Hello everyone.  My wife and I are building a steel, custom home with a snap-lock style roof.  The roof was designed to be screwed directly onto the purlins and insulated with a 2 layer, mineral wool, non-faced, R-38 insulation.  What we are trying to understand is- do we actually need any other materials to act as a weather barrier (XPS with taped seams) or are we ok here.  Our hvac system has been custom designed by Energy Vangaurd so we feel comfortable that the interior air will be conditioned appropriately.  Our main concern is being able to afford the entire build, and as everyone knows, there are many metal roofing assembly designs out there, and many of them are extremely expensive and multi-layered.  This will be a custom home, not a barndominium, which we plan to live in for the rest of our lives.  We cannot afford the ‘Cadillac’ design, and are looking for the ‘Toyota’ design; a design that will function effectively and not end up causing us leakage and/or mold issues.  We are trying to decide which one of the following methods would work best;

Option 1:  2-layer, unfaced, 12″ thick, R-38, mineral wool insulation installed within the purlins with snap-lock style roof screwed directly to purlins (No weather barrier)

Option 2:   2-layer, unfaced, 12″ thick, R-38 mineral wool insulation installed within the purlins with 1″ thick FOAMULAR XPS with taped seams with snap-lock style roof screwed directly to purlins with a mesh underlayment such as 5/16″” thick Delta-Trela or Solitex UM attached ON TOP of the XPS (for drainage plane and to keep the metal panels off of the XPS due to heat issues in the south.  XPS can only handle 150 degrees of continuous heat).

Option 3:  2-layer, unfaced, 12″ thick, R-38 mineral wool insulation installed within the purlins with 1″ thick FOAMULAR XPS with taped seams with snap-lock style roof screwed directly to purlins, with 1 x4 strapping to raise the metal roofing panels OFF of the XPS, and allow for drainage.

I would love some advice on this topic.  Thank you. Roger