Planning a partially conditioned barn build and thinking about using a “flash” layer of closed cell spray foam on the underside of the metal roof to air seal and insulate and then ridged EPS insulation between the trusses below that to save cost.  The EPS would also be my “finished” ceiling (this is a barn).

My thinking is that the EPS would be set in place tightly, but with no attempt to airseal. As I’ve thought about it, my concern is that the gaps between the CCSF and EPS and gaps at the edges of the EPS will allow air movement between the layers. The EPS over the CCSF would lower the temperature at the center of the sheet and moisture in the air could condense between the two layers.

Should I treat the CCSF as exterior insulation and follow the ratio rule for hybrid roof insulation (41% for CZ5)? Or since there isn’t moisture susceptible wood sheathing between the two layers is this irrelevant?