You must waterproof all the walls of the shower before laying the tiles. A tile backing or cement board is usually not waterproof, although many are water resistant. The most effective and reliable solution is a separate waterproofing membrane on the primed shower walls before tiling. Water containment in shower walls is important and a requirement.
Waterproofing your walls may cost you a little more, but it will save you thousands of dollars in the future. Water, water everywhere? Bathrooms are, by nature, humid spaces. But you need to make sure that water doesn't seep through walls and cause mold or other damage. To prevent leaks and excessive moisture accumulation, take care to waterproof the wall, floor and ceiling in the bathroom.
The tile is mostly waterproof. Natural stones will not, but porcelain tiles for the most part will not absorb water. Ceramic tiles installed on walls will spill most of the water. Grout, on the other hand, stinks.
Grout is sand that is bonded with Portland cement (with some exceptions: epoxy grout, unsanded grout, etc.) Therefore, it acts like sand and absorbs water as it flows across the surface of the grout. This means that when building your shower, you need to consider what the water will do when it passes through the grout and reaches the back wall. The problem is that the tiles and grout themselves are not waterproof. The floor must be installed together with a waterproofing system, or whatever is behind them will get wet.
Custom Building Products requires two coats of its Redgard liquid waterproofing on all waterproof surfaces. Waterproofing can serve as a moisture barrier, but a moisture barrier does not necessarily qualify as a waterproofing barrier.