Careful with radiant barrier. Done wrong, you can melt off your shingles during the summer. Not saying don't do it, but find out how much it will raise the roof temps and see if the max temps will remain within specs of your current roofing.
Even without a radiant barrier, I had some shingles soften enough to start sliding off my roof this summer. Record high temps here combined with the original builder not putting in enough roof vents, and a few shingles that weren't put on exactly right started shifting and sliding down when they softened. I had to put $500 into replacing a bunch of shingles and I had the roofer put in 3 more vents while he was up there. It's still not quite enough for the other parts of the roof to be properly ventilated but it's better than it was.
