[Porsche-O-Phile]

38 is awfully high. Check the code requirement for your area. While more is better in terms of energy efficiency, you do have to consider the implications of paying for something you're not going to realize any (or much) benefit from. Also consider how long you're going to live there and whether or not you'll gain benefit on a life-cycle cost basis.

A residential job I did recently required R30 between rafters or R23 continuous. And this is a pretty cold climate. It also varied based on the compsition of your building envelope (more glass = higher roof insulation requirement, at least in my case).

That said, also keep in mind that codes are MINIMUMS. You can (and should) exceed them if it's to your benefit to do so.