I’m building a home in climate zone 5 and have an average temp of Dec, Jan, Feb of 20 degrees. 

the wall assembly I’m considering is 2×6, 24 inch on Center with 2inch zip R sheathing.  I know in climate zone 5, the chart says that with a cavity R value of 19 the exterior insulation should be at least 7.5.  However after doing the calculation, Interface temp. = Indoor temp. – [(Indoor temp. – Outdoor temp.) X (Cavity R / Total R)]

I found I will actually need a higher exterior insulation R value.  Currently the zip R I’m using is R9, so I can’t fill the entire cavity up to R19 without causing the interior face of the zip R foam to fall below the dew point.  My proposed solution is to fill the 5.5 inch 2×6 cavity with 1 inch closed cell spray foam against the zip R foam, effectively bringing the “exterior insulation” up to 15.  Then I plan to fill the remaining 4.5 inches with a 3.5 inch Roxul batt.  This would keep the foam definitely above the dew point, but would also leave 1 inch of vacant space between the drywall and the roxul batt.  Would the roxul batt stay secured in place against the foam or would it need to be secured?  Thanks.  I’ve added a mock up to show what I mean.