How could I insulate a thin hot roof and bring it to current Southern CA code?
I have resolved cases like this (in Minnesota, no less) in two ways:

Keep the &quot;hot roof&quot; but add polyiso insulation, as you described. In our case we put down two layers of ~3&quot; panels (which gets you more than R-25), then OSB. Obviously it took some long fasteners to do that. I'm not sure why it was suggested that you put down OSB first. Of course the shingles need to be removed, leaving you with a suitable deck, I'd think. One challenge is working out fascia structure. There's not much to fasten to with the foam, so you need to embed backing lumber or fasten to the new sheathing and original decking.

Overlay scissor trusses with modern energy heels. This gives you space for blown insulation or batts, which you'd probably install from above before you put sheathing on the roof. Scissor trusses leave a very short space that's difficult to work in otherwise. This approach also provides a vented attic, which is good for efficiency. The drawback might be that you have to deal with siding on the gables.


Both are substantial and somewhat expensive projects, but if you want a modern, energy-efficient solution they're worth it. Chances are your home's value will increase comparably to cost.