Insulating a Concrete Slab Incorporating PEX
TJames4
| Posted in Green Building Techniques on
Hello,
I’m looking to pour a basement 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.
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Replies
Don't use the chairs, they are a pain. Let the wire mesh lay flat on top of foam, wire tie pex to mesh. When they make the pour they take their rake, hook the mesh and just pick iit up off the foam a bit. It doesnt need to be in middle of slab. If they cut control joints they don't need to be any deeper then a half inch. The pex should have 40 or 50psi of air in it as all of this is going on. There should be a gage on the temporary pex manifold to monitor. Another hint, take pictures and measure where your pex is before the pour. By using the mesh you have a grid to keep pex straight and to be able to figure out where it is in case you need to drill in anchors later.
John, The control joints are particularly important if mesh is used as it constrains the ability of the slab to shrink and makes it more susceptible to cracking. To be effective the joints must be at least 1/4 of the slab depth, so for a 4" slab than means at least 1" deep. Using rakes to lift the slab leave it at varying heights and may well lead to the pex being high enough to be damaged by the cuts.
This is one reason why I don't use mesh in heated slabs. Use #3 rebar on a 12" grid and support it on 1" dobies. The top of 1/2" PEX is always 2" deep if the slab is 4+" thick, and it won't float during the pour assuming you use adequate wire ties. This also places the reinforcement where it should be structurally. I don't buy the theory about the concrete rake lifting the mesh, either, since you're walking on the mesh a few feet from where you're trying to lift it.
What a great stream of thoughts! Thank you all. David- What is a "dobie?
Nevermind- I googled a bit. A 12" x 12" grid from the #3 re-rod? Not 12" spaced parallel rows, right?
I see your discussion on this here: https://www.greenbuildingadvisor.com/community/forum/gba-pro-help/23750/where-place-vapor-barrier-concrete-slab-grade-radiant-floor-and-i and there it sounds like you have 12" on center parallel rows
Ted, If you are going to use rebar it has to be in a grid.
Thanks Malcom. That was confusing to me. I now see this referenced a lot as a solution, but they all seem to me to be describing parallel rows of re-bar.
Thanks for clarifying that. Seems like an unearthly amount of re-rod!!
EPS is both cheaper and greener than XPS in this application. If you want to staple the PEX to the foam you'll need to use Type-IX (2lb density) for staple-retention rather than Type-II, (1.5lbs density), but either is fine.
Put the vapor barrier on top of the foam rather than under it if you're putting any finish flooring on it. Under the foam it'll trap puddles of excess water from the pour and take forever to dry through the foam, raising havoc with any subfloor or even paint bond on the concrete above. Having the vapor barrier on top of the foam doens't preclude you from stapling the PEX to the foam, but it does make it slightly more awkward. There's no need to try to seal those 10,000 staple holes- vapor diffusion is a permeance x cross-section sort of thing, and those little rips & punctures only add up to a few square inches.
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Ted, It is a lot of rebar. I'd be tempted to go with a larger spacing - say 16" o.c. or