Knee to vault bay to attic transitions – interior polyiso? – GreenBuildingAdvisor

Hi, I’ve been reading and learning a lot, and have a few clarifying questions as I embark on an insulation effort on our 1922 craftsman home, located in northern CO, at the border between zone 5 and 6. Winter indoor humidity is 30-35%, without humidification or dehumidification. Current attic spaces are virtually uninsulated (2-4″ of patchy loose fill fiberglass from the distant past, no air sealing to note).

The house is somewhat interesting in that it has a central half-floor, with a vented attic above, and shed roofs to the sides. But instead of the knee walls going from the interior of the house directly to the attic space, there is an intermediate partial-attic storage area that was framed in and “finished” with hardboard and 3/8″ drywall. There is a narrow door that accesses this area, then a hatch into the “real” attic. So two layers of knee walls. My plan is to make this space part of the conditioned interior of the house, but not the “real” attic area – two devil’s triangles to deal with.

This also leaves about a 5′ long section of vaulted roof bays that join the lower attic triangle to the upper vented attic, which is only partially accessible. However, a benefit is that this vault bay is above the intermediate attic space, not a normal room.

Attached is a sketch of the existing situation, as well as a proposed insulation fix for the kneewall/vault bay area. The upper attic will remain vented, and get air sealed and blown loose fiberglass in a typical manner. The lower attic will get the same treatment. It is the interface between the two and the vault bays I’m most concerned about.

I have three questions about it:

1. Because the vault bays are 2×6, and only partially accessible, how do I keep my fiberglass loose fill or batts up in there, and how do I maintain ventilation (or do I need to)? Does there need to be a clear continuous ventilation channel from the lower attic to the upper attic?

2. To get closer to the code minimums, I was considering putting polyiso on the interior side of the knee wall and vault bays, just securing it over the existing hardboard, and using a product like Thermax to avoid needing a thermal barrier. This is technically “storage” but very infrequently accessed. 4.5″ of fill in the bay + 2″ of polyiso seems to get me ~R27 on the full assembly. Am I missing something here? Is this a silly way to get a higher R-value in this attic storage space?

3. Anything else I should be keeping in mind while up here? Old house, tight spaces, trying to do this one time.

Thanks – Sam