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The placement of the vapor barrier is critical in preventing moisture issues within walls. In a wall assembly with brick on the outside, insulation in between, and sheetrock on the inside, the vapor barrier should be located behind the sheetrock, which is the interior side of the wall.

This configuration serves to minimize the likelihood of moisture from the interior air passing through the insulation and condensing within the wall cavity. By placing the vapor barrier on the warm side of the insulation (which is the indoor environment), it effectively blocks moisture from entering the wall system during colder temperatures when the indoor air is heated.

If the vapor barrier were placed behind the insulation or in the middle, it could allow moisture to accumulate in the wall cavity, leading to potential mold growth or structural damage. Similarly, placing it outside the brick would expose it to moisture from the outside, which can also lead to other complications for the wall structure. Thus, positioning the vapor barrier behind the sheetrock ensures that it provides the best protection against interior moisture infiltration.