Part 4: Surface waterproof sheet membranes for tile showers
This post will focus on waterproof sheet membranes for bathroom tile showers. Although some manufacturers have recently jumped into this space, Wedi and Durock to name two, this particular area is currently dominated by two main players: Schluter and Noble.
We’ll discuss where to use these products, perm ratings, advantages and disadvantages, and all the usual info. Also to be included in this category are the polystyrene foam building panels, such as Kerdi-board and Wedi building panels, since the waterproofing method is essentially the same concept as it is with waterproof sheeting.
How waterproof sheet membranes work
Waterproof sheet goods for tile showers are put on the surface of the shower, much like a liquid waterproofer. They stop the water at the surface of the installation before it is able to soak in. Where sheet goods really shine, compared to liquids, is with perm ratings. As mentioned previously, the sheet membranes come in at >1 perm to qualify as a vapor retarder, while most liquids are in the 2-ish range.
The foam building panels install right over the framing of a wall. Some are a little flexible and it’s probably not a bad idea to put some additional blocking in the wall where the seams are to help take the flex out. I don’t believe any of the manufactureres require this, however. The sheet goods need a surface to go over. Cement board or Fiber cement board make good backings for this. Schluter says it’s OK to apply their Kerdi fabric right over sheetrock. I’m not sure if the other manufacturers will approve this, or not. You’ll have to check the instructions.
Advantages of waterproof fabric sheets
On paper, waterproof fabric membranes are king. They stop water at the surface level, right behind the tile and have the lowest vapor transmissions. The fabric sheet is already a predetermined thickness so there’s no using a wet film gauge and wondering if you put the waterproofing on thick enough like you would with a liquid. Also unlike liquid waterproofing, there’s no applying multiple coats and waiting for each coat to dry before tiling.
So with all these advantages, what’s not to like?
Disadvantages of waterproof sheet membranes
There’s two big disadvantages to waterproof fabric: cost and overlaps.
Waterproof sheets are the most expensive way to build a shower of the three forms that have been discussed, and the foam building panels are even more expensive than the sheets. People usually have budgets with their bathroom remodels and they don’t usually want to spend all their money behind the surface of their shower. When faced with a decision of a Kerdi-board shower or tile floor heat, which one seems more appealing?
The other big disadvantage is when you have to overlap your sheets. Assuming you are using the fabric not only on the walls, but on the shower floor also, you will have three thicknesses of fabric that are overlapped. These overlaps get in the way when you are trying to install tile over a flat surface because the surface is no longer flat. Most good installers will take some time to flatten out the walls and the overlapping of the fabric screws that all up. If you are installing larger tiles, it won’t be as big of a deal, but if your wall tiles are smaller on your shower, this is going to be an issue.
I believe Schluter’s Kerdi fabric is the thinnest at 8 mils thick. You can then use their waterproof seam fabric and preformed corners (4 mils thick) to try to minimize the amount of buildup from the overlaps. Noble, although being the best on the stat sheet, has the thickest membranes. Their Wall Seal is 25 mils and can only be used on the walls, which means that you have to use NobleSeal TS (30 mil) on the shower pan. The TS has to be flashed up the walls with an overlap in the corner and the Wall Seal would overlap over it. This means that you would have the thickness of three membranes (0.30 +0.30 +0.25) to deal with in the corner.
What does it all mean?
Waterproof sheet membranes offer the best of both worlds: waterproofing and vapor protection at the surface level. However, they will cost more and you will have to deal with the problems that the overlapping fabric cause.
Check out the other parts of this series here: