A vapor barrier in your shower will save the walls of the shower or the bathroom. It’s something that’s vital to protect your house. There are a number of reasons for having a vapor barrier in the shower but they all revolve around making sure moisture doesn’t penetrate where it can do damage to the fabric and structure of the house.
Where there is hot water, moisture will gather on cooler solid surfaces. Your shower generates a great deal of moisture, not only in the water but also in the steam that comes from the hot water. That is why kitchens and bathrooms have gloss paint as it prevents the moisture penetrating through to the drywall, where it would simply soak in. Over time the drywall would crumble. In that case the paint forms a vapor barrier.
Bathrooms tend to be warm places, and anywhere that warm air meets colder air, moisture forms. A vapor barrier shields the place where moisture can form. The vapor barrier holds the condensation and stops it penetrating further. The volume of moisture will decrease as the moisture returns to the air, as long as there’s adequate ventilation on both sides of the vapor barrier.
Types of Vapor Barriers
A Type 1 vapor barrier is mandated where the wall construction has exterior sheathing that has high resistance to moisture. It’s also required in showers and other high-moisture areas. A Type 2 vapor barrier is used where there’s no expectation of high humidity.
Given that moisture will continue to penetrate any material until it’s stopped by a barrier, it’s extremely important to have the vapor barrier on the warm side of any wall. In an outside wall, for example, it will stop the moisture from penetrating the insulation. Put in a vapor barrier in any interior wall where there will be moisture. You should also install a vapor barrier in all exterior walls. A shower is where you’ll find the highest level of vapor.
If you’re using greenboard or drywall for the walls of the shower, you’ll need to put the vapor barrier between the greenboard and the water. Failure to do that will mean that moisture will soak the greenboard and it will gradually crumble, as will your shower wall. A good vapor barrier should be 10- to 15-millimeter polyethylene. This will be thick enough to hold the moisture effectively.
In a shower you can’t have the vapor barrier on top of the greenboard. That will require a waterproof membrane as a vapor barrier that can handle ceramic tile on top.
You still need a vapor barrier with cement board, although in this case the barrier can go behind the board. Cement board is heavy enough and breathable enough to take the moisture without a problem. By having the vapor barrier behind the cement board you stop the moisture going further into the wall.