Mold on Kerdi Caulk

[openhand]

Hi All- A customer has called me regarding mold growing on this one year old project. It is a new acrylic tub and fresh tile, with Wedi backing. They told me they've re-applied the white color match Kerdi caulk I left behind, three times because of mold growing at the tub rm/tile connection. The inside corners of the wall tile is also Kerdi caulk and they are fine- the mold problem is only happening at the tub rim/tile connection. Tile/tub connection at floor is the same product and no problem there. they are using Concrobium to prep the joint before re-applying. I've used Concriobium before but in a mister to remove mildew smell, not sure if it's a good product to use in this way. I don't know what their cleaning person is using to clean the tile surround and I've asked them to find out. Any ideas from anyone before I go take a look would be much appreciated! I've used Kerdi caulk in this way before with no problems and I'm really puzzled.No mold elsewhere in the bathroom.Thanks!

[cx]

What is "Kerdi caulk," Clark? Don't think I've heard of that one.

[Tool Guy - Kg]

…and can you ask them if they are using a bar soap or a liquid soap?

[openhand]

Sorry- Mapei color matched sanded keracaulk. I'll as about soap. I also asked for a photo of the tube of caulk but am having trouble uploading pics. Slow connection here. Thanks!

[openhand]

I'm having trouble loading photos- I've been resizing to about 750k and when I try to upload I get an error that says I'm not connected to the internet. I ased for a pic of the caulk they were re-applying and it's DAP Alex Fast Dry siliconized latex painters caulk. That's a whole other issue- I did use the Mapei sanded color match caulk when I built the surround. But still the question of why the tub rim to tile connection is getting moldy? And regarding soaps, here's direct from customer: I use bar soap, usually from farmers markets/stands… but the kids use Dove body wash/liquid.

I do use a lot of bubble baths or bath bombs also…"

[openhand]

Trying to send pic of mold

[John Bridge]

Hi Clark,

I've used that caulking a lot, but it's not silicone, which is recommended.

The caulking needs to be completely dug out and redone with silicone. And beyond that, mold can still form if water is allowed to remain on/in that joint. I recommend to everyone that the shower should be towel dried after use.

[openhand]

Thanks so much for the reply. Since this joint has already been filled with “siliconized” latex caulk I wonder if pure silicone will have difficulty sticking. I have used Lexel before and although it can shrink and require two lifts, it will stick to itself (unlike silicone) and makes a stronger bond. When you wipe it after it cures it doesn’t feel like it might get dragged out of the crack. Does this sound like a good idea? BTW- this is a family of four and their only shower/tub, so this thing really gets a workout.. there’s also an old exhaust fan with a long run and I’m going to recommend they upgrade it. The toweling off is a great tip and I’ll pass that along. Thanks again and I hope to hear some of you weigh in on Lexel!

[Tool Guy - Kg]

I don’t think 100% silicone would have trouble sticking. It’s not the same as trying to go over old silicone.

But that shouldn’t stop you from trying to get rid of 100% of the old stuff before starting with silicone.

[openhand]

From the same company (Sashco), this product is offered. But I'm still concerned about getting the area 100% dry before application, and this is latex based. I can't find a good spec sheet on it either. I've also seen mention of the mildew inhibitor in this type of product losing strength over time. Can anyone weigh in on this? I'm surprised this isn't a hotter topic among tilers! I was hoping for lots of hits! Oh well..
https://www.sashco.com/products/cleanseal/

[Tool Guy - Kg]

In the area in question, you’ve got a horizontal application, which is naturally going to stay wet longer than a vertical application. Add in bar soap and you’ve got food and moisture to grow mold. Latex caulks take longer to dry after becoming wet, so there’s a longer opportunity for mold to grow before it dries out completely. A 100% silicone caulk will repel water and dry faster than any latex caulk (“siliconized” caulks are still latex), cutting down on the time mold can grow. It doesn’t mean mold can’t grow on it, but drying faster cuts down on the opportunity mold has a chance to grow between shower uses.

I don’t know which Sashco product you are referring to in your last post.

As far as mildewcides being used. They are used in some 100% silicones. I’ve talked to a chemist at one company and he explained that they use the maximum amount of mildewcide in the formulation without being forced by the government to reclassify the caulk as a pesticide (which would require additional government regulations that they don’t want to get involved with). The maximum amount of mildewcide, however, only lasts for about 6 months.

[ss3964spd]

Since mold is growing in those joints I would definitely want to remove all the old caulk first. You'll likely find there is more mold than what you can see on the surface.