Attic conversion in Philly: Is R15 fiberglass plus 2 layers of foam board a good plan?

You need to double-check that those are actualy rafters and not just ceiling joists before you start hauling drywall up there. In those old Philly houses, a 2x6 at that span was usally just meant to hold up a plaster ceiling, not the dynamic load of a home office with furniture and people walking around. If the floor feels bouncy now, it’ll be a nightmare once you finish it. I'd have an engineer take a quick look to see if you need to sister some larger lumber to those joists first.

Your insulation stack is actually pretty solid for hitting R38 in a tight space. Two layers of 2-inch polyiso gives you about R-26, and the R15 batts get you to R-41. But skip the **faced** fiberglass. Since polyiso acts as a vapor retarder, putting kraft-faced batts behind it can create a moisture sandwich where condensation gets trapped against your roof deck. Go with unfaced batts and focus all your energy on taping the seams of that first layer of foam board with high-quality foil tape to create your primary air seal. One thing you’ll run into is how to hang the drywall. Screwing thru 4 inches of foam directly into the rafters is tough and often leads to sagging or popped screws over time. The best move is to run 1x3 furring strips over the foam, screwed into the rafters with long structural screws. This gives you a solid base for the drywall and creates a small service cavity for any wiring you need for those mini-split outlets or lighting. Don't forget to **air seal** the top plates and any wire penetrations coming up from the floor below before you lay down your flooring. A mini-split will handle the temperture fine, but if you're leaking conditioned air into the soffits or through the floorboards, you're just throwing money away. Also, make sure your baffles go all the way from the soffit to the ridge vent without any gaps, or you'll end up with ice dams during those freezing January stretches.

That furring strip idea makes sense for the drywall, but does adding all that foam and wood put too much weight on the roof? I'm already worried about my floor joists like you mentioned. Also, when you say the baffles need to go from soffit to ridge, do I need to worry about the foam blocking the airflow if I don't leave enugh of a gap? I'm trying to figure out if those cheap plastic baffles are enough or if I need somethig beefier to keep the polyiso from squishing them against the deck.

Keep an eye on that polyiso R-value. In Philly, once it drops below freezing, polyiso acutally loses some of its punch compared to XPS or EPS. It's still a great choice for space-saving, but don't be shocked if it underperforms a bit during a January cold snap. To compensate, make sure you're using canned foam around the perimeter of every foam board. Foil tape is good for the seams, but those old rafters are never perfectly straight, and tape won't bridge the gaps against the wood. A bead of foam in those gaps will do more for your comfort than the extra R-value ever will.