In my ~6800degree-day central MA climate, foam-insulation under between the rafters behind the kneewalls proved to be the SOLUTION TO, not the cause of ice dams!

Ice dams are created by heat transfer through the roof decking to the snow above- the lower the heat-transfer (by increased R-value), the lower the ice damming potential.

Adding vent space between the insulation & the roof deck allows some cooling of the roof deck, but it would be the rare situation where that would be more effective than increasing the R-value from below. It's somewhat common in my area to cure ice dam issues by applying an inch or two (R5-10) of XPS above the roof decking when re-roofling/re-shingling.

There's very little down side to making the roof deck the pressure & thermal boundary of the building envelope. There are fewere penetrations to seal than the typical attic floor situation, and by sealing up attic vents parasitic thermal bypass air currents disappear.

Half-pound foam will allow leakwater to drip through, making detection & location easy, 2lb foam will not. Being waterproof, 2lb foam diverts the water to the lowest pressure gap, and being a class-I vapor retarder, limits the drying capacity of the structural wood. I use both in my place though, but limit the 2lb foam to the cathetral ceiling of the attic spaces with mere 6" rafters, to max out the R-value. Areas behind kneewalls get half-pound foam, cellulose insulation on the floor, and fiber-faced 3" iso board on the backs of the kneewalls themselves to achieve a higher net R-value. (When I re-roof I''ll be applying 3" fiber-faced iso above the roof deck as well.) Then if I get a roof leak in the cathedral-ceiling parts it'll most-likely show up as a stain line at the top of the kneewall below the leak.

3" fiber faced iso above the roof deck needs mastic/caulk sealed joints, and a nailer-layer of OSB above (held in place with long screws to the older roof deck, with the OSB seams offset from the iso-board seams) to have something to nail the shingles to. Just like foaming under the roof deck, the iso then insulates the nails, impeding thermal transfer from the interior to the snow layer via the nails. In hot climates in particular (but not exclusively) iso is somewhat preferable to XPS/EPS in roof-deck applications, since it can handle much higher temperatures without degradation.