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Unread 02-11-2022, 03:31 PM   #1
MesaTileworks
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Thinset/Primer for Warmup Underlayment

Howdy Folks,
I’m getting ready to help a buddy install some Warmup DCM-PRO underlayment for a heating system. It’ll be tiled with 3” marble hex tile. Warmup says modified *or* unmodified thin-set above and below the underlayment is fine. Any thoughts on mortar to particularly use or avoid with this product, given that the floor will be heated? I have on hand some Mapei modified mortar designed for ceramics, but can get a pretty good range of other products from my local shop.

Related question: is there any point in priming the subfloor (BCX ply) prior to putting down the underlayment and if so what kind of primer would y’all recommend?

Thanks!
-Matt
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Unread 02-11-2022, 03:40 PM   #2
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Matt, when you're bonding to a plywood subfloor, you don't have any choice at all. It must be a modified mortar meeting ANSI A118.11. Period.

I know of no reason for a primer in your application.

My opinion; worth price charged.
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Unread 02-11-2022, 03:48 PM   #3
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If the manufacturer says either modified or unmodified will work I'd be inclined to believe them. Schluter says the same for Ditra Heat; modified that meets ANSI A118.11 or unmodified that meets ANSI A118.1. I used an ANSI A118.11 modified to install my Ditra heat.

I wouldn't bother priming the ply, but I would dampen, but not wet, it a little with a sponge just prior to combing on the mortar.
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Unread 02-11-2022, 06:30 PM   #4
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Schluter says you can install Ditra Heat over plywood with other than an A118.11 mortar, Dan?
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Unread 02-12-2022, 07:46 AM   #5
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They do, cx, using an unmodified mortar meeting A118.1. Same when installing regular Ditra.
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Unread 02-12-2022, 09:36 AM   #6
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Need to submit a correction; While Warm Up may say that an unmodified mortar can be used to install their mat over a plywood subfloor, Schluter does not say the same for their product.

On page 4 of the Ditra Heat installation guide, under "Setting and Grouting Materials", they list modified and unmodified, but in detail DH-W16-T-16 on the left side of the page it clearly shows "Latex p.c. mortar" (modified) should be used to install the product to an OSB or plywood subfloor. Page 4 of the guide for regular Ditra uses slightly different wording but is the same message.

Thanks goes to CX for fact-checking!
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Unread 02-12-2022, 07:45 PM   #7
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Modified it is the and I won’t bother priming but will bother thorough cleaning and then dampening the floor. That seems like a good idea. Thanks for the advice as always, gentlemen!

Just finished the niche and excited to get started on the floor…

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Unread 02-12-2022, 08:36 PM   #8
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And for bonding to plywood, the requirement if for the mortar to meet ANSI A118.11. Don't know if there are still any A118.4 mortars that don't also meet A118.11, but if there are, they are not what you need.

Schluter does make an exception when installing their foam shower trays and allows an un-modified mortar (ANSI A118.1) to be used over wood subfloor. I believe the thinking there is that the thinset mortar is more of a bedding material in that application than a bonding mortar. The indicate that they are being helpful and not requiring two types of mortar for a shower installation with their products.

My opinion; worth price charged.
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Unread 02-16-2022, 10:16 AM   #9
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Related question: for the mortar that will bond the tile to the Warmup underlayment, are there any products that are particularly good for a heated floor? My understanding is that the changes in temperature subject the whole assembly to stressors that are not present in a “normal“ unheated floor. If there were a type of mortar that was particularly good in this situation I would go for it. Otherwise I will just trust the Mapei representative who I spoke to yesterday who said that their modified mortar is suitable.

(The tile that will be set on the floor are 3” hex stone and with. 4x12” stone border.)
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Unread 02-16-2022, 02:02 PM   #10
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DCM pro is an uncoupling membrane, right? so that will isolate your tile from the underlayment.. I think most buckling worries are due to wood/underlayment expanding/contracting due to moisture changes and not heat.

Tile/cement conduct heat pretty well so there wont be much in terms of thermal gradients.

