Mold Theory Question - Shower Tile/Pan


Old 08-23-16, 12:23 PM
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Mold Theory Question - Shower Tile/Pan

Another post got me thinking about this and although it'll be a few years before I tackle a bathroom remodel I figured I'd get the question out of my thoughts hopefully.

I realize grout is cementitious (sp?) and will weep some amount of water through. I understand the function of the shower pan to collect what what does seep through and drain it into the weep tiles of the floor drain.

My question is, why does not the small amount of water start a growth of mold between the grout/tile and the shower pan. I wouldn't think there would be any really way of the moisture in between to really dry out as it would be getting a fresh shot of moisture every time someone uses the shower. Without good airflow I would think it would stay moist for a long time. I would assume body yuck or soap would provide food for the mold.

Since I don't have an expanding colony of mold coming up through grout lines into my shower I must be missing something. Just not sure what.

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Old 08-23-16, 12:57 PM
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The pan is sloped so it drains thus there is less water and even less food than you think in that space.
Old 08-23-16, 04:53 PM
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Mold can and does develop on vertical surfaces if humidity is high enough and other conditions are met. (If you don't clean your shower mold will develop everywhere without "standing water"). These conditions are surely met in a dark, high humidity environment that I would think exists below a mud bed/grout.

I wouldn't think the draining of water on the slope of the pan would impact the environment.

Just having the grout lines/thinset damp would be enough to promote mold.

Again, I'm not saying this doesn't work. I'm just curious why it does. I know I'm missing something.
Old 08-23-16, 05:49 PM
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This is a good topic, and Im not sure anyone has completely figured out the "best way" to prevent mold... but I think a lot of it has to do with where the vapor barrier is located.

For instance, do you put vapor barrier on the studs... and then cement board... and then tile?
Or is it better to omit the vapor barrier, put up cement board, paint on Redguard (a vapor barrier and waterproofer)... and then tile.

I think current building science is leaning toward the latter.

The same could be said for vapor barriers in walls with drywall.... in some cases, remodellers are finding mold behind the poly vapor barriers... so more and more I think vapor "retarders" (between 1 and 10 perms) are encouraged which allow moisture to pass either way, depending on the needs. For instance, latex paint is a class III vapor retarder.

A little off topic, but similar.

Back to grout... I think some walls will get mold behind the tile and in the grout, but I think a lot of that comes from failed grout letting water, mold and air in. Mold is generally not going grow if the spores aren't there... i.e. if the area is sealed, fairly mold spore free... and new air is not introducing new mold spores into the area... I think it will not grow between layers. But if you have an open crack that is letting water wash surface mold behind... then you have a problem.

Mold on/in the grout usually results for someone not cleaning the shower often enough or from not letting it air out after use. But I think certain building practices can exasrebate the problem.
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