THE NATURE OF WATER VAPOR

Consider two closed rooms adjacent to one another. If the partial pressure of the water vapor in room #1 is greater than the partial pressure of the water vapor in room #2, then the water vapor will travel through the wall into room #2 regardless of the composition of the wall.
Let's take this hypothetical example a step further. If the absolute humidity of the air in room #1 is greater than that of the air in room #2, then the water vapor pressure will be higher in room #1. Therefore, when drying room #2, the problem of water vapor coming through the wall from room #1 must be considered.
A vapor barrier can slow down the passage of vapor from wet to drier areas, but it cannot keep water vapor out; it can only slow the rate of penetration.
The choice of a vapor barrier is based on the degree of dryness required in the controlled space; the efficiency of the equipment being used for drying; and the cost of construction.
Commercial vapor barriers, moisture resistant construction material,p paints an other coatings, offer a variety of design alternatives. Manufacturers of vapor barrier materials can supply specific information on their products.