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Old 03-16-2019, 09:57 AM   #1
Gradyy
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pex retrofit over slab under tile

I have a 2000 square foot insulated [under and sides] slab with a 6 mil vapor barrier. I want to add pex and tile and keep the total thickness to a safe minimum. What are my options? I don't really want to go with gypcrete. If I use an additional 6 mil separation barrier between the existing slab and the new mud bed, will I have any issues with the double vapor barrier? The existing slab is well cured - 1980's.

Any thoughts or suggestions are appreciated.
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Old 03-16-2019, 11:10 AM   #2
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Welcome, Grady.

A geographic location in your User Profile will help in answering some types of questions.

I presume we're talking about a Slab On Grade concrete foundation "slab?"

Where is the polyethylene in your current setup?

What insulation is in place?

This is PEX tubing you're considering adding? For hydronic heating purposes?

Lemme stop there 'till we're sure what you're doing.
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Old 03-16-2019, 06:52 PM   #3
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Yes, slab on grade. two rooms, a 30x40 and a 24x24 that share a 10' opening.

slab is 15" thick for 4' wide around the perimeter and where the two rooms join
8" thick everywhere else.

6x6 wire and rebar reinforcement

2" rigid foam insulation under and around slab

6 mil poly over foam, under slab

6" sch 40 pvc trunk line in rear perimeter "thickness" with 4" pvc branch lines from FHA wood furnace to floor registers.

Hydronics was unknown in this area in 1980 - to me, anyway so Yes, mainly for heat but I thought I'd add a domestic supply line or two as well.
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Old 03-16-2019, 08:14 PM   #4
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Once again, Grady, references such as "in this area" mean a great deal more if there is a geographic location in your User Profile, eh?

'Specially when there are references to "FHA Wood Furnaces." It's always been my understanding that the FHA would not accept a wood burning appliance as the sole source of heat in a home.

And that very interesting foundation slab design. Also something I've never encountered.

But to your question. Without knowing what you mean by, "I want to add pex and tile and keep the total thickness to a safe minimum," I'm not sure you're gonna like any of the options. The absolute thinnest industry accepted method I'm aware of is to encapsulate the hydronic tubing in a self-leveling underlayment with the minimum thickness 1/2" above the top of the tubing. That is a lot of SLC and there are no inexpensive versions of that product.

Second thinnest would be encapsulating the hydronics in poured gypsum with a minimum of 3/4" above the top of the tubing. That method also requires the use of a membrane over the gypsum.

Most reasonable, best performing, and probably least expensive would be a reinforced mortar bed over hydronics pre-filled using the same mortar (deck mud, in the vernacular). Requires a cleavage membrane over the filled hydronics, then a minimum of 1 1/4" of deck mud with welded wire reinforcing mesh in the vertical center of the mud.

Any of those fitting into your ballpark expectation of "safe minimum" thickness?

My opinion; worth price charged.
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Old 03-16-2019, 10:34 PM   #5
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I did update my profile but I can't even manage to find that page now to see what the issue is. In any case I'm in the New England area - central NH to be exact.

By FHA, I was referring to forced hot air - sorry, I was trying to be brief.

My atypical slab is a "floating" slab on the idea of an "Alaskan" slab. Around here 4' deep 8" thick concrete "frost" walls on a concrete footing are typical for slab construction. The slab never cracked except for very small hairline cracks in the areas where the screed supports were removed and filled.

I'm resigned to not liking how ever thick the floor needs to be. Oh well.

What about 1 1/2" of concrete made with a small aggregate like 3/8 pea stone or even smaller like "rice" stone to encapsulate and cover the pex and then the uncoupling membrane and thin-set?

Someone suggested a 6 mil poly under everything to keep the layers from bonding as they may expand at different rates and cause the thinner top layer to crack.
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Old 03-17-2019, 06:24 AM   #6
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How about Schluter Bekotec.

https://www.schluter.com/schluter-us...OTEC/p/BEKOTEC
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Old 03-17-2019, 08:36 AM   #7
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Grady, click on User CP in the dark blue bar near the top of the page, look to the left for Edit Your Details, scroll down to Location.

