The vertical wall seams crack more readily because the house shifts and moves. Ive tried grouting only and going back to caulk with sanded grout caulk or silicon only after a crack develops, hoping in some cases it will not. Sometimes it does not crack. Some contractors caulk all seams as a matter of course, no callbacks, safer. I liked the idea of sanded grout caulk to match your grout color It has not held up for me in wet conditions, such as the top of the bathtub where the wall tile starts, etc. , definitely not along the shower floor where the wall tile starts. It has held up in other areas though, and looks nicer when it all ties in. I agree, mildew on silicon is unsightly. Every bathroom has different conditions, including ventilation, moisture, etc. Some people have to clean out and recaulk the silicon every year or two. Whether its allegedly mildewproof silicon or not.

Catbuilder, I don't know the answers to most of your questions, since I did not do the work, nor did I witness the work done. He did not show me the type of caulk (he does this for a living--I trusted him to use what was appropriate). He did not report seeing any visible mold (I asked.) There was no delay between removal and recaulking. It dried for at least 24 hrs, maybe a little longer--I cannot recall. I presume the surround was *adequately* waterproofed, but again I did not do the work. I saw that they did do the waterproofing.

Okay. My opinion? First, about LIQUID water getting behind the tile. It can happen. It doesn't always happen, but it can happen. It's more prone to happen with a lousy tiling job or a lousy grout job, or with a failed installation where the grout is pitted or cracked. In those cases you literally have open paths for liquid water to get behind the tile. What happens more often is that the grout can wet and you can get vapor drive. I'm not going to get into specifics, suffice it to say that the when moisture vapor gets into a wall via this mechanism...it usually comes out of the wall via the same mechanism during a drying cycle. Typically, any moisture that gets behind the tile/grout is absorbed by the cement board. If there is no poly behind the cement board, the moisture can continue through the cement board and into the insulation or into the framing bay. But that'll only happen if the rate of moisture accumulation during a wetting cycle is greater than the rate of moisture evacuation during a drying cycle. It takes a broken installation for that to happen. With poly behind the cement board, the cement board will hold the moisture until it can dry back out into the bathroom during the previously mentioned drying cycle. If, due to failed grout or cracked tile, more water gets behind the tile than can come back out during a drying cycle? Then yes, over time the moisture can travel down the poly/cement board wall and hit the tub's flange area. If the joint between the bottom course of wall tile and the tub flange/poly is open, it can dry out through that opening. If that area is caulked, you'll typically have problems with the caulk. The caulk might soften or re-emulsify. The caulk might be perpetually discolored. The grout may always appear damp at the bottom of the wall. Some installers caulk that entire joint. Some don't caulk it at all. Some leave weep holes every few feet in the run of caulk. To be honest, excess moisture behind cement board is usually not a problem in most installations, unless the installation itself is flawed. Is a topical membrane on the face or cement board better than poly behind the cement board? Yes it is, as it limits how far moisture can penetrate into the wall. But with proper detailing, there should be no negative issues with a cement board and poly installation, even with the bottom tile-to-flange joint sealed with caulk. Again. My opinion.

Have good ventilation in the tub and use it. Use a silicon or siliconized latex caulk in the tub instead of a latex caulk. Less likely, make sure there are no cracks or pinholes in the grout that are allowing water easy access behind the tile. Check the tub deck along the walls and corners. Make sure it's not holding water against the caulk after you are done showering. Some plastic tubs can be deformed and have a slight slope towards the wall instead of shedding water into the tub. If your deck edge holds water, then after you towel off it might be prudent to towel off that area.