Enclosing a deck.

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First of all, Chicago isn't a terrible place to do what you want to do. More humid climates can cause a bigger problem.

Like I said, apply a vapor barrier topside and then plywood, 1/2" if you can't find something smaller.

We are missing insulation of the floor, though. That floor is going to get mighty cold. If you insulate the floor, you'd get a couple extra months use out of it.

I don't think the cedar will have any shorter lifetime for the application, it won't get rained on, and it won't get uv.

By leaving the skirting as is, the deck will be damn cold, but well vented. This goes back to the question about whether to insulate.

The best protection against having a moisture problem is ventilation. If you decide to enclose it more so you can insulate the floor and keep it warmer longer, then you'll be best served by applying a vapor barrier on the ground below the deck.

Also, if you can physically accomplish it, you can apply the barrier on the underside of the deckjoists. The main thing is you don't want to trap moisture, that's bad. If the deck gets wet anywhere, the plastic is a sure disaster.

You could put pergo down over 1/2" floor, or the 1/4" floor (if you can find it). For your application, you could probably get by with 1/2" regular plywood, but the green will help the floor last a lot longer.

Stopping air movement is bad. Moisture rises up from the ground, trapping it is bad. Unfortunately, there are climates where the RH is 80% all the time. In those cases, you'd have to box in the deck, and run a dehumidifier underneath it.

Also, in your case, the plastic will help to keep earthy smells out of the room.