Schluter on phone told me that can only use Ditra if the hydronic tubes are BELOW the plywood, but I didn't ask why.

The reason why is if the heat tubes are just below the Ditra, as they would be with Quik-trax, or similar systems, there's not enough support directly beneath the Ditra for the tile. Keep in mind-- Ditra's basically a flimsy piece of plastic. It has no structural strength at all. If there's give underneath it, you're going to have a problem with tiles cracking. As for heat transfer from under the plywood, most good systems ARE mounted from underneath these days. Additionally, the water lines are about 2-3 times the size of the ones used with the kind of system you're talking about.

Thermal track has an aluminum face. Are the Thermal Track reps telling you that you can thinset the Ditra directly to the aluminum and the thinset will adhere to the aluminum?

As to the PEX expansion, the track will contain any linear expansion of the PEX. But if the grooves are grouted, then I could see tangential expansion of the pipe cracking the grout. I don't know if there would be enough movement to pop tiles, as the thickness of the Ditra might absorb the expansion.

Personally I wouldn't tile directly to the Thermal Track.

As to the PEX going below the subfloor, yes, it's a fine application. You'll usually make two runs of tubing per 16" joist bay, then add insulation beneath the tubing. I use foil-faced rigid polyiso insulation. I shoot for a minimum of three times the R-value below the tubing than the R-value above the tubing.

If you have hydronic baseboard heat, realize that typical baseboard water is in the 160-180 degree range. For radiant floor you'll want that dropped to roughly the 110-120 degrees.

Mongo

Mongo, I *think* warmboard is the one with the aluminum face. With thermal track [which is actually made by a couple different companies, and comes with different flooring rules from each], the aluminum is under the ply.

The warmboard has a greenish coating on it, allegedly allows stuff to stick. It's the one that's supposed to replace the subfloor instead of just going over it.

If Thermal Track is made by a couple of companies, that explains it. The brochure I have shows an aluminum face and their recommended installation for tile is cement backer board over the Thermal Track, then an iso membrane on that and tiling on the membrane.

Warmboard itself does "allow" you to thinset Ditra directly to the Warmboard, but Schluter won't warranty an installation like that. Schluter recommends a transition material between the Warmboard and the Ditra. Warmboard used to have a bit of double-speak on their website and in their literature. Not sure if they are still that way. They first tell you to install in accordance with TCNA guidelines, etc, or IAW the manufacturer's instructions, etc, but then they give you shortcuts around those guidelines and say "they've never heard of a failed installation" when taking those shortcuts.

For those not familiar, the big difference between Warmboard and Thermal Track is that Warmboard in an of itself is a structural subfloor and radiant track combined into one material. Thermal track is installed over an existing subfloor.

I installed Warmboard when it first came out. Nice product, but not worth the $$$ or added steps required during installation or in protecting it after the installation. At least not in my book.

Mongo

Thanks Bill, Mongo and Oruboris! It sure looks like we'll just keep it simple and safe and put the pipes below the plywood.

Mongo, can you clarify two things for me?

1. You stated that the radiant floor requires a lower temp for the water (110 approx vs. 160 approx) than our rest-of-house baseboard heat. Is there something extra that needs to be done by the plumber due to this, or are you just letting me know? (As this floor heat would have to have its own zone/ own zone valve.

2. You mentioned R value of insulation below PEX should be 3 times the R value of the insulation above. Do you mean that some insulation is placed above the PEX (but below the plywood), or is the "above" referring to the R value of the plywood, Ditra, various thinsets and tile? If the latter, how would I know what that R value is?

Thanks!!

Lainie

1) If your plumber is familiar with rfh, he'll know what to do. Sometimes it's a heat exchanger to pull the heat off the boiler, then a mixing valve to add cold to the hot to drop the temp, then a t-stat and circulating pump to move the water through the loop. For below floor, use half-inch PEX.

Dos) Yes, the "R above" is the subfloor, underlayment, backer board, tile...wood is about R-1 per inch. Most wood floor or tiled assemblies are about R-1.5 to R-2.5. If you use 1" of foil-faces polyiso (R-7 per inch) that'll work just fine.