A.Frank Vigil responds: The answer is simple. Or, more accurately, the question is irrelevant since codes in most states with mild climates require a vapor barrier on the side which is warm in winter — the inside. However, it is important to remember that walls built in this climate must have the potential to dry.
Mild (or "mixed") climate zones are those areas requiring both heating and cooling for several months of the year. During hot, humid, summer months, the predominant vapor drive will be from the outside in during the sunny daytime. At night, the wall should dry to the outside, if given the opportunity.
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