Unvented low slope cathedral roof assembly – GreenBuildingAdvisor

Hello.

I am attempting to restore a 1952 MCM home in Saint Louis – Zone 4A.

When I bought the house in 2022 I had planned to simply move in and live happily ever after, but then I found out it had mold issues.  I am trying to take the right steps to make the house healthy again, but it is much harder than I expected.

It is 1170 sq ft on a slab foundation.  The roof is a 2:12 pitch cathedral ceiling with no attic.  The siding is the original vertical redwood T&G over something labeled “water resistant gypsum”.  There is a full wall of clerestory windows in the front.

At this point the house has been fully gutted and remediated, and the new framing, plumbing and electrical roughs are complete.  I replaced the ducted HVAC with a whole-house mini split system.   I had a new mod bit roof put on as well.

At some point during this process I learned that my municipality does not allow any type of spray foam insulation.  I wasn’t too worried about it because I have been following the advice of multiple healthy home builders who do not encourage spray foam either.  One of them (who actually builds in Chicago) advised me to insulate the ceiling as a “hot roof” assembly – see attached plan.

The plan calls for 7.25″ Rockwool batts “dense packed” into the rafter cavities followed by an Intello Plus smart membrane system.   The walls will be 3.5″ Rockwool followed by a CertainTeed SmartMembrain.  The plan has been approved by both my local fire chief as well as the county building plan reviewer.

I had just purchased the Rockwool batts and was doing a little research on installation techniques when a GBA article about unvented roof assemblies caught my attention.

If I understood everything correctly, it sounds to me like I should have also planned to either (1) add rigid foam under my mod bit roof above the decking, or (2) add rigid foam under my decking before the mineral wool, or (3) add spray foam under the decking before the mineral wool.

What I don’t understand is – if the IBC requires a layer of air-impermeable foam against my roof deck – why would my municipality approve my plan as is?  I have to assume that they understand the moisture/mold/rot risks of getting the assembly wrong, right?

I would be very grateful for feedback on whether my “hot roof” plan has any merit at all without the foam.

Thank you in advance,

Debbie