Minimum insulation for walls is now R-19, and a low-slope roof wants an R-44. The walls aren't that much of an issue since I designed the walls with 2x6 studs instead of 2x4, which leaves a cavity of 5-1/2 inches. Without going to more exotic (and expensive) insulation materials, I can use EPS (pink foam board) inside the wall cavity and achieve well over an R-25 (R5 per inch thickness), but the roof...
With an R-5 per inch, I need almost 9 inches of foam thickness. My cavity is currently 7-1/2" deep, and I should leave a minimum of 1/2" of airspace between the insulation and the roof deck. So I need to add two inches of depth below the roof joists to fit in that insulation.
Most likely way that will happen is to run some 2x3 stringers across the joists in the ceiling of the porch. That will give me an additional 2-1/2 inches of depth and still leave me something mechanical to screw the wallboard into.
Oh, yeah, wallboard. I have to use gypsum wallboard on the interior walls and ceiling. minimum 1/2" thickness on the walls, and minimum 5/8" on the ceiling. Fire rated. I can put stuff over that if I want to, but the wallboard has to be there.
I don't mind the wallboard so much; I was planning to do dual-layer wallboard anyway for sound insulation. I'm just surprised that it's required. And I found some new paper-free wallboard (mold grows on paper, not gypsum & plaster) available at 'Nards.
But the oddest part of this is that the porch isn't heated.
I was considering heating the porch. Adding an in-floor radiant heat to the existing slab wouldn't be extremely hard, just tedious and uncomfortable. It would mean crawling into the crawlspace with a boatload of PEX tubing and epoxy and metallized insulation, and it ain't gonna happen soon. But I did design with this in mind-- there are even stub-outs on the heating system that I added just for this purpose. It would have its own zone with a thermal balancing valve to maintain the slab at a constant temperature to avoid cracking. And I wouldn't have to heat it much: if I were to keep the porch at 50 degrees or so, it would still be a pretty marvelous place, even in the dead of winter. I might actually have more of a problem cooling it in the summer, so I'm considering what it would take to build in an air conditioning insert while I'm still framing, and whether it's a good idea.
I also have to figure out the wiring. Not just the electrical mains, but low-voltage stuff as well. Doorbell, sensors, security, LV lighting, that kind of stuff. Most likely I'll run flex conduit where I need to and pull the wires later, but I do know some stuff that I want already and it would be nice to have that in place.
After all, I am a very technical boy.