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Old 07-09-2019, 08:06 PM   #1
paredown
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Electric heat mat with dry pack mud

I'm helping a friend with a kitchen remodel. We have an out-of-plane floor.

The electrician set the mat on the plywood, and the tile guy has gone over the floor and leveled with a drypack mud layer with mesh reinforcement.

The plan is then to thinset and lay tiles on the drypack.

Two questions:
1. is this procedure likely to be durable?

(I could not convince the homeowner to use Ditra for crack isolation, or Ditra Heat for a tidier install--it is some mat system.). Floor is 3/4" ply screwed, 2x10 16"OC for base, span I would have to check).

2. Given that it is already done, does it matter where the heat sensor is placed?
The tile guy set the sensor in the drypack, between wire runs. The homeowner/general thinks that it should have been placed in the thinset layer, directly under the tile.

Will the lower placement make any difference to the functioning of the floor heat?

(Long time no post--I've been doing other stuff and not messing around with tile....)
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Old 07-10-2019, 06:01 AM   #2
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How thick is the drypack, Dean?

I'd want to go back to the manufacturer of the mat to find out if the mat will function correctly under whatever thickness of drypack there is. No idea how well drypack conducts heat. If the sensor is closer to the heat wires it seems to me the drypack will reach the set temp before the tiles ever do, and in general I think it'll take quite a while for the tile to reach the set temp. That mud is going to act like a huge heat sink.
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Old 07-10-2019, 08:11 AM   #3
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Quote:
Originally Posted by Dean
...tile guy has gone over the floor and leveled with a drypack mud layer with mesh reinforcement.
As Dan asked, how thick is this mud bed and is the "mesh reinforcement" actually expanded metal lath fastened to the floor over a cleavage membrane or is it a welded wire mesh placed in the vertical center of a mud bed at least 1 1/4" thick over a cleavage membrane?

And I agree with Dan that the only useful answer about the placement of the temperature sensor must come from the manufacturer of the heating system.

Properly prepared deck mud is not a particularly good conductor of heat as would be a more dense masonry product or natural stone or ceramic tile.

My opinion; worth price charged.
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Old 07-10-2019, 05:02 PM   #4
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I was busy doing outside work, so I did not see the full mud bed laid, but my guess is that it ranges from 1.5-2" at the lowest corner, to around 3/4" at the thin edges--we did a rough check with a laser before they started.

The 'huge heat sink' part is probably the most worrisome, since we also took out the hot water radiator to re-jigger the cabinet layout.

The lath (which I did not see them set) was the simple expanded metal style (not welded). I'm assuming that they stapled it?

I'll see if I can figure out the mat brand and check the thermostat specs.

It's one of those situations where the owner was keen on using a tile guy that she has worked with as a GC; personally, I would have been much happier with a careful check for deflection, SLC to level and Ditra for crack isolation like youse guys have taught me...

Thanks for the answers as always.
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Old 07-10-2019, 05:29 PM   #5
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Ditra is not a crack isolation product, Dean.
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Old 07-10-2019, 08:06 PM   #6
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CX, I misspoke--or typed faster than I was thinking. Chapter and verse clarified my thinking:

"Tile has been successfully installed for thousands of years by incorporating an uncoupling layer, or forgiving shear interface, within the tile assembly. Schluter®-DITRA membranes provide uncoupling through free space, which allows for in-plane movement that effectively neutralizes the differential movement stresses between the substrate and the tile, thus eliminating the major cause of cracking and delaminating of the tiled surface."

Stated much better, me thinks. And i do appreciate the correction for sloppy thinking/writing.
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Old 07-10-2019, 08:11 PM   #7
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Just keep in mind, Dean, that the ceramic tile industry has no standard for "uncoupling Membranes." A manufacturer can make any claim he wants for that type of product. The only actual requirement is that it must be capable of a shear bond to the substrate of 50 psi or better.

Crack Isolation membranes, on the other hand, do have a ceramic tile industry standard (A118.12) so you can more accurately select a product for a particular application.
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Old 07-11-2019, 06:24 AM   #8
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From my heated floor shopping experience, Dean, all the residential grade floor warming t-stats are going to be quite similar, as will, too, the heat cable output. One can only run just so many amps through it.

My guess is that the HO will find it takes a loooong time for the tile to reach the set temp. And it may never, depending how deep the sensor is in the mud bed.
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Old 07-11-2019, 09:26 AM   #9
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Most of these heating cables are about 3 to 4 watts per foot or 10 watts per square foot. For example my 26 sqft 80 linear foot heating strip was only 240 Watts. Most baseboard heaters are going to generate a thousand Watts. I think the temperature sensor anywhere in the middle of this mud-bed, tile mass is going to be fine. I think it will take 3-5 hours to heat up this floor so you may have to keep it on all the time if you want warm floor at breakfast. Sure the temp sensor might be higher temperature than the tile surface but you can take that into account by setting a higher set point on the thermostat. The main thing is this temperature probe will provide an input for the temperature control circuit. I did not plan on my 240 Watts adding any heat to the room just taking the chill off of the floor by the toilet and in front of the sink and maybe when you step out of the shower.
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