As I torture myself over whether I need more insulation to maximize efficiency, I wonder if the cost of the insulation will be made up through reduced tonnage needed for the hvac system, or am I spending money I could put into having nice hardwood floors? And then I suddenly remember my intention of adding 36 PV panels and solar water-heater, and I ask myself (and you guys) isn't it okay to just have my open cell foam walls and roof, given that I'll be generating the electric I'll be using? Can I be guilt free if I don't add exterior foam board? Will I forever regret that my cathedral ceiling only has an R value of 22?

I would investigate open cell foam closely. From the reading I have been doing on this forum there are reports that open foam allows moisture to pass through the foam and into the roof sub-structure be it plywood or OSB. If I can find the article it expresses it pretty well. My guess is that open cell depends on where you live. Lets say Arizona is unlikely to produce this problem but Florida most likely yes.
Here is the title of the report and short extract. I would suggest reading it then decide or gather more data.
I was not aware of this issue and was thinking open cell foam too because it is cheaper than closed cell...
This extract should help you get to the report which was referenced by a member online recently...

Application of Spray Foam
Insulation Under Plywood
and OSB Roof Sheathing
Building America Report - 1312
September 2013
Aaron Grin, Jonathan Smegal and Joseph Lstiburek
Abstract:
building science.com
© 2013 Building Science Press All rights of reproduction in any form reserved.
Unvented roof strategies with open cell and closed cell spray polyurethane foam insulation sprayed to the
underside of roof sheathing have been used since the mid-1990's to provide durable and efficient
building enclosures. There have been isolated moisture related incidents that raise potential concerns
about the overall hygrothermal performance of these systems. This project involved hygrothermal modeling
of a range of rainwater leakage and field evaluations of in-service residential roofs using spray foam
insulation. Explorations of eleven in-service roof systems were completed. The exploration involved
taking a sample of spray foam from the underside of the roof sheathing, exposing the sheathing, then
taking a moisture content reading. All locations had moisture contents well within the safe range for
wood-based sheathing. One full-roof failure was reviewed, as an industry partner was involved with
replacing structurally failed roof sheathing. In this case the manufacturer's investigation report
concluded that the spray foam was installed on wet OSB based on the observation that the spray foam
did not adhere well to the substrate and the pore structure of the closed-cell spray foam at the
ccSPF/OSB interface was indicative of a wet substrate.

Start with the basics. Location drives all climate specific decisions. IF you live in San Diego, exterior foam is probably not needed- if you live in Frenchville, ME, you are going to want it. If you have natural gas heating vs #2 fuel oil the line between these two changes. There are alot of drivers of these decisions. Also, think hard about what is easy to do later and what is hard to redo. Hint: retrofitting exterior insulation and better windows is hard to do. Give us a bit more info and we can help you make better decisions. Cheers, Eric

R22 cathedral ceiling might be OK for a SCIP roof in Hawaii, but you'd regret it forever in Minnesota, eh?

Foam insulation isn't exactly the greenest stuff around, though it certainly has it's place. Open cell foam is a lot greener than closed cell, where you can get away with it. But it's not nearly as green as cellulose, or even rock wool , R for R.

I'm south of Atlanta, Georgia. I've decided to add foam board to the roof above the cathedral ceiling, and above the scip side of the house. We aren't going to do it where the roof is flat, as we are afraid it will reduce it's weight-bearing capacity (I'm planning for a rooftop patio). Still trying to determine if I can afford solar AND mini splits. If I can't, I'm leaning toward solar panels and a high efficiency regular heat pump for hvac.

Mini-splits shouldn't cost any more than a heat pump. Our Mitsubishi Hyper-Heat units cost less than a central system.

Posted By jdebree on 30 Sep 2014 07:08 AM
Mini-splits shouldn't cost any more than a heat pump. Our Mitsubishi Hyper-Heat units cost less than a central system.

Yabbut, you probably haven't been told you need 7 ductless heads , eh?   (IIRC that's what had been quoted here.)