Moisture on Outside of Bathtub

[cwtx]

We have an odor in our linen closet that smells like moisture, musty, etc.
Four months ago we replaced our tub with a new Kohler Villager cast iron bathtub. The installation is in a three-sided alcove with tile surround walls above the tub. Tub, tile and plumbing are new, but in same configuration as before. On the other side of the wall where the plumbing and drain are is a linen closet, with an access panel you can open and see the underside of the tub, the drain, and the valve. The closet has started smelling really musty in the last few weeks. There are no visible drips, moisture on the floor, leaks from drain or supply pipes. The only evident moisture is that when you touch the outside of the tub, (the cast iron part behind the access panel - not the finished part in the bathroom) it feels moist like there is condensate on it. It's all over the tub, not just near the drain and overflow. The house is a 45 year old one-story, central heat/air, unfinished attic. Is it normal to have condensate or moisture on the outside of the tub like that? Is it a problem? Any ideas what could be causing the musty smell in the linen closet? Bathroom is used by two teenagers who take really long showers, but there is an exhaust fan to remove moisture. Weather has gotten cooler in last few weeks, so we are not running AC as much as before, and don't need heat yet, so inside humidity is higher in general.

[jadnashua]

When the walls were tiled, how were they prepared? Best practices call for cbu (cementeous backer unit - i.e., cement board) on the walls with either a surface waterproofing or a vapor barrier behind it, lapped over the tiling flange of the tub. Tile and grout are NOT waterproof, and moisture, especially in long, hot showers will penetrate the walls and condense in the cooler confines. This can lead to exactly the symptoms you describe.

Also, is there an exhaust fan, and if there is, is it used? To rid the room of excess moisture, it could take the fan a 1/2-hour after the shower is over or maybe even more, if it was a very long and hot one.

One way to help is obviously, the use of the fan, but also to require they wipe the walls down in the tub/shower area after finishing.

If there was no vapor barrier behind the cbu, or if drywall or greenboard was used on the walls of the tub surround, you are on borrowed time.

[leejosepho]

Assuming it would not be possible to apply a coat of foam insulation to the underside of the tub, I would look for a way to power-ventilate or dehumidify the space around it.

[cwtx]

The tile is installed on DensShield brand backer board. The mfg.'s info says it has a built-in water barrier in the form of a gray acrylic coating on the surface next to the tile, which is supposed to prevent moisture intrusion into the wall cavity. It also has a "moisture resistant core" Per mfg. instructions, no moisture barrier was used. Our contractor has had good experience with the product, but I don't know how long it has been around.

The fan gets used during showers, but probably should be left on longer afterward.

The current door on the access hole does not fit as tightly as the door that was there before remodeling. Would air leakage through that access hole between the finished closet and the unfinished cavity around the tub be enough to cause the condensation and the musty smell? The condensation went away as the outside temperature warmed up today.

[cwtx]

When we compare the pictures taken during construction to now, the back (non-tiled) side of the DensShield looks different. It was all a somewhat mottled light brown. Now there is a dark brown band that extends from the bottom edge where it meets the tub up about 6 inches. We can only see one end, but the band goes all the way across the end. My guess is that the DensShield is absorbing water and wicking it up. It looks like they put a bead of clear sealant on the tub, then pushed the edge of the DensShield into it. The instructions say there was supposed to be at least 1/8 inch gap between the bottom of the DensShield and the tub flange, but the gap appears non-existant. The contractor put a flexible, grout colored sealant between the bottom edge of the tile and the tub, but it also is not 1/8 inch wide. Does this sound like a problem and could the DensShield be absorbing water? Could the wet DensShield be the source of moisture in the area, which is then condensing on the tub surface?

[cwtx]

Thanks for the experienced people who respond here - it helps to hear someone other than our contractor.

I need some input on what my contractor is planning to do to identify our problem. The shower has not been used for four days, and the DensShield walls are drying out slowly. (Weather is cool and humid here, so AC and Heat aren't running much.) The contractor plans to apply "clear waterproofing" to the tile and grout joints, and caulk again between the tub and tile. He says if we still get moisture on the back of the DensShield when showering after doing that, it means the moisture is coming from somewhere else, not the tile installation. I'm not buying it, because I think the "waterproofing" is a temporary fix, and besides, the problem built up over four months so it might not show up the first few times the shower is used. Is my thinking correct?
He also says that as long as sealant was put between the tub and the DensShield, it doesn't matter if there is a gap between the two now. We still think they put a bead of sealant on the tub, then pushed the end of the densshield down into it, leaving the densshield touching the tub and all the sealant pushed out on the back.

[jadnashua]

I'd get some more opinions over at www.johnbridge.com.

It does sound like you are getting some wicking if the stuff is discoloring. And you're right, it does say the stuff should not need an additional vapor barrier. I didn't read it carefully enough to see what they require for sealing the screws, but did see they want the typical fiberglass tape on all seams. That tape, unless it is different than those used for cbu, won't have any waterproofing capability, though, it is only to tie the two sheets together structurally.

They claim the core is water resistant...a cbu is not affected by water. The jury is still out as to whether this is a superior product to a cbu. It is being touted as a replacement for greenboard, and it is probably better, but greenboard was a joke, so I'm glad it is out of the national code (although not adopted everywhere).

Even if you had installed cbu instead, you still caulk the seam at the tub, so not sure. If it truely is essentially flush with the tub surface, squished into the caulk during installation, then it would generate a cup and hold water in the end. If it was installed, then caulked, that would be less likely. Don't know if that would make a difference.

A grout sealer will not prevent water absorbtion, and most tile will not absorb one. If it is painted on and left (contrary to the instructions for those I'm familiar with), it will leave the tile surface streakey. Those I've used say to buff off any that is not absorbed before it gets a chance to dry. A good sealer will limit water absorbtion, but not stop it. It will repell stains, but not stop them.

[leejosepho]

The contractor plans to apply "clear waterproofing" to the tile and grout joints ...

Tell him or her you would like to see the manufacturer's specification and application information sheets for that product, and that you will be looking for another tile installer's verifying opinion about its use in your specific situation before proceding with the proposed "patch and please" ...