When properly applied, liquid waterproofing membranes for the shower must last at least ten years before they need to be resealed. While basic waterproofing membranes should last at least ten years, they can last much longer if chosen and under the right circumstances. On the contrary, poor quality waterproofing will rarely last the distance. The last thing anyone wants when building a new shower.
A professionally installed quality waterproofing sheet membrane should last a long time. Most waterproofing products come with a minimum 7-year warranty, but if applied correctly (depending on which membrane you choose), a membrane should last much longer than that. As far as I know, there is no official distinction between these two membranes in the tile world. In fact, the term moisture barrier is not used in the TCNA manual.
Instead, they use the term vapor retarder. A fully sealed solid aluminum frame would be completely waterproof, similar to a solid stainless steel niche, however, the Kohler pilaster has a cover that is attached to the top of the niche frame. However, Densshield has a permanent rating of 1.5, which is not suitable as a shower wall waterproofing membrane, in my opinion. The many liquid waterproofing products don't require a fabric reinforcement, but there are still a few on the market that do.
One of the additional advantages of sheet-type waterproofing shower membranes is that they can also serve as decoupling membranes. This type of product can be considered a waterproof paint for the shower because the most common application method is with a paint roller. What you SHOULDN'T DO is assume that since one waterproofing strategy for the shower base is good, two strategies will be even better. The board itself is water resistant, so you don't have to install a cement board and then waterproof it on top.
This video shows exactly how to properly waterproof a shower complete with a mud shower tray, using your Kerdi waterproofing system. These old school “waterproofing systems” were often incorrectly installed, causing gaps in the membrane of I wonder if I can use the Redgard liquid membrane on the drywall AND a waterproof decoupling sheet on top of that. It also comes with a custom shower drain designed specifically for this system, so it is easily integrated with its waterproofing membrane. It was designed this way so that the waterproofing membrane and the tiles can overlap this flange, creating a dripping edge.
There aren't many stated requirements when it comes to the definition of a waterproofing membrane for use in a shower environment. Given your extensive knowledge, I wonder if you can comment on the effectiveness of a transparent waterproofing membrane on (rather than under) shower tiles. To apply liquid waterproofing membranes, contractors use roller brushes or spray machines to cover a surface with them. I'm not sure I agree with the use of silicone to bond the metal panels to the waterproofing membrane.
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