[Porsche-O-Phile]

Another thing to consider is whether the R value is a total assembly (continuous) or just between the joists/framing. Big difference. A lot of codes only prescribe a net assembly performance which means you might "need" a big R number (like a 38 for example) to realize a lower R number (like 30, for sake of example) over the assembly - accounting for the fact there's no insulation where the framing is.

There are all kinds of ways to get high R values and efficiency. As has been stated, light-colored roofs are a good choice and there are a bazillion kinds of "cool roof" assemblies out there.