I am in the middle of a full tear-down remodel of a small house in San Francisco and I have a question about roof venting. The house has a flat roof made of 2 x 6s. The entire roof is 20' by 40'. Currently, the roof has vents all around. Previously there was no insulation. I'd like to put in 6" fiberglass insulation but am concerned about the air not being able to move if the insulation fills in the entire space. Is this a valid concern or should I be fine given the climate? I can put in 4" insulation and leave 2" of free space, but I would prefer the additional insulation power of 6".

Thank you.