My ceiling is closed near 100% - all plaster ceiling holes sealed. We use a central air ventilation system from airegard for fresh air. But that is not a must but it helps if your place is permanently a/c-ed. The heat transfer you talk about will be neglible for most circumstances. But still a good idea to keep unwanted houseguests out

You should absolutely vent the space under your roof tiles or use a tiling system that breaths so disperse the hot air too. Monier has a solution for that.