Robert wrote me and asked:
I have some 26 guage green metal. I want to use it to re-roof my home. I want to install 2×4′s every 2 ft on top of the 3 tab comp singles, and install the metal to the 2×4. Do I need some kind of rigid board insulation or radiant barrier? Also, do I need to put down tar paper on top of the comp roof?
I would put down underlayment, yes.
Radiant barrier is up to you. A radiant barrier facing the airspace could help with energy efficiency.
You may want to consider vertical battens and then horizontal ones, and then ventilate the resulting chamber.
Assuming you have a vented attic, if you want to add insulation, do it on top of your ceilings not up on the roof at this point.
Make sure that the panels you are installing are okay to be installed over battens.