My opoinion is if your are going to bring the attic into the condition space then go with a lifetime roof and make a roof with mutlitple layers to prevent water problems.

I think if you want to bring the attic into the conditio space then more attention needs to be paid to the roof. I would think you would want to make the outside of the roof decking the air and water barrier. Then place more than 2 layers of ridig foam at your desired thickness. Then put a a water barrier on top and then a lifetime type of room. On the inside of the roof decking you could insulate with cellulose or fiberglass and then dry.