Zone 2 Conditioned Attic … Or Not – GreenBuildingAdvisor

Zone 2 – Phoenix, AZ

Hey all, I really need help with my roof design … both living space and garage.

Some details:

-Single level home

-0.5:12 slope roof (basically flat)

-Living space attic is about 40’ (L) x 62’ (W) x 2’3” (average Height) = ~5580 -Cubic ft 

-HVAC lines will run through the attic space

-Built-up roof using polyiso rigid foam on top of decking then TPO

My initial intent was to have a conditioned attic space because of the HVAC lines.  The more I look into it, the expense is going to be pretty steep and problems with foam make me question which path to follow. 

From what I can tell, there’s no way to get R-43 ish without also applying some sort of foam to the inside 

R-Values

-Rigid foam (FOAMULAR NGX F-250 @ 2”): R-10

-Open cell foam: ~R-4 per inch

-Closed cell foam: ~R-7 per inch

If I’m figuring right, one of the options is,

4” of the foam and 6” of open cell spray to get me at R43. That equals a BUNCH of money.

Should I just consider a vented attic and bump up the AC units’ tonnage to manage the delta between the cool air inside the duct and attic air. ::sad face::

Then, for the garage.  It won’t be conditioned.  Just vent that attic in there?  I’m wondering if it’s even worth putting more than a couple inches of rigid foam on top of the decking.  

I’m really at a loss for how to move forward. Are there any professionals who are familiar with zone 2 construction willing to help me figure out what the best options are considering the variables?  Or point me to who I should be talking to?