Insulation board under metal roof - Full log, no attic cabin

Help, please!

We are replacing an exposed fastener metal panel roof with a new exposed panel fastener or standing seam. The roof is currently attached directly to 1" by 6" T&G which is also visible from inside. We see areas with fasteners punching through. There is no roof insulation nor roof ventilation other than what is provided by the metal panel ridges.

We want to install an insulation board, 2", 3" or 4" and then install the metal roof. We are leaning toward an exposed fastener panel, given cost and the need to install boards atop the insulation with standing seam. Will there be condensation issues, or other problems we can anticipate or mitigate with the installation noted above? I have read several other posts, which noted issues though none about direct install, no ventilation.

Roof is about 900sf, 9/12 pitch, located in Utah mountains, not accessible in winter, snow build on roof given positioning is very unusual, no AC, heating is wood burn stove, cabin use is <2 months a year in fall, summer and spring.

Michael,

Here is what I would do if it were my roof.

I would remove the existing roof covering and cover the entire deck in peel and stick membrane (i.e. ice and water shield). This will air seal and give you a nice barrier against ice dams.

I would then build a modified SIP (Structurally Insulated Panel) system out of 2x4s and 1.5" foam. I would install 1.5" foam down, clamp the foam in place with the 2x4s running horizontally and at the eaves/rakes, and then 1.5" of foam in between the 2x4s that are 16" on center.

This gives you a nice attachment point. Keep in mind, you will need custom rake and eave trim to make this work.

Lastly, while an exposed fastener will work, I would be worried about the application of that under a bunch of heavy snow. The fasteners are going to be leak points in the assembly. I would see about a clip or mechanically locked standing seam.