From my own heated tile floor use, I'd say you're unlikely to heat your floor above 100F, and your 'hot' shower water is likely around 105F.

I think the max limit on electric wire heat is around 120-130F on the controllers; if there were any limitations for thinset mortars, your heating cable installation manual would have instructions/warnings.

People use similar thinset mortars on fireplace surrounds (which are hotter) without issue, you could seek out guidance for thinset mortars specifically recommended for tiling near heatsources, but I think that's unnecessary.
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Unread 02-16-2022, 03:02 PM   #11
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I'm guessing that system you're using, Matt, is one in which you wind the wire through the profile of the mat, similar to DitraHeat? If so, I'm not at all sure it's the same thing at all as having a membrane of some sort on tops of the heating system to protect the tile installation. Especially since we don't know what the "uncoupling mats" do and there is no known testing for their capabilities.

Be that as it may, I don't think any of those radiant heating systems produces enough heat to have any meaningful effect upon the thinset mortar, so long as the mortar is allowed to cure before energizing the heating wires.

Quote:
Originally Posted by Phil
Tile/cement conduct heat pretty well so there wont be much in terms of thermal gradients.
Having seen infrared images of electric radiant heating systems in use, Phil, I wouldn't agree with that. Depending upon the spacing of the wires, you might have some pretty dramatic heat gradients in that floor.

My opinion; worth price charged.
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Unread 02-16-2022, 04:09 PM   #12
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Quote:
Originally Posted by PhilWA
Tile/cement conduct heat pretty well so there wont be much in terms of thermal gradients.
Quote:
Originally Posted by CX
Having seen infrared images of electric radiant heating systems in use, Phil, I wouldn't agree with that. Depending upon the spacing of the wires, you might have some pretty dramatic heat gradients in that floor.
Hey, everything's relative. Maybe more important would be the thermal expansion coefficient for the cement and tile.

Just so I'm not trying to mislead anyone, here's some comparative thermal conductivity (w/m K) per google:
fiberglass insulation 0.043
wood 0.12-.04
concrete 2.25
steel 45
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Unread 02-17-2022, 07:38 AM   #13
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Quote:
Originally Posted by cx View Post
I'm guessing that system you're using, Matt, is one in which you wind the wire through the profile of the mat, similar to DitraHeat?
Yep. It’s similar to Ditra Heat. Plastic underlayment and the wire gets snapped into grooves in the mat.

If I’m reading y’all right, the heat generated by the mat is not likely to be enough to compromise the thinset over time. The last one of these that I installed was a bit of a different setup and had a SLC poured over the wires, so I was just curious about the mortar.

Thanks y’all!
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Unread 02-17-2022, 07:46 AM   #14
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I believe I've read here, somewhere, that one manufacture, perhaps Laticrete, offers a mortar specific for use with a tile warming system. I don't believe it is touted as having a better bond, but more that it helps distribute the heat more evenly. Of, wait, here it is. Not a mortar, but an additive for use with some of their mortars. https://floorlife.com/products/latic...SABEgJY__D_BwE

The Schluter branded floor controller I'm using tops out at 104*.
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Unread 02-25-2022, 11:40 AM   #15
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Grout Question

After confab with Warmup and Mapei and taking y'all's advice, I think we have the mortar question sorted. Thanks!

The next question is about grout. I'd planned to use Laticrete Spectralock (have had good experiences with in the past once I learned how to work with it), but Warmup recommends a "flexible grout"—they don't have any more specific recommendations. My sense is that Laticrete epoxy is strock but provides zero flexability.

The fella at the local tile shop suggested either Mapei Flexcolor CQ or Laticrete's urethane-based Ready-To-Use grout. I have used Flexcolor once before and it was OK, but I wasn't as happy with the result as I have been with Spectralock.

Does anyone have thoughts on either of these grouts?

One possibility would be to use the Laticrete urethane grout over the warmup and epoxy everywhere else, but obviously, there might be issues matching the color and texture?

Open to suggestions.
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