Many of the bathroom remodeling projects we have completed over the years have been in homes that were only 5 to 15 years old. By far the most common problem I have encountered has been tile installed over drywall in shower/wet areas. This is the case so much of the time that when people tell me on the phone that they have tiles they can push in with their hands in the shower, or that the tiles are falling off the wall or seem ‘squishy’, I immediately get the image in my mind of the very common 4″ ceramic tiles installed over drywall. And in most cases, this is exactly what they have.
This method of tile installation is simply bad practice, yet it is by far the most common situation I see in bathrooms that we tear out. The reason this is such a common practice by new home builders and remodelers is only for one reason – it is cheap to do. There is no other benefit than cost.
Many of these installations have water damage in the wall, on the studs, wet drywall, spots of mold/mildew in the wall, ants, and water-loving bugs present in the wall (at least this is what we have found in many occasions). Another issue is that at the bottom of the tiled shower wall (where the shower base is caulked against the shower wall), there is an endless black discoloring of the caulking. Even if you re-caulk this joint, in just a few months the caulk discolors and turns black again. The problem is that water does seep through grout joints and gets the drywall damp. This constant wetness with repeated showers every day doesn’t allow anything to ever dry out. (Please also see the value of having a good quality Bathroom Exhaust Fan)
The bottom line is that drywall (and this includes moisture resistant drywall) should never be used behind tile in wet areas (showers and tub-shower combinations). The preferred method is to use something like Hardi-Backer or concrete board, though with these products it is important to correctly build in the proper vapor barrier as without a proper vapor barrier water vapor will migrate through the wall and under certain temperature conditions, the dew point (inside the wall assembly) causes condensing of this vapor inside the wall (on outside walls). Done correctly these products will work just fine. I have seen too many installations in which Hardibacker and concrete board was used but with the improper techniques that did not follow the manufacturer recommendations and industry guidelines.
We prefer “Wedi Board” as a tile substrate in showers. The “blue board” insulation that makes up the core of Wedi Board provides a continuous layer of insulation. The benefit of this is that this closed cell insulation is 100% waterproof, but it also provides a “vapor stop” to keep water vapor from entering the wall cavity. This is especially important in a shower located on an outside wall, but also helps even if the shower is on an inside wall. The beauty of this is that you could actually install Wedi Board in a shower and not tile the shower at all (not that you would do this) – and take showers for years with just the Wedi Board in place (no tile) and you would have a 100% leakproof shower. This means that by tiling over the Wedi Board, you are simply providing an aesthetic look to the shower but not relying on the tile itself to do the waterproofing of the shower. The tile does add a layer of waterproofing protection, but the real heart of the system is in the Wedi Board behind the tile. Compare this to the common practice of tiling over drywall and you get the idea that you have a totally different shower by using a product that is meant to function in the shower.
The Wedi Board/ Wedi Shower system includes special bonding agents (referred to as “wedi caulk” though is much more than caulk) that waterproofs the seams in the corners as well as the joints where the 3×5 board pieces join together. This binding of the pieces throughout the shower also help keep the whole system acting as one unit behind the tile, helping to keep tile and grout from cracking.
– Scott Hall