Water Barrier
Whether you are applying or installing a water resistant barrier behind or over your shower walls, it will be the determining factor for securing your wood frame from moisture damage. As part of your waterproofing strategy for your installed shower walls, you have two choices, first is to install a water resistant barrier behind your tile substrate, and second is to apply a liquid membrane or install a sheet membrane over the tile substrate. Your local building codes will dictate what is required or advised.
Install Behind Substrate
The traditional method has been to apply a material, such as a 15-lb or #15 felt paper, some building codes require a minimum Grade B building paper to qualify as a water resistant barrier, or a plastic sheeting with a minimum 4-mil thickness, behind the tile substrate to act as your water resistant barrier. Using the traditional method of waterproofing your installed shower walls would not be duplicated to the exterior of the tile substrate.
Install Over Substrate
With the advent of modern waterproofing materials, the water resistant barrier can not only be applied or installed over the tile substrate, it further acts a solid bond material when applying adhesive and setting your tile. The extra added advantage of applying a liquid membrane or installing a sheet membrane is that, not only are both considered a water resistant barrier, but they also server the purpose as a vapor barrier. The water resistant barrier over the tile substrate would not be duplicated behind it.
Moisture Sandwich
A moisture barrier should keep moisture from entering behind your installed shower walls, more importantly, it should encourage it downward into the shower pan. The worst case scenario for your waterproofing strategy is to be redundant behind and over your tile substrate by applying or installing a water resistant barrier. This creates, what the industry calls, a “moisture sandwich”, where moisture makes its way behind the tile substrate but does not have adequate breathing room and temperature conditions foster mold growth. The rule of thumb is to only choose one place to apply or install your water resistant barrier.
Seams, Joints, and Fasteners
Wether you choose to apply or install the water resistant membrane behind or over the tile substrate, your seams joints are a vulnerable aspect of your installed shower walls that require careful attention. If you choose to install the water resistant barrier behind the tile substrate, you should use a combination of a fiber mesh tape and a liquid membrane to cover the seams, joints, and fasteners to prevent moisture from entering behind the walls. The liquid membrane would not be applied to the entire tile substrate, therefore it should allow for breathability.
If you choose to apply a water resistant barrier in the form of a liquid membrane over the tile substrate, you want to reinforce it using a fiber mesh tape at each seam and joint. If you choose a sheet membrane as your water resistant barrier, you can mud pack the seams and joints with a thin-set mortar, apply the cut to size fiber mesh band over them, and secure with a thin-set mortar over it.
Water Barrier Needed
You might hear of stories where someone remodeled their bathroom and after removing the shower walls, they were astonished to discover that a water resistant barrier was not present, not even a piece of plastic. In those cases, besides luck, atmospheric conditions outside and inside the shower area never differed much in a season or every in a day that would introduce or effect the presence of moisture. You most certainly need a water resistant barrier, not just because local building codes will require it, but more importantly your waterproofing strategy is part of the moisture management that you are subscribing to. Your waterproofing strategy for your installed shower walls should not be an exercise in how much risk you can tolerate and luck you can accumulate.