Using the U-Factor for roof assembly & amp; exterior foam – GreenBuildingAdvisor

Wow, this community has been so helpful and just want to say thanks!  This will be my 1st question, probably of many, so go easy.

We are in the midst of an entire home remodel, down to the studs.  We will, for several reasons, be replacing the roof deck and roof.  My plan is to turn our current vented attic into an unvented conditioned space by adding exterior ridged foam along with interior rockwool.  A hybrid approach.  We live in zone 6a and had planned for r-26 on the outside and r-23 on the inside for a total code compliant of r-49.  I was planning to install 2 layers of 2” polyiso (r-13.1) for r-26.2.  While not the most economical, it was about the only way I could see meeting code with our build and was nearly set on our plan.  Until….I started reading about the U-factor alternative.  By following the U-factor alternative I could use 2 layers of 2” EPS (r-7.7) for a total of r-15.4.  By doing so, this would get me to a U-factor of .025, which is below the code maximum of .026 and save me probably $3000 in materials.

Sounds great.  But…I’m wondering, while this will meet code and save me money, will this present problems with the roof decking since my exterior foam is not 51% of the total r-value as stated in “The Ratio Rule for Hybrid Roof Insulation”.