Vapor Barrier
A vapor barrier is a material that will be installed onto the wood studs of your shower area and retard vapor that manages to enter through cracks in joints and also will encourage moisture produced from condensation to travel downward into the shower pan.
Materials
You have several options to choose from when choosing a vapor barrier and each affects your waterproofing strategy. First, you have vapor barriers that are installed behind your tile substrate, common materials that are used due to their availability and cost are, a continuous plastic sheeting with a minimum thickness of 4-mils, 15-lb or #15 felt paper, some building codes require a minimum Grade B building paper to qualify as a vapor resistant barrier. The alternative to a vapor barrier that is installed behind the tile substrate is a liquid or sheet membrane that is applied over or installed on to the tile substrate. Depending upon your choice of tile substrate, such as a backer board material, some manufacturers will not stress the requirement of a vapor barrier, such as Hardiebacker or Durock, in these cases it is left up to the local building codes and industry standards to determine the best choice. While other manufacturers advise against it, such as DensShield, which has a built-in moisture barrier.
Install Behind Substrate
For vapor barriers that are installed behind the tile substrate, the installation is quick and easy but there are some considerations to keep in mind. First, whether you chose a plastic sheeting or 15-lb felt paper, when installing onto the wood studs, it is suggested that you use a minimum amount of staples to minimize any entry points for moisture. Ideally, the plastic sheeting should be one continuous piece, but where two pieces need to be seemed together, a water resistant tape can be used to adhere overlapping edges. If you choose to install 15-lb felt paper, the horizontal pieces should be continous that start from the bottom up, then overlap each other a minimum of 2 to 4 inches, encouraging moisture to exit into the shower area. Where a seam connects two pieces, there must be a minimum of 6 inches of overlap.
Install Over Substrate
A vapor barrier can be applied or installed over the tile substrate using a liquid or sheet membrane. There is a clear advantage of using a liquid membrane, it is relatively easy to apply over the tile substrate, along with the use of a fiber mesh tape, you can secure joints and changes of plane. A sheet membrane has just as much of an advantage for ease of installation, but you gain the further benefit of continuity of thickness. The seams and joints still need to be covered with a fiber mesh tape but you will be left with a sturdy surface.
Moisture Sandwich
The industry standard, as well as most local building codes, do not encourage the use of a vapor barrier behind and over the tile substrate. The materials that are most commonly used today for the installation of shower walls all either completely deter moisture penetration or encourage it to pass through, a poor understanding of both could leave you with what is called a “moisture sandwich”. If moisture reaches your vapor barrier behind the tile substrate it should have the opportunity to evaporate, this process is hindered when a second vapor barrier is applied or installed to the outside of the tile substrate. In this case, there is less breathability to encourage evaporation, which ultimately could cause mold growth.
A standard argument would be that moisture would never reach behind the tile substrate if a liquid membrane is applied or a sheet membrane is installed over the tile substrate. If this is the case, than a vapor barrier installed behind the tile substrate is not needed, and you would therefore avoid the dreaded “moisture sandwich”. It is important to note that the wet area is inside the shower area, not behind the shower walls, therefore you need protection leading outward, not protection leading inward.
Vapor Barrier Needed
In some cases, you may not have a choice, your local building codes will determine what is or is not required. If you choose to apply a liquid or install a sheet membrane over the tile substrate, you would then be adding a vapor barrier by default. Have a firm understanding of the materials that you intend to use so you can determine how a vapor barrier fits into your waterproofing strategy.
Atmospheric Conditions
The condition that will influence your decision most as to whether you should install a vapor barrier, besides your local building codes, is going to be the atmospheric conditions outside of your home and inside your shower area. Vapor and condensation behavior is going to differ in different regions where temperature differences are greater during a season or much less during a single day, building materials are going to react differently due to these differences, and the immediate area in your shower upon being exposed to vapor will behave accordingly. You will know the conditions in your shower area better than anyone else but you will need to have a firm understanding of the materials you intend to use in order to have a successful installation of your shower walls.