A. Two coats of a latex vapor-retarding paint or an oil-based enamel will provide reasonable protection against moisture diffusion into the wall. But diffusion is often not the problem. The most important thing is to seal the wall as much as possible to prevent interior air from infiltrating the wall cavity. This will involve sealing the baseboards to the floor and wall and around electrical boxes, windows, doors, and other penetrations.
If you add 1/2-inch foam, the inside surface of the sheathing could get cold enough to reach the dew point. The foam sheathing you add should be at least 3/4 inch thick, and preferably, 1 inch thick. This would provide enough insulation so that the dew point in the wall section would be in the foam and not closer inside where it might cause...
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