Indeed, when the sun is on the roof, it's like a radiator inside. I purchased R3 poly bats, which I had to split in half so they're around R1.5. Backed and held in place with masonite board. It now looks like this inside:
It's a pretty rough job but the effect is a huge improvement in comfort when the sun is beating down. Foolishly I went along with the offered skylight and of course this isn't desirable in full sun so I've placed the 65W solar panel over it and it covers it pretty well.