We are DIYers who installed a subfloor plate system in our remodel and are preparing to lay flooring, engineered wood in some rooms, tile in others. Our tile adviser tells us we need to put down a layer of plywood then thinset the cementboard to it, but our heating system designer believes the cement board can be directly glued and screwed to the subfloor. We'd prefer the cement board have direct contact with the tubes and fins, but don't want to build a system that doesn't adequately support the tile. Any advice or direction would be appreciated.

I'm a little confused about the type of radiant heat you installed.Is it like quick track installed on top of the sub floor or is it aluminum panels installed under the sub floor?

If it's the Quick track variety you can put the cement board right over the top of it.I wouldn't glue it down as adhesives may react with the pex tubing.Most guys use thinset and screw the edges.

The closer you get the tubes to the surface of the floor the better.An extra layer of plywood will only further insulate the tubing from the finished floor.

This sounds like it may be a Warmboard system. If so, the backer board is attached directly to Warmboard with screws and thinset. More detailed instructions are found on our website (warmboard.com) or you can call 877-338-5493. Regardless of system, John is basically right, little thermodynamic good can come from putting more plywood on top of the aluminum plates.

Thanks for responding. We have oriented strandboard sleepers over the subfloor with aluminum plates holding the PEX tubing. It's very much like warmboard only not quite so easy to install.

The sandwich method.Kind of a homemade version of Quicktrack.

I would put the cement board right over it with thinset,nailing it into the sleepers.I don't see any need for an additional layer of plywood.

well, how wide are the gaps between the sleepers? is the issue that the installer is afraid the cement board won't stay firm over it because the gaps are too wide?

Typically it's installed with a 1" gap between sleepers, and I've never heard any reports that this was a problem under tile that requires more than the cement board to provide stability. If your gap is wider, I don't know whether that would cause a problem or not.

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Northeast Radiant Technology, LLC
-=RFH Design, Supply and Consultation=-
RPA certified Radiant Designer
http://www.NRTradiant.com
rob@NRTradiant.com

Thanks, Rob.
Yes, it's a standard 1-inch gap. What happens when the metal plates heat and expand in such a system? Do they make a grinding noise against the thinset? Or does the thinset inhibit movement?

well, we use one of two (or both) methods to reduce expansion and movement in the first place; PEX-AL-PEX tubing in sandwich applications, and reset controls with dynamic mixing that minimizes water temperatures (this is good for efficiency and comfort and performance as well).

The plates themselves don't move a whole lot; the tubing is the major concern. PAP reduces this by 90%, and minimizing water temps makes it pretty much a non-issue as long as there is a *little* room for the tubing to expand at the bends.

I'm not actually sure if thinset is strong enough to prevent movement on its own, but I've never had a problem using the above methods to avoid noise/expansion problems.

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Northeast Radiant Technology, LLC
-=RFH Design, Supply and Consultation=-
RPA certified Radiant Designer
http://www.NRTradiant.com
rob@NRTradiant.com

We use reset controls on ALL our radiant systems and have NO issues with expansion noise.

From my experience the biggest issue with expansion noise is rapid change in temperature.When the temperature goes from 70* to 120-140* in 2.2 seconds there is a rapid expansion and noise.A reset control will raise the temperature slowly and eliminate any noise.

Reset usually has a lot of factors that recommend it, we use it ourselves on probably 95% of our systems, along with indoor feedback (thank you tekmar RTU's!)

However low mass sandwich with PAP really doesn't have much of an issue with expansion, temp rise or not, nor with flywheels, so when we do omit feedback/reset, those are the jobs we do it to without worry. Though the efficiency still makes reset/feedback pretty attractive on midsize jobs or larger.

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Northeast Radiant Technology, LLC
-=RFH Design, Supply and Consultation=-
RPA certified Radiant Designer
http://www.NRTradiant.com
rob@NRTradiant.com

A couple follow up questions to this post after seeing it. I am in the early stages of planning a radiant floor install where it will be also be a "sandwich" system. 3/4" OSB subfloor, 3/4" plywood sleepers with an "omega" shaped plate on top with the 1/2" pex in the slot. Spacing of plywood sleepers will be 3/4"at tubing. Questions:

1) I can get "Barrier Dura-Pex" made by CPI (Consolidated Plumbing Industries) at a good price. Is this the same as the PEX-AL-PEX that NRT.Rob refers to.

2) For tile installation I wanted to use 1/4" Hardibacker. If I read this thread correctly I can put the thinset directly over my sleepers? Will thinset stick to aluminum plates? What about where tubing is - just fill in thinset around tubing in groove or avoid this area completely? I would think I couldn’t avoid or the gap might cause a little flex causing trouble with tile later.

I took a look at that tubing but can't find anything that indicates that it has an aluminum layer in it; you might want to call the manufacturer and see. If not, it's the not PAP or PEX-AL-PEX.

We have specified 1/4" hardboard under tile normally. Someone else may be able to comment on direct thinset over plates and sleepers, I'd be interested in the answer myself since we've never done that and I'm not a tile expert.

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Northeast Radiant Technology, LLC
-=RFH Design, Supply and Consultation=-
RPA certified Radiant Designer
http://www.NRTradiant.com
rob@NRTradiant.com

Guest 2:
If you go to Warmboard.com's very helpful web site, you'll find the Tile Council of America's recommendation for putting backerboard over radiant systems. I believe it says you butter the thinset onto the backerboard, rather than spreading it over the plates and tubes.

We did a small mockup (thinset backerboard to a 2-foot by 4-foot sandwich of osb, metal plates and PEX) this week just to see how it all came together. It looks pretty danged solid.

Went to warmboard and it did have directions. Thanks! Was basically what you said. I just need to put the thinset on the backerboard (not the sleepers).