Standing Seam over Rigid Foam – Asked a thousand times but still need clarification – GreenBuildingAdvisor

Good afternoon everyone. I know that this question has been asked a thousand times and answered more than that but I haven’t found an exact answer that seems to fit our use case. I’m building a farm office/workshop for myself but this is not a regularly occupied structure nor does it have any plumbing. It’s a 2-story structure with the “office” above and a small farm workshop below (with an overhead coiling door). I’m experimenting with various details gained over years of practice and thought I had this all figured out. As I was talking to a friend who builds a lot of smaller but well-performing houses, I mentioned my detail and he said “Oh, you’re done it backwards”. Here’s what I’ve done and why he said that.

The roof structure is 2×10 rafters with 3/4″ plywood sheathing. On top of that is a layer of self-adhered Henry Blueskin PE200HT. My plan has been to install 2″ of rigid XPS foam over that with vertical and horizontal 1x furring (see attached roof detail) to allow for snap-lock standing seam clips to be installed at 16″ O.C. both horizontally (16″ 24 ga. panels) and vertically running up a 4:12 pitch roof. My intention was to install Comfortbatt (or similar mineral wool) R-23 in the roof cavity inside to effect a net R-34 for the roof. I was planning on this being an unvented roof assembly with an ERV and mini-split for environmental control. We’re in Nashville, TN.

He suggested that with all of the trees in our area he was worried that the metal roofing could easily be penetrated by a falling branch or similar. He offered that I should have installed the insulation directly against the sheathing and then used a second layer of sheathing on top of the XPS and then apply the underlayment on that layer of sheathing and then the clips could affix directly to (or through) the upper layer of sheathing. In all honesty I’m not sure how many exterior insulation projects he’s done as he is a very strong proponent for I.C.F.

Here’s my dilemma. The structure is built. The (rather expensive) vapor-permeable PE200HT is already installed so it’s not coming back up. I own the XPS so I’d like to use it and not have wasted the $$ on that.

Any thoughts on the best way to proceed here or any I stuck installing an upper layer of (thinner) sheathing as a cover board of sorts with a roll of underlayment on that? My two primary concerns are: 

1. Will the original detail work as designed with the understanding that I’m relying on the thickness of 24gauge steel to prevent damage?

2. Will the XPS defeat all of the work I’ve done to try and let vapor escape (and let the sheathing dry)? I thought that with the correct interior finish applied (zero V.O.C. latex) and the R-23 Comfortbatts, the sheathing can still dry to the interior.

3. If the second layer of sheathing is suggested on top of the furring (or is the furring  needed at that point?) would the underlayment on that layer need to be vapor open?

I’m aware that this amount of roof R-value is not enough for code here but this is technically an agricultural structure and is really more of a test-bed for details than it is a structure for the ages. If convicted hard enough, I could be convinced to install a second layer of 2″ over the XPS to result in a net R-45.

Thanks for any comments or help you can offer.

-Paul Marshall