Goedjn wrote:

Here in the southern US, not leaving a place for ventilation will cost
you a roof job about 10 years before it should.

Why would you say that? Seems unlikely to me, if there's no chance of
condensation from moist house air leaking up under the roof.

You don't stop water-vapor on the far side of the insulation,
you stop it on the near side.

You stop it on the warm side. Plastic film under insulation would do that.

Nick