Zone 2 conditioned attic … or not
vitamin
| Posted in Green Building Techniques on
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.
And then I watched this BS* + Beer’s Spray Foam Insulation video!
Now my head is whirling. With the serious costs of foam spray + dangers, my head went to other options.
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?
Very much appreciated.
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Replies
Start with this article:
https://www.finehomebuilding.com/project-guides/insulation/five-cathedral-ceilings-that-work
That article sent me down the rabbit hole I needed I think.
I'll plan on using 2" of R10 rigid above the roof decking and an R30 or R38 rockwool on the underside and where the truss heels are. Then use foam where penetrations to the outside occur.
And, the install is of Rockwool is something I can do to save on some labor costs.
Thank you.
In zone 2 you only need R5 exterior rigid for condensation control. If you already have r10 above, you can insulate underneath with batts, you don't need any spray foam. You can also loose fill the lower section and batts for the taller side in the rafter space.
The attic space needs to be sealed from outdoors, that can be done with rigid insulation between the rafters over the wall plates sealed in place with canned foam or with spray foam.
I'm in much colder climate where air handlers are universally inside the house, I'm amazed that attic air handlers in a vented attic are even allowed in such hot climate.
Thank you very much. I shoulda been more clear. The air handler is in a conditioned mechanical room but the ductwork is in the attic.