me to my questions:  Do I need any additional protection (self-adhered ice/water barrier) between the zip and the poly-iso?  How about a rainscreen product like HydraGap or Cedar breather?  Should I use a rainscreen on the outside of the poly-iso under the metal roofing or siding?  The metal cladding company I am using said that in a dry climate like ours they do not require any rain screen behind the metal panels as the panel profile creates enough gaps for air to move and dry things out.  However, this isn’t the case between the poly-iso and the Zip sheating.