You can use anything you want to pour around your hydronic tubes, but only if you really like cracks in your tile would you try to pour any kind of concrete there. Can't guarantee the cracks in the tile, but I'll be happy to guarantee cracks in the concrete and would bet very heavily in favor of the cracked tiles.

The methods I pointed out are what the tile industry finds acceptable. You don't wanna do that, it's certainly OK with me, but the reinforced deck mud method would be my first choice. And I would want some type of crack isolation or uncoupling membrane on top of whatever I used.

My opinion; worth price charged.
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Old 03-17-2019, 08:56 AM   #8
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Welcome aboard, Grady.

The other thing is that it's impossible to finish poured concrete to the smooth flat surface that can be had when using deck mud (dry pack).
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Old 03-17-2019, 04:19 PM   #9
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CX, the main reason I was thinking about concrete is just familiarity with it - although I never did a thin over-pour. My tile setting has been limited to thinnest on cement board. I have zero experience with a dry pack mud bed so that wasn't going to be my first choice. I did see RH-110 that said 3/4" minimum of concrete over pex tubing.

Why would the concrete crack in this application? Because it is thin? because the radiant heating would make it move? Even with a membrane between the existing slab?

I came here because I'm looking for answers - not because I think I already have them. I appreciate the frankness.

Maybe I better study up on mud beds.
Any hints? - or is that worthy of it's own thread?
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Old 03-17-2019, 07:15 PM   #10
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1. You'll note that RH110 presumes your hydronics are embedded in your structural foundation slab on grade. For you that ship has sailed.

2. Yes, and yes, and yes. The very thinnest recommended thickness for poured concrete is two inches and you'll find that that will end up with some cracks, too, even without an embedded heating system. I've tried it for stained concrete in remodeling situations, with reinforcing mesh and control joints cut in withing 24 hours and it cracks. Looks fine for a stained concrete application, not so much if a ceramic tile installation over it cracks, too.

Could you pour two inches of concrete over your hydronics, let it cure for a month, install a full coverage crack isolation membrane and get by with it? I dunno, maybe. Would I try that? No. See post 8 for another good reason. Keep in mind that while it is possible (rarely seen or done, but possible) to finish poured concrete to a flatness of 1/4" in ten feet, if you're planning large format tiles (more than 15" on any side) you will not be able to finish your concrete to the required flatness of no more than 1/8th" deviation from intended plane in ten feet nor 1/16th" in two feet. Ain't gonna happen. With deck mud you can do that, though.

3. It's worthy of lots of threads, all of which we have. Type "mud work" (including the quotation marks) into the Advanced Search feature and ask for Titles. You'll find some to start with. Might wanna pack a lunch on accounta the top one on the list has several thousand posts.

[Edit] Here's an an old article from our own Dave Gobis that'll give you a little history of the art. Not sure you can open it without registering, but it's the only place I could find it.
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Old 03-17-2019, 09:43 PM   #11
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Thanks for the link - interesting reading - has me rethinking things.
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Old Today, 12:51 PM   #12
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F111 says: " where radiant heating pipes are laid over
the slab, screed fill flush to top of pipes before placing a cleavage membrane and reinforced mortar bed."

I assume that means with the same mortar?
No membrane between that and slab?
How to keep reinforcing wire in middle of bed - chairs?
How long would it take an experienced mudder to do 2000 sq. ft.?
What multiplier would you use for a newbie?

3/4" for the tube screed plus the 1 1/4" minimum for the mortar bed makes 2" plus the height of the tile would give me a finished minimum thickness of about 2 1/2".

RH110 doesn't state a minimum slab thickness, just a 3/4"minimum over the tubing. Other concrete regulations state a minimum thickness of 3 times the diameter of any pipe embedded in it so about the same thickness.

Maybe I'll try the mud and tile in the kitchens and baths and some decorative concrete in the large areas. I'm still undecided.
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Old Today, 01:44 PM   #13
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1. Yes. That would be method RH117 in your Handbook and you'll find all the details there.

2. See #1. I would personally be inclined to bond the first mud layer to the slab with a slurry of thinset mortar or pure Portland cement.

3. Start with it on the floor (on top of your cleavage membrane in this case) and pull it up into the center of your top mud bed. By eye is the only method I've ever used to fix its location.

4. No idea.

5. See #4.

My opinion; worth price charged.
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