1.5in zip-r applied to topside of exposed roof system – GreenBuildingAdvisor

Scratching my head a little.

Not my project.  A friend has purchased for his home: a 1960’s chalet with an exposed roof system.  As the rafters are exposed, there is no cavity insulation and no outbound insulation.  2×8 t+g applied to the topside of the rafters as a deck and finished ceiling.

His intent is to apply 1.5in zip r to the topside of the t+g as a insulation and nailbase for metal roof.  Of course, it’s too little insulation to add value.  My concern is condensation on the inside face of zip-r: this is climate zone 6.  But, given zero cavity insulation, the ratios of rigid to otherwise work.  As the assembly is open, there is good inward drying potential.

Zip-r was ordered and is onsite.  No back pedaling.  I have encouraged 8 inches of carefully detailed exterior rigid, including the zip-r, but he may not have the bandwidth.

This is somewhat of an outside-of-the-box question for me as I usually consider high(er) performing assemblies, not low!