Vent Porch Ceilings Below Open Decks?

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I'm re-building an unenclosed 3 level porch-deck. The level 2 and 3 decks are waterproofed with an EPDM membrane over plywood over 2x8 floor joists. I will install beadboard panel ceilings on the undersides of these decks.

I plan to prime the back side of the beadboard ceilings - and vent the ceilings to allow any moisture condensation to escape. Are back-priming and venting necessary? (The owner thinks I'm going overboard here since there is no heated space, but I assume the interior joist spaces will heat up and cool down at a much different rate than the outside air, and will therefore produce a decent amount of moisture from condensation).

I'd appreciate any suggestions.

Thanks,
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