I’ve been searching the forums for a few weeks looking for advice on how best to move from an insulated floor in my attic to an insulated ceiling, but noticed that some of the most helpful advice I found and was about to follow was from posts in 2012… so I thought I would ask my specific questions to see if advice has changed in the past decade.

I have a 100 year old stone home in Kansas City (zone 6).  Unlike most homes in KC, my house is built on a slab with my HVAC air handler and ventilation in the attic.  For reasons I won’t bore everyone with, I recently removed all of the insulation (which was blown in on the floor of attic) in an 800 sqft section of the attic where my HVAC mechanicals reside.  I would like to turn this space into a conditioned area with an insulated roofline in this area so that my HVAC will run more efficiently (and hopefully I can install a much-needed humidifier that will be inside the insulated envelope) as well as create space for storage.  This will never be a living space, so I’m not concerned about the finish.

My question is, what is the best-bang-for-the-buck DIY solution for insulating my attic ceiling?

A couple of details:
– Currently, this section of the attic is ventilated with a turbine near the roof ridge.
– 2 x 6 rafters (5.5″ actual)

From the advice here:
https://www.greenbuildingadvisor.com/question/rigid-foam-attached-to-underside-of-rafters-good-idea  I was leaning towards not furring down my rafters but simply installing high-density R-15 batts in the rafter cavity and then gluing 2″ polyiso foam boards across the rafters, taping the seams, and 1″ fur strips to screwed to rafters to hold them in place.

As for having a turbine vs a ridge vent, I thought I could run some 2x4s a foot or so down from the ridge of the roofline and just insulate between the rafters to that point (and along the new 2X4s) so that air could flow through that cavity to the single turbine.

I also need to figure out how to seal off the conditioned envelop but also allow access to the non-conditioned sections of the attic.  My thought was I would just get some thick iso boards (like 6″) that would be rigid enough to stand-up and tape off but allow me to remove it for the occasions where I might need to get to something in the non-conditioned spaces.

With the best-bang for the buck in mind, a couple questions:
1. Should I look at rockwool instead of high density fiberglass batts?
2. Would it be worth it to fur down my rafters for any reason?
3. Does it matter what surface material should be on interior face of polyiso boards?
4. Any advice on my ideas to seal off the conditioned envelop but also allow access to the non-conditioned sections of the attic?

Thanks!