One of the more common problems affecting 2nd level additions is the increased solar heat gain coming from the rooftop in summer. When the roof heats up, it radiates down into the space through whatever insulation envelope has been put in place.
To deal with that, you can create a space between the hot roof deck and the insulation, as well as vents at the top and bottom to allow the hot air to naturally flow out.
An additional step that I'm taking is to create that air space using an insulation board coated on one side with a radiant heat barrier. That should help to inhibit the transfer of heat in the first place.
The next level of insulation is going to be a closed cell spray foam, a material with great R value and ability to seal against air and vapor transfer; so this extra step might not be necessary. However, I'd hate to find out that it is once the first summer rolls around.