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Old 01-19-2008, 01:13 AM   #1
cwp
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Preventing heat loss through a cement slab and other questions

I am going to install 400 square feet of 18 inch porcelain tile, and an electric floor warming system, over a 30-year old cement slab that is currently partially covered in carpeting. My goals are to maximize heat in the rooms at a reasonable price, as well as plan an installation that will allow level tiles for a DIY'r like me, and of course, an installation that will last several years without cracking. I want to prevent heat loss into the cement slab and maximize heat in the room because there is no other source of heat in the rooms (living room and kitchen) besides a wood burning stove. I live in the northern California foothills where the outdoor temp rarely drops below 32 degrees F. I have researched the options until my head spins. I would love it if someone could recommend what I should put on each layer, from the slab up to the porcelain tile, to make this work, that overcomes the problems I address here, and doesn't cost a fortune. Here goes: OVER THE SLAB AS AN INSULATOR: Cork is fairly inexpensive but per the manufacturer, requires backerboard above it if the tile size is greater than 12 inches; backerboard alone doesn't make much sense as an insulator because it is made of the same stuff as the slab, I don't trust just using thinset to attach it to the slab, and using power nails into the slab seems like overkill and the heads might protrude into the heating wire; Ditra sounds like it might work instead of backerboard but the makers of Ditra admit it has no thermal properties and it demands unmodified thinset so it is incompatible with what most porcelain manufacturers recommend for setting porcelain tiles with modified thinset; Wedi board is very expensive and has not been tested with self-leveling cement over it, which is the only way I can insure my tiles will have a level surface given that I will be installing an electric warming system; FLOOR WARMING SYSTEM: Cable is a lot more reasonably priced than mats, and as a DIY'r, I have more time than money, so I'd rather use cable, but the most reliable product I can find (Flextherm) only produces 12 W per square foot, while some mats produce 15 Watts. Will I have enough heat with 12 Watts per square foot? OVER THE FLOOR WARMING SYSTEM: I have read on other threads that some people have had trouble leveling the floor with the usual process of thinset over the cable/mat (even with the 2 step process). I therefore like the idea of self-leveling cement better, but it also seems to have its problems - how do you quickly apply self leveling cement to 400 square feet? Can you do it in pieces? Besides, self-leveling cement seems to be about 4 times as expensive as modified thin set. Instead, I have seen some suggest that you can or should put Ditra over the cable/mat before you lay the tile - why? Is that necessary? Does that help to level the floor before applying another layer of thinset and then the tile? And if you do, once again, there is the problem that Ditra requires unmodified thinset while everyone (except Ditra) insists that porcelain requires modified thinset. I am confused and frustrated. We are getting to the point in our project where we have to make these decisions, and quickly. Please help.
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Old 01-19-2008, 09:32 AM   #2
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Custom Building Products makes a product called "easy mat". Looks like little balls of styrofoam. It will offer an r value and will not require epoxy grout like the cork. Use the thinset version, not the peel and stick version.
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Old 01-19-2008, 10:04 AM   #3
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I used flextherm in a small bathroom and it worked well (still happy with it). I believe that flextherm has a couple of different recommended spacing options for the heating cable. Is it still only 12W / square foot if you use the tighter spacing?

We used SLC (small room and small mosaic tiles), but based on your constraints I would (if I were you) try to get by with the thinset only approach. Since you have both a large area and large tiles to work with, you should be able to refine your technique to get good results with some patience and practice. Doing SLC in a large area is tricky (and expensive), and there are plenty of stories of people messing up their first attempts. With the thinset method, you can take your time and work on a small area first.
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Old 01-19-2008, 10:48 AM   #4
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Thank you to both of you for responding. I thought of Easy Mat but my local Home Depot stores don't carry it except a few in the 3mm small sheets. I called Custom Building Products, the manufacturer, and they recommended that I use the 12 mm stuff for my purposes. I told them I couldn't find it locally (Sacramento area) and they said they would get back to me with a vendor who sells it in my area, but they never did.

Flextherm still recommends a maximum of 12 Watts per square foot (2 Watts per linear foot laid 2 inches apart) for their Green surface wire. Right now, the only product they have for higher wattage is for embedding in concrete in new construction.

You may be right about avoiding the SLC - what you say about people messing up is exactly what I feared.
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Old 01-20-2008, 01:29 PM   #5
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Since you are using big tiles over that radiant floor, your tiles aren't going to be sensitive to the same small bumps that a 1" or 3" tile would be. In your case, based on what I've read so far (doing a heated bath myself with a mix of tile size), you may be OK with thinsetting your tile directly to the wire. You can take an extra step and thinset the wire first then set the tile over that, but it doesn't seem to me that you need to.

Another product to look at is WEDI. It is a closed cell extruded ridgid foam board with a cement face on both sides. You treat is like regular cement board, but it has a good R value (R4 vs. cork at R2). It's waterproof too, if you need it to be.

http://www.wedi.de/usa/products/buil...nel/system.php

This is there big partner in your area:

GSTA - Genesis Specialty Tiles & Accessories
8324 Luzon Ave
Sacramento, CA 95825
Phone: +1 916.387.8500
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Old 01-20-2008, 01:45 PM   #6
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Welcome, cwp. Please give us a first name to use. If you go by those initials, put that in a signature line by going to UserCP, finding Edit Signature, and entering it there.

And please, please, please break those long posts into a few paragraphs to make it a lot easier to read.

Custom has a new and much different Easy Board now and I don't know if you'll be able to find the old stuff you're looking for. That may be why the delay in their response. And I don't know if the new Easy Board is indicated for thinset installation over concrete. Wedi is, if you can find it.

Most of the heating system manufacturers don't recommend using any such insulation at all. It seems like a good idea to me, but the actual test results (I think Noble Company has some?) make a fella wonder if it's worth the bother at all.

Tiling directly over the open wire heating system, as suggested by xxPaulCPxx (who will also please enter a first name in his signature line) is not at all likely to bring you any measure of success. That's a tough installation for an experienced tile mechanic. I think you'll be wise to find some way to fill over the wires first, either with SLC (done successfully by many of our DIY visitors) or with thinset. Getting a suitably flat surface with the thinset, especially over a large area, is also a very tricky proposition.

My opinion; worth price charged.
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Old 01-21-2008, 02:43 AM   #7
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From Michael Byrne's JLC Sept 2004 article entitled "Laying an Electric-Radiant Tile Floor"
Had tiles 10 inches or larger been specified for the floor, I
would have installed the tiles directly over the electrical resistance
matting with a medium-bed latex-modified thinset
mortar, and saved the time and expense of encapsulating the
matting with a self-leveling compound (SLC). At 10 inches or
greater, a single tile can bridge over three or more resistance
wires, which helps to keep all the tiles on a smooth, even
plane. Smaller tiles, especially those that may only be able to
bridge one wire at best, tend to tip to one side or the other,
creating an uneven surface finish.
There are a number of DIY people here who've had success doing SLC on a first time pour. I don't think a large pour like a kitchen will go well for a first timer.

Tell ya what, the easiest way to see if you need to pour SLC is to test a thinset only patch yourself. Go to Home Depot and find some crumby 18" tile, and pick up a sheet of drywall too. While you are there, find some rope, twine, or even steel cable that is the same thickness as your heating mat cables (roughly). In your driveway, tack the cables down to the drywall in a similar pattern to your mat. You might need to buy a glue gun to help. Try thinsetting those cheap tiles directly over that. If your results look fine, you don't need to risk a SLC pour.
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Old 01-21-2008, 02:46 AM   #8
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WEDI
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Old 01-21-2008, 06:44 AM   #9
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ok CWP, this is what I think....

Installing radiant heat over a concrete slab is perfect. Don't give it a second thought about heat loss, it ain't a factor. The "heat" is actually the radiation that your body feels from the source and just a slight amount from conduction. Any conductive heat loss from the cables will only increase the heated mass and make you feel more comfortable. I'd also take into consideration that "heat" rises so anything under the mats is kinda a waste.

The total amount of watts for heating your home is the amount calculated from the National Electrical Code. If I remember correctly it's a figure based on W/sq.ft.

SoCal? geezzz...not a big deal dude, To give you some idea, I'm heating a 600 sq.ft. house in -30 degree weather with 12 Watts per sq.ft.! In your case the minimum Flexitherm recommends will be OK.

I'd use SLC. it's a "real" job to use but makes for easy tile setting. Before getting started ask for a few pointers here and don't sweat it (the sweat will come when you use the SLC )

- pete
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Old 01-21-2008, 08:21 AM   #10
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Quote:
Installing radiant heat over a concrete slab is perfect. Don't give it a second thought about heat loss, it ain't a factor. The "heat" is actually the radiation that your body feels from the source and just a slight amount from conduction. Any conductive heat loss from the cables will only increase the heated mass and make you feel more comfortable. I'd also take into consideration that "heat" rises so anything under the mats is kinda a waste.
Well, this isn't entirely accurate. The cable system conductively transfers heat to the tiles (the cables get hot, and transfer heat to the tiles since they are in contact with each other via thinset). The warm tiles then radiate heat to the rest of the room indirectly.

So CWP is right to be concerned about heat transfer to the slab (ie earth).

The question is, how much of the cable (or mat) output gets transferred to the slab versus the tile, and if this amount is relatively low, is there a payback to install insulation or not.

If I was you CWP, I would NOT install insulation, and here's why:

Building code only requires you to insulate a new building slab around the perimeter of the building (approx 24"). This is because there is very little heat transfer between the earth and your house in the middle of the slab...the earth under the house (at least the 1st few inches) tends to maintain a consistent temperature (a certain # of degrees or so cooler than your house). So I think that most of the heat ouput from the cables/mat will go into your house and not your slab.

A slab above a crawl space is a whole different story....
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Old 01-21-2008, 10:23 AM   #11
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Thanks, everyone, for all the advice. It really helps give me a perspective. Just to be on the safe side, I think I'll go with Wedi - I do have a local supplier - and that way if insulation helps at all, we'll have it. However, that means that SLC is out, since WEDI isn't tested with SLC (they WEDI rep said to try it would be "an expensive experiment."

Does anyone out there have experience with WEDI on a cement slab?

And as a first timer with the electric cable, I think I will go with the 2-step thinset method - too many of the pros say that is the way to go to ignore it. Does anyone have any tricks for leveling thinset over the cable? What about guiding a level over two equally raised runners on either side of a patch of thinset, and filling in the low spots, or something like that?

Thanks for your help.
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Old 01-21-2008, 10:34 AM   #12
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Well, I'll let you know the results of my "expensive experiment". I got the same reply from them last week - I bet you talked to Tom. I'm doing a small bathroom, 5x5 with lots of different tile sizes on the floor.

I look at it this way, in every way they want you to treat WEDI like it was cement board, except when it surpasses CB in capabilities. You can pour SLC over CB, I see no reason why you couldn't also do it over WEDI.
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Old 01-21-2008, 12:26 PM   #13
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Quote:
To give you some idea, I'm heating a 600 sq.ft. house in -30 degree weather with 12 Watts per sq.ft.! In your case the minimum Flexitherm recommends will be OK.
Figuring that runs about 8 hours a day

600 sq feet x 12 watts x 8 hours x 30 days = 1.73 million watts a month.

At $.10 /Kw that would be $172 a month to heat the floor? Yikes...
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Old 01-21-2008, 01:29 PM   #14
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IMO
Dump the cables and use a mat system like Nuheat. Save all the time of trying to get the cables on the floor and then pulling them back up when the catch on a heel or trowel and fighting the tile installation. Getting the mortar even over them and tile level
Mortar the pad to the floor and then set the tile right over that.
Eliminate the time and expence of Wedi, SLC, etc.
You do know that the under tile heating systems warm the tile and not the room?
Good Luck
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Old 01-22-2008, 06:56 PM   #15
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Hi Chad..

The figure of 12 watts per sq.ft. is accurate . Your assumption that the cables are energized for 8 hours a day is erroneous, the true number is dependent on outside temp and the insulation system of the house. To make it more clear, I use about 2kWh a month, which in Québec for 1700 sqft. of house is peanuts!!!

Doubts about the cost benefits of radiant heating? I'd be open to any of your thoughts about that (really).

I used cables over plywood but I'd probably lean more towards a mat system on a slab (without Wedi).

-pete
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