Bedroom addition under existing porch roof insulation advice

A homeowner is building a bedroom addition under an existing porch roof and asked an online building forum for advice on insulating the ceiling. The roof has a shallow slope, roughly 1-2/12, with no attic or crawl space above it. Stucco is applied directly to 2x8 roof trusses, and the existing ceiling gets airflow from soffit vents that run up through a center beam and back down to soffits on the other side, with no ridge vent. The current insulation looks like a blown-in fibrous material, leaving only a few inches of open space for that airflow.

For the new walls, the homeowner plans to use "cut and cobble," a method of cutting rigid foam boards to fit snugly between framing, topped with fiberglass batts. They'd rather not do the same for the ceiling. One option is closed-cell spray foam, priced around $2,500. The other is to build a new vented attic space by framing a 2x4 ceiling below the existing roof, insulating that new ceiling, and adding a solar-powered fan to pull air in through the soffit and out through the attic space.

This is a homeowner's open question rather than settled advice, and only one reply had come in so far, simply asking where the house is located, since climate affects which approach makes sense. Anyone tackling a similar low-slope, no-attic addition over an old porch will likely face the same tradeoff between spray foam and building a small vented cavity.