Pouring a slab as follows:

6" washed 3/4" stone- compacted
2" XPS
vapor barrier

Next I'd like to tie the PEX to mesh and pour. That's the problem. Masons hate this, PEX can get cut when control joints are cut. Chairs holding up the mesh just get crushed or are a trip hazard.

If the PEX is attached to the foam, not thinking you can put the vapor barrier over the foam / PEX, plus the PEX is under 4" of concrete, and I understand it should be within the 4" of concrete.

Seems this very point is a sticking point in the overall theory.

Thanks for any insights.

Isn't 3/4" stone considered "self-consolidating? I'm not sure you can "compact" it.

I put my vapor barrier on top of the stone and the rigid insulation on top of that. BTW, someone will be along soon to tell you that XPS isn't very environmentally friendly and they would be correct. Use EPS.

I also use #3 rebar instead of "mesh" and tie the radiant tubing to the top of that so it is in the upper half of the slab. This improves efficiency and strength.

I didn't use control joints because I didn't care about the hairline cracks. You can lay out the control joint locations in advance and have the tubing dive down under the reinforcement at those locations to keep them away from the saw blade. I would also recommend visually verifying the depth of the control joint before it is cut as some of those boys aren't too bright.

You can also embed some sort of a "soft" divider which will direct the concrete to crack along a certain line, but it won't be as straight on top as a saw cut.

ICF is correct. Use EPS and tie your PEX to the top of the rebar or mesh. We like 6" x 6" Welded Wire Fabric (WWF), or rebar/WWF combination when structurally required, and just lift it using a hook as the pour is accomplished. We often have control breaks, but we always plan for them in advance by first minimizing having PEX in control break locations when doing the PEX CAD layout and always properly sleeving and holding low any PEX that needs to cross a control break. One should keep in mind that if a benign "hairline crack" becomes an "uncontrolled break", any PEX in this location will be at increased risk of failure.

However, there are many designers/installers that don't have the ability to do this or simply don't want to take the time to do this. They will frequently just staple the PEX to the EPS. Besides the reduced efficiency, we believe these installations are more prone to failure if they experience freezing temps. Nevertheless, this is frequently done and the increased failure rate and efficiency loss is apparently considered an acceptable trade-off.

Thank you all-

I'm liking the #3 on dobies. rows 12" apart.

Great stuff on the EPS. Thank you. Is there a brand or two that is preferred?

The topic of PEX and control breaks is interesting. "Sleeve and hold low." I may have tee shirts made...

I'm liking the #3 on dobies. rows 12" apart.

Have you seen the plastic rebar "chairs"? Construction supply houses have them. I think they are superior to traditional concrete block and wire tie dobies.

I've never seen a slab using #3 bar for reinforcement develop an uncontrolled break. I have seen slabs using mesh do so however.

Running the #3 bar at 18" oc seems to work very well in terms of laying the PEX.  That way, you can have 9" tubing spacing pretty easily.

A lot of guys don't like the chairs. I can see how they would be the nuts. I originally spec'd them, but after the resistance we're doing dobies. At this point whatever the crew likes is the better choice.

Sailor - have you ever seen pex damaged at a control or expansion crack? The reason I ask is that no one around here ever sleeves expansion joints.

I have seen joints in both thick and thin slabs open to 3/8 of an inch, such that you can see the runs of pex, just because someone thought water was their friend. Never a problem with the pex. Well within its elasticity even if the slab stretches the pex by 1/2". As long as the pex is in line, no problem. Concrete does not bond to pex.

When you dip and dive the pex under a control joint, what happens? The floor slabs move apart as it shrinks and put tension on the pex. Now, since the pex is no longer linear and the pex can be very warm and flexible and the tension wants to straighten the pex, the bottom of the pipe pushes up against the top, which can't move because of the sleeve/concrete and flow is restricted or cut off.

As hybrid infers, most mesh is useless. Consider the tensile strength of shrinking concrete and that of the wire. Most 4 gauge wire becomes 12 gauge at the joint.

To place reinforcing without chairs of some sort is a waste of time. I use to insist that the placing crew have one man dedicated to the hook and make sure the bar was all lifted, only to have the guys with the screed and the bull-float guy stomp it all down again. I have never, in all the thousands of feet of concrete slab that I have removed, witnessed properly placed reinforcing that was not set on chairs. I suspect pretty soon we will see specs requiring the scanning of reinforcement prior to sign off, just like we require concrete strength tests.

While concrete blocks can work as chairs, they weaken the slab, especially if place in line to each other. Plastic chairs have very small areas of disruption.

A lot of guys don't like the chairs.

Because it makes it harder to stomp the bar grid back down?

We have always used 24” sleeves on PEX passing through control breaks. This allows the PEX to stretch over this 24” as opposed to only the control break dimension. We figure it is cheap insurance and we have always done it as it is recommended by Sigenthaler. We have never had PEX damaged by adhering to this practice. I am encouraged to hear that you have NOT had any problems when NOT using sleeves. This indicates that PEX is much more resilent/robust than we have expected.

If you have proper control breaks, you can use WWF (if this meets all your other structural design requirements) and you should not experience any uncontrolled breaks. We don’t have anything against using chairs. However, if you have a good concrete crew that can lift the WWF/rebar, you don’t need chairs.

I wish the chairs were better received by my mason. I have to admit it looks like it would be impossible not to trip. Yet obviously they are used by the thousands daily.

When you go 18" oc with the bar, footing isn't really a problem. Didn't realize that was the source of the resistance.

••••This allows the PEX to stretch over this 24” as opposed to only the control break dimension.••••
Pex stretch is never restricted to "just the control break dimension." As soon as there is any tension, pex de-bonds from the concrete as its outer dimension shrinks marginally. In essence, all pex in concrete is "sleeved" once it goes through a few heat cycles.
If you ever break up a floor that had pex embedded in it you will find that the pex falls free from the concrete as you break it, unlike deformed bar, which must be broken free bit by bit so it can be recycled.

I suspect that how well concrete bonds to PEX may depend on the concrete used. I can't say that we have a lot of experience breaking up HR slabs. We have broken up a few slabs that we used for integrated HR/PS testing and found the PEX to be very well bonded to the concrete. In fact, this made breaking up the slabs quite a chore. In these slabs, the PEX was fully embedded within 4000 psi concrete. I wonder if just stapling the PEX to the EPS insulation somehow results in less PEX concrete bonding. Perhaps since the lower portion of PEX is not embedded, the heating cycles can break the concrete bond? Again, we have never questioned or deviated from Siegenthaler's recommendations with regard to this sleeving subject. Very interesting discussion/information.

ICFHybrid- I think the spec was for 12"OC PEX. 18"OC would give more foot space, however.

You really should not deviate from your HR design. If it calls for a 12" spacing, that is what you should install. The spacing and the resulting circuit lengths are an important design aspect so as to obtain the required upward heat flux to align with your expected heat loss. 12" is also the recommended minimum spacing for barefoot friendly residential floors...so your toes are less likely to sense the colder areas between the PEX runs.

Is there anything that's done to make this easier to walk through? As I understand it, this would be a 12" x 12" grid. Could it be moved to 12" x 18"? Keeps PEX at 12"

Thank You

I use Crete -Heat and Helix it gets rid of both problems. No mesh and less chance of a crack so the control joint doesn't have to be deep

We use 6" x 6" WWF. I don't think 12" x 12" would provide adequate reinforcement. That's the fundamental problem when using WWF...you pretty much have to walk on it! So we just walk on it (wearing shoes that won't damage the PEX) and we lift it into the center of slab immediately after the concrete has been poured a section at a time. The WWF and PEX will stay exactly at the level you raise it. What you do NOT want to do is raise it any higher than it needs to be because there is no way you are going to be able to lower it. This likely sounds much more difficult to do than it really is. However, this is easy to accomplish for a good HR concrete team.

We are beginning to use helix with ICF…but still use rebar for the lintels. We have played with helix in HR slabs to reduce the cracking potential and to decrease R-value (which is goodness for a HR slab). However, you still need to fasten the PEX to something…either WWF or rebar.

I like the idea of setting the PEX exactly where it needs to be. Chairs of some type are the best way to go, technically.

Is there a best practice that would allow these guys to survive this 12" x 12" pattern? Like a type of chair that works better than others?

Can I do 12" x 18"?

This is super valuable insights and very helpful to me.