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Old 04-23-2005, 10:20 PM   #1
oregoncoast
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Why use Wedi AND Ditra??

My husband and I are in a spin about this question.
I told him about Matt Clark's concrete slab bathroom and how he layered Wedi-Warmly Yours-Ditra before laying tile. He is saying if we use Wedi we do not need Ditra and that to use both is overkill and over our budget. I need to find out WHY we might need Ditra (as in what it does in this application) before he will consider it. Please HELP!
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Gayle
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Old 04-24-2005, 09:21 AM   #2
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Hi Gayle, if you are going on concrete you really don't need either. The ditra would provide some extra insurance of tiles cracking if the concrete has a few cracks. The wedi will just raise the floor higher.
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Old 04-24-2005, 09:33 AM   #3
bbcamp
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Wedi is used as a thermal break so the heat will stay in the room, not sink into the slab.

I so no reason to use Ditra over the electric heat in this case.
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Old 04-25-2005, 06:32 AM   #4
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I used the Ditra as an isolation membrane. When you heat the tiles, they WILL expand...it's physics. The Ditra allows the tile, grout and thinset to expand as a unit, without affecting their adhesion to the underlying concrete slab. I was sold on it...and since I did it myself, it only added about $150 to the entire bathroom project. I got it locally, it was relatively cheap and I actually needed the 3/16" or so it added in height.

I'm not gonna say it was the only way to go or that my project would have failed had I not used this stuff. What I will say is it was easy to install and adds some insurance to a project that will total about $6000 (more than twice that, had I paid to have this done). Looking at it that way, it was less than the sales tax on the vanity my wife has chosen...but that's another story.

I don't sell the stuff, but I am a believer...for whatever that's worth.
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Old 04-25-2005, 12:01 PM   #5
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If you must cut out one or the other, I don't know which one is the first to go. I do believe you need one or the other or both.

The three posters above all make good points. I did install heat cables directly onto concrete, and it works. My sister did the same in Southern California and it works. We are both dissatisfied. We would have like to have known about some form of slim underlayment that would separate the concrete structure from the tiles and thinset. Because of the waste (time, money) and the ongoing aggravation. Sounds like we needed Ditra or Wedi or both.

When we put heat cables on top of wood structures we loved it. People we know have put heat cables on top of concrete with a thin layer of *anything* and they are satisfied with the result.

To tell you "no tiles have cracked yet after three years" does not address the issue of insurance that Ditra provides. So I am in favor of Ditra.

From what I hear of Wedi, I like it.

Elsewhere in this forum experts have pointed out that keeping level across floor thresholds is not a key factor and SHOULD not be a key factor. They say this is the first thing to sacrifice if you have to do something right. And when I have visited people's houses where they have a 1/2" bump they are not bothered by it. They say it is not an irritant.
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Old 04-25-2005, 12:07 PM   #6
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Old 04-25-2005, 12:41 PM   #7
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I can tell you that the heating mat I installed went right over the slab, covered with SLC, and since I had some cracks in my slab, Ditra went over all that--however, I can think of no reason to have the Wedi. I live in MN, and although I had some reservations about how well the mat would work to heat the floor in winter months, it performed like a champ, without any thermal barrier whatsoever. However, I leave my thermostat at a certain temperature constantly, with no shutdowns at all. So I can guarantee if it was a cold start, that it'd take the better part of the day to heat BOTH the slab and the tile surface.

Just because you don't have a thermal barrier doesn't mean the heat'll all transfer down. It just may take a while if you like to turn it off.

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Old 04-25-2005, 01:11 PM   #8
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On a long term stable state basis your heating cables will compensate for the temperature of the ground coming up through the concrete. Whether you run them at high or low is your choice and its cost is small in both cases. The heat loss is directly related to the thermal bridging down to the ground.

Having seen your other posts, now I can say you have a concrete slab that, if and when heated, will send its heat down into the planet Earth. In Canada people have started insulating under the bottom slab; they put styrofoam down before pouring. And it works. It prevents thermal bridging. And in Canada electricity costs only a few cents a kilowatt-hour.

Any heat-separation membrane is better than none. Just like wearing one layer of clothing outdoors (vs none). No need to buy a product designed specifically to be a heat insulator; it just separates because it is there.
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Old 04-25-2005, 01:36 PM   #9
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Trowel-on membranes like RedGard and Laticrete 9235 also provide some isolation. While they might not be as good as Ditra, I think that they are less expensive and certainly add less height.

Another option might be to use a premium thinset like Custom's new MegaLite. It claims to be able to handle cracks of up to 1/8" without a membrane.

Either of these might give you a measure of protection from expansion and contraction without going the full Ditra route.
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