When I did the cab insulation and and sound proofing project, the only metal I left exposed/uncovered were the reinforcement "struts" or ridges that run under the roof and sides sheet metal to form the rigidity for the cab - which are covered by the headliner. And then also the exposed metal trim around the doors, because those are actually visible when the door is open or closed.
In retrospect, I would have covered those struts with some of the sound matting because that would have prevented a lot of the condensation buildup. I didn't cover them because acoustically-speaking covering those struts doesn't do much but from an insulation/moisture barrier perspective it clearly does.
I'm very familiar with the soundproofing mats/rolls having used it for years and done many vehicles with it but - If we are really using hindsight 20/20 - since I had the cab completely torn down anyway, I wish I had used one of the new spray-on insulation and sound-proofing products like lizardskin or something like that.
I think that would have solved the problems I was wanting to solve - insulate and soundproof the cab significantly - while also solving the problem I didn't realize was as bad as it is - condensation in the cab.
If I ever find myself needing to take the headliner and side panels out of my cab again (god I hope not) I will probably add some matting and potentially fill in the struts with expanding foam.
Anyone that is considering tearing down their cab area to better soundproof and insulation - I would DEFINITELY suggest they use a spray-on product which will serve all purposes better.