Kerdi Wicking

**GCloud** · 02-20-2011, 04:51 PM

So I've installed a base, covered it with Kerdi and all seems good as far as the flood test goes.

But I've got a beef with the Kerdi. Here's why.

The layer of fleece wicks water.

I'm testing a strip of kerdi submersed in water with water soluble ink marks on it. See pics. After one hour the bottom mark is gone. Completely. We'll see what happens over the next while.

My question is this...

A strip of kerdi hung vertically allows capillary action, so how can I expect proper drainage of water on a slope of 1/4" per foot? I've increased this slope to exceed local code, but I still don't see this water going anywhere.

Sure the bulk of the water is going to drain, but the thin layer interwoven in the fleece is not going anywhere fast. Sounds like a nice little breeding ground for mold and mildew, no?

I've just come from a situation of mold and mildew and I really don't want to go back to that.

**LLigetfa** · 02-20-2011, 05:16 PM

I think the Kerdi fleece will be encapsulated in the mortar and that there will not be any way for mold spores to get anywhere near it and no source of food for the mold to eat.

**jadnashua** · 02-20-2011, 05:38 PM

First, you should never have liquid water on the Kerdi once installed. On the seams, you need to ensure you have the proper overlap and you've got intimate contact with the majority of the thinset smushed out. It a little disconcerting, but it does work with many thousands of installations around the world. The fabric itself is hydrophobic and pushes water away from it. The fleece needs to accept the thinset, and needs to not repel it to hold things together.

**GCloud** · 02-20-2011, 05:41 PM

Good points. Maybe another test would be to coat this strip in thinset and see if water wicks then. I'll start that one when I mix my next batch of thinset.

**jadnashua** · 02-20-2011, 05:48 PM

That won't tell you much...take two strips, over two inches wide, and put thinset on the two and bond them together like you would with a seam. Let the thinset cure (at least a day) and see what happens. Water will wick a little way, but nowhere near the extent of the overlap. A shower has the tile on it, bonded with thinset, and very little water penetrates. The kerdi will not allow any to get to the backing, which is the point. With Kerdi on the interior of the shower it will not let any moisture into the backing materials. Kerdifix is designed for places you can't put thinset, like sealing around pipe penetrations, tub/wall joints, and things like that. If it makes you feel better on the seams, as long as you get full covereage, it should be fine, but it's otherwise a waste of money on 'standard' seams.

**GCloud** · 02-20-2011, 06:15 PM

Jadnashua, can you expand on your first point about never having liquid water on kerdi once installed?

How does one flood test if there can't be liquid water on the kerdi? I feel maybe I'm not following your comment..? Do you mean "should" like it's unlikely to happen? Or "should" like I should avoid water contacting the kerdi at all costs?

I understand what you both are saying about the thinset binding with the fleece. It makes sense that the thinset will 'seal' this fleece layer and stop the capillary action.

Cheers
Grant

**jadnashua** · 02-20-2011, 07:58 PM

Feel free to flood test, the Kerdi doesn't care about being wet, but the point is, in a properly installed shower, once it is covered with thinset and tile, it's unlikely there'll be any liquid water that gets to it, at least not standing water. Even if it does get damp, it's not like anywhere near the hydrostatic pressure when you flood test...just a little dampness, and then, not much or for long. Think of a 'traditional' shower that has cbu behind the tile...unless you have a defect in the construction, it won't be soaking wet, either. But, since there's more mass and thickness, if it does get wet, it will take longer to dry out. The beauty of any surface membrane is that there's almost nothing that does get wet and it dries out faster.

**GCloud** · 02-20-2011, 08:42 PM

I see what you're saying. Makes sense. Thanks.

Grant

**johnfrwhipple** · 02-21-2011, 10:04 PM

Grant I'm testing this exact point in my home.

I find that the Kerdi is wicking a lot. With thin set and with out.

This wicking is quite odd as so many say the Kerdi is Hydrophopic...

I am going to re-test my result and document them much better. My Kerdi Box I flooded out a few weeks back is slowly loosing water and I feel it's wicking away into the box material and evaporating. Not enough to show a leak but a faster rate than regular evaporation.

My new test box will be sealed from evaporation and I will showcase my findings. From the testing I have done to date I would advice anyone to use Kerdi Fix for their connections fleece to fleece.

JW