hmmm Generally, in warm climates air born moisture condenses/comes from the /inside/ of the house (I believe especially air conditioning, showers, cooking, etc.)
As I understand it, in general, presuming rain water isn't getting into it, an insulated wall, with zero vap barrier, only rots/molds from moisture vapor penetration if there is not enough air flow in the wall cavity to dry it out. So as long as you have enough airflow to move the moisture out of the wall cavity before it can settle on the wood, there should be no problems...
If I'm not mistaken, which is entirely possible here cause I'm not getting any brain pictures on the subject - just a vague memory, wood siding and poured/sprayed poly foam have rather similar vapor permeability characteristics??
I'd research "Perfect Wall," "Passive House," and "house vapor protection layer" to get the scoop on insulation techniques. (Unfortunately, I have too much projects on the docket to do it myself atm.)