any non-cementitious material between your cables and your slab will lessen the amount of thermal energy that the slab absorbs. Imagine you wear no clothing outdoors, or a thin layer of clothing, or a thicker layer of clothing, or two layers, or three, or... this is a perfect analogy. You choose if

but, first Q:
what is the slab on top of?

in other words, choose whether you want to warm the slab, or don't mind...
or whether you want to lessen the amount of thermal energy that the slab absorbs.

If under the slab is a ceiling in a lived-in area, and if the building in general is well insulated, you don't lose heat... you just send warmth into the bones.

Usually, people like to know that the energy from the cables is going more into the tiles on top than the slab underneath.

Hope this helps.

If the cork can be compressed at all with pressure, I'd think long and hard before installing tile over it...

cork has been used for a long time. Web search cork.

No argument about the long and widespread use of cork David. The tile pros over on an un-nameable tile forum (JB) seem to go both ways on this topic.

With regard to the use of Redgard as a thermal break - they all seem to agree that Redgard or similar waterproofing membrane won't affect the heat transfer in any appreciable fashion.

It's an interesting topic - heat transfer and heating properties of solid materials. I'm wondering just how much of the heat would be absorbed by the slab, what the additional heating load requirements would be once the slab reaches its heating equilibrium point, and the old issue that you yourself have pointed out in other threads - heat is heat is heat.

Thank you David and Dseng. The slab is the foundation of the house. I do hope to avoid heating the slab. What do you think about using 1/8" inch cork and Red Guard? 1/4" cork seems like it might be too thick. I'm not sure how these products would compare to Ditra.

In answer to the OP's question re: Ditra Ditra will not act as a thermal break. In fact Schluter recommends installing Ditra on top of electric heat mats.

e.g. Plywood sub floor > heat mat > optional SLC > thinset > Ditra > thinset > tile

Works great - my feet enjoy a bathroom floor done exactly this way every morning.

kendog2, you have a concrete subfloor not a wooden subfloor so disregard dseng.

yes you may use Ditra as a shield against side-to-side cracks opening and yes you may install heating cables on top. There are installers doing it. Many years ago i saw an installer's web site that showed it. I have no idea if Ditra will one day be recommended or "sanctioned" by its manufacturer for this specific manner on install, i.e. of layering the heat cables on top. Don't sweat it. Yes, study more, search more, read more, but don't be concerned in any big way.

Other companies make other products, that are good as anti-crack membranes to go on top of concrete. Noble is one. Custom Building Products is another. Web search these companies with the word membrane or underlayment. Hint hint. The Ditra manufacturer is very good at getting internet people to write about Ditra. Hint hint. Hope this is clear. The other membranes are like rubbery sheets. Cork is even better.

I wouldn't use cork on the slab under your floor heating.

First, you may not need anything at all. I used a NuHeat mat on an uninsulated slab (below grade, not on-grade) with Ditra on top and it heats perfectly well. How much energy are you going to lose in a smaller bathroom slab? Presumably you will put the floor heat on a programmable thermostat that allows it to run only a few hours per day in the winter months.

Second, if you do want to insulate the slab (and I can understand this) use a better insulator than cork and something that is made specifically for tile. Use either Wedi-board or Kerdi-board and thinset it to the slab -- both have foam cores that insulate well and are designed for floor tile loads (and wall tile). Wedi-board has a cementious coating and Kerdi-board has Kerdi waterproofing. The foam will help retard water vapor migration from the slab up to the floor heating cables and temperature sensor. After you lay the heat mat put Ditra on top (protects the heat mat in case you ever need to chisel out a broken tile).

Most importantly remember that very little heat flows sideways through the tile when using an electric warming mat. This means that you must be sure to place the heat mat under the cabinet toe-kicks and near the toilet, but keep it away from the toilet wax ring. The manufacturer will have a recommended distance for this.

Billy

Thank you all so much for taking the time to discuss the details of the different options available. It seems that there are so many ways of doing things and products that I was unaware of - sounds like there may not be only one "right" way. David, I'll take your suggestion to do a little more research because the shower tile will be installed first. I'm sure it will be weeks before we get to the bathroom floor.

Dseng and Billy, maybe you're right that it's unnecessary to insulate the slab. If not using insulation won't significantly affect our electric bill or the temperature of the floor, then we probably shouldn't bother. We live in the High Desert area of southern California. Very little moisture here and low temperatures are almost never colder than 22 degrees in the winter.

Thanks again!

Our slab was below grade and the ground temperature stays around 55 degrees. If you have a slab on grade it will get much colder, so keep that in mind.

Also consider the age of the house and whether you have insulation under your slab.

Billy

kendog2, you wrote: "Dseng and Billy, maybe you're right that it's unnecessary to insulate the slab..." This is wrong. Both wrong. The first guy was assuming you had a wood subfloor, not a slab. So take back what he said, disregard it. The second guy said to insulate with something even bigger and better than anything yet mentioned: foam boards known as Wedi and Kerdiboard. billy_g did write that maybe you wouldn't need to insulate at all. BUT you have a crack in the slab. So, some crack isolation membrane is very very good to have, or else a crack will occur in the tile! Do not tile onto the slab. Use a membrane.

B.t.w. the shower needs a membrane, and it has to be sloped too.

Billy,

I think the slab is on grade but I don't know for sure. The house was built in 1990. I have no idea if there is insulation under the slab but ince this is southern California, I'm guessing there is none.

Thank you David. I do recognize the need to use a membrane under the area that is cracked. I just wasn't sure it was necessary for the entire floor. However, one thing I hadn't considered is that, aside from the heat loss issue, we live near a fault line. It would probably be wise to use a membrane to reduce the risk of tile cracking during an earthquake.

The shower is covered with Kerdi membrane. The floor is sloped but we haven't yet installed the Kerdi on that area.

"If you have a slab on grade it will get much colder"

That depends son how far from the perimeter of the slab the area is.

If the BR is many feet from an edge underneath it will have a rather stable temperature.

a membrane on top of a slope. Good.

Summary. Cementing heat cables onto a slab = the heat ends up mostly in the slab (a large heat sponge, heat sink, thermal mass) and some of the heat ends up in the tiles above the cables. The cables distribute the heat 50 50 half going up half going down, but then the warmed up tiles send their heat into both the room and the slab. The slab continues to suck heat out of anything warmer than it, until such time as (theoretically) the slab's temperature matches the other thing warmer than it. (that would be the tile surface). It never happens: the slab just keeps on sucking for ever and ever. The ground underneath the slab sucks heat too.

Suffice to say that any membrane will act as a heat transfer slow-down agent, i.e. an insulator and not as a heat transfer conduction agent.

if the slab is at a stable temperature, or if its temperature varies with the seasons, it doesn't matter. It's not relevant to the statements in this post.

Bickeyee, the bathroom is on the outer edge of the slab. David, what you're saying makes a lot of sense. I'll do more research to decide what type of membrane to use. Thanks so much.

Interesting banter , but I think all you "pros" missed the most important element in this equation:

"where there is a vertical crack in the foundation. There's a drop of about 1/8 to 1/4" so we plan to use a leveling compound before we begin any other work"

ken - how large is the crack AND are you saying that there are two parts of the slab that have up to 1/4" of difference in height ???

If so, you, probably don't want to put tile down at all! That is a huge crack/ cleft to cover and control. I haven't looked at the Ditra spec lately , but I'm pretty sure they won't warrant a 1/4" gap- nor is any other product.

Do you know what's causing the slab to crack or move? Have you remedied that cause ???

Oh, better check the specs on the leveling compound too! Some of those don't play well with other tiling / adhesive products .

Clean crack, isolation caulking first.

syntrio, hope they figured it out in the last 5 years, lol.