Chris Friesen wrote:

The roof rafters are 2" x 8". For that size, I should use R20
(6") in order to keep 2" for ventilation on the top of insulation.

Why not put up a good plastic film vapor barrier under 8" of insulation?

You're supposed to leave space for airflow between the insulation and
the underside of the roof.

Maybe not, if there's no chance of condensation from moist house air
leaking up under the roof.

Nick