Is a thick, unvented hot roof a good candidate for spray foam?
You need to make the foam your vapor barrier, which means using closed-cell foam. Secondly, you need sufficient foam in order to prevent condensation inside the wall. Thirdly, you do not need to fill in the space, but you need to provide enough insulation to bring the total to the required minimum for insulating ceilings. Lastly, foam is nasty when it burns, so you'll need to put an ignition barrier (such as cellulose, hardboard, or drywall) directly in contact with the foam.

The required minimum amount of foam for the various climate zones of the United States/Canada are:


Zones 1-3 — R-5
Zone 4C — R-10
Zones 4A, 4B — R-15
Zone 5 — R-20
Zone 6 — R-25
Zone 7 — R-30
Zone 8 — R-35


Since you are in zone 4 or 5, you'll need a minimum of R-20, which is about five inches of foam.

I would fill the ceiling to slightly more than this minimum with foam, install drywall, and then fill the gap with blown cellulose insulation. You would need to remove all of the drywall.

You could also add ventilation to this knee-wall/cathedral ceiling by using mid-roof vents such as The Edge Vent or The Smart Vent installing a baffle to prevent insulation from blocking the air flow, and filling the walls with blown insulation or faced batts (with the paper against the drywall). This would likely be much cheaper than foam insulation, but not insulate as wall.

Insects (such as ants and termites) can tunnel through foam insulation.