We make it a point to have several sheets of Styrofoam of varying thickness on site, usually 1.5" and 2" works out,for our framers to insert in these future inaccessible spots. Until the roof is installed these walls sometimes get wet which is why Styrofoam works better than fiberglass.

Thanks for the great idea...We are just getting ready to frame so if you have anymore I would love to hear them.

Advanced framing with two-stud corners eliminates the problem. But you have to plan the build that way.

Here is a link that might be useful: Building Science Corp.: Advanced Framing