What happens to water behind tile?

[mc510]

I just had a professional tile contractor install a tile tub/shower surround and tile floor. They paid a lot of attention to installing waterproof backer board, which they pointed out is important because grout isn't 100% waterproof. Makes sense.

After the project I started to wonder, if it's expected that some water will get through grout and behind tiles, what happens to it after it's trapped there? Google tells me that it's bad to have water behind tiles, can create mold etc, and needs to be addressed. Makes sense.

So now I'm wondering why isn't the objective to have a backer board that retains 100% structural integrity when wet but will allow water to pass through and evaporate off the back? Well, that would be bad too, if it's more than a very very tiny amount, just moves the mold into the walls.

Anyways, the more I think about this the more confused I get. I have no doubt that the way that it's done is correct, I just don't know why! Anyone want to educate me?

[cx]

Welcome back, Mike.

What is this waterproof backerboard they used?

The two most common ways to contain water that gets through your shower tile installation are 1) to use a water resistant membrane, such as roofing felt or polyethylene sheeting, over your stud wall behind your backerboard and lapping over the tub's tiling flange on the drain side and 2) to apply a direct bonded waterproofing membrane to the face of your backerboard which is lapped over the tiling flange on the drain side.

With the first, water that penetrates the tile installation will either pass through the backerboard in vapor or liquid form while also drying by evaporation back through the tile installation. The liquid water that enters the backerboard will also gravity feed to the lower areas. The water vapor that penetrates the backerboard will, most likely, condense when it encounters the moisture barrier and also gravity feed to the lower areas.

A problem with that is that the ceramic tile industry requires that the gap at the bottom of the tile installation, between tile and tub, be filled with a flexible sealant. I personally think this a bad idea, but that's the current requirement.

With the second method, water that passes through the tile installation, either liquid or vapor, will encounter the waterproofing layer. There it will either gravity feed lower or evaporate back through the tile surface. That portion that does not evaporate between shower uses will eventually feed to the bottom and encounter the same problem described above.

Depending upon your particular environmental conditions around the shower envelope, a very small portion of the moisture vapor will actually penetrate the envelope and find its way to the wall cavities, where a different discussion (between us, not the vapor molecules) would need to take place.

That's the simple answer. Beyond that it can get as complicated as you want it to as you enter into the endless discussions available in residential building science.

I hope one day the industry will come to its collective senses in the matter of sealing the joint between tile and tub, but it'll not happen in my lifetime. Other than that, if you use one of the industry published methods of tub/shower construction, you can make a shower that will outlast you.

Stand by for differing views.

My opinion; worth price charged.

[MesaTileworks]

This is super interesting and actually something I’ve wondered about a lot. Thanks for the thorough info, CX!

Do you reckon a better solution to be to grout the intersection between the tub and the wall, let it crack, and then know that any water can drain out? Or would it be better to use something flexible (to prevent cracking) but water-permeable to allow trapped water to escape?

It’s my understanding that most tile, whether ceramic or natural stone, are water resistant but not waterproof, and that we expect as certain amount of two-way transmission of liquid water and water vapor. Same goes for grout, but maybe less so for epoxy? From working with it, I have the sense that epoxy grout is *much* more water-resistant than cementious grout, but I’m not a chemist and don’t know how you’d quantify that.

What I’ve wondered is: if we expect water to get under the tile and we use some kind of waterproofing membrane, that means the water and setting compound are basically trapped together. What happens to thin-set over time if it remains damp? Does it hold up to this and keep its bond strength and structure? (Assuming normal conditions and good installation so minimal presence of water).

[Just In Tile LLC]

I've also wanted to understand this exact process completely as well, from what I've seen on the building science of exterior cladding is even when you you have different methods of water control, the main one to get right in all instances is drainage. Human error being applied to all water systems your best bet is still how and where do I direct moisture flow when it is controlled and when it is not.

Hydrostatic pressure, or to simplify it, the amount of force liquid water exerts on a surface when it comes into contact with it is a big deal. Someone put it like this, a tent in the rain is just fine, once a tree limb rests against the tent material you'll have a drip inside the tent.

So according to what I've read on exterior systems, it would be better for us to use some sort of impervious dimple mat that is fleece covered so our mortar doesn't contact the mat completely to allow for a drainage plane behind the tile, and to have all the bottom weeps as open as possible so water can flow unimpeded to the tub/pan and make it's way to the drain.

[Davy]

Matt, as long as the thinset was mixed, installed correctly and allowed to dry completely, it will remain bonded even when submerged under water. Mold isn't as big a problem with cement based products like thinset. We see a lot more mold with mastic when used in showers.

[John Bridge]

Time for me to re-state what I've been preaching for many years about shower maintenance.

If you dry the shower with a towel after each and every use you will never have a problem with moisture behind your tiles, and your mold problems will be minuscule as well. The small amount of moisture that might penetrate the grout during the course of a shower will evaporate back out the way it entered.

[mc510]

Thanks everyone for an enlightening discussion!

@cx, they used a very lightweight board for the walls; I didn't catch the brand name but I did note that it said "waterproof" on it. They put fiberglass tape and pink goop over the joints and over all of the horizontal surfaces (niche, and a stoop at the foot of the tub). Oh, and roofing felt behind the backer board. On the floor they used Hardiebacker; I suppose they covered it with the pink sealer but wasn't home to watch on that day.

@John Bridge, very sensible advice to wipe down the tiles after a shower ... not that easy to get my kids to actually do it, but it's good advice!

[cx]

Originally Posted by Matt

Do you reckon a better solution to be to grout the intersection between the tub and the wall, let it crack, and then know that any water can drain out? Or would it be better to use something flexible (to prevent cracking) but water-permeable to allow trapped water to escape?

Matt, I've grouted that joint for years. The grout will crack, every time, and I'm aware of that. If it gets to be more than just a hairline crack, as it does sometimes, and the customer complains, I'll go back and re-grout. Customers are told beforehand that it will crack and why I'm grouting it anyway and for the most part they've just accepted it.

With today's newer sanded 100 percent silicone sealants available, I'd be inclined maybe to caulk the joint, leaving several gaps for potential drainage, but I've not done a tub/shower for years and hope that remains the case.

Originally Posted by Matt

Or would it be better to use something flexible (to prevent cracking) but water-permeable to allow trapped water to escape?

Good eye-dee. When you've got such a product ready for market, please do give me a call.

Mike, I'm a bit concerned about that shower, but if you want to discuss the specific project it's best you start a project thread in the Advice forum and post some during-construction photos.