Shower vapor barriers provide you an extra layer of protection against both internal and external water damage, leaks, condensation, mold, and mildew. From the outside, the barrier keeps condensation and water leaks outside the shower where they belong. Conversely, the vapor barrier keeps the sub wall, which is the wall behind the fancy tiling, from being damaged by the added effects of water, soap, and other caustic chemicals. Quite frankly, the shower vapor barrier will probably save you thousands of dollars over its lifetime. When properly installed, the shower vapor barrier can be as important as a well-placed shower curtain or bath towel.