Screws for 2×4 sleepers through exterior roof foam – GreenBuildingAdvisor

I am planning a chainsaw retrofit and adding 5-6 inches of polyiso foam to the roof with 2×4 sleepers for vent channel.  The 2×4’s lock the foam in place but I am curious if I should be using fully threaded screws like Simpson SWD or truss screws to secure the 2×4 so they can’t fall down the smooth screw shank of a standard HeadLok or equivalent if the foam shrinks.

At second thought, the top layer of sheathing secured to the sleepers would probably mitigate the issue I’m worried about.

If anyone who has done a roof like this can comment on their experience with fastening this assembly that would be great. Thanks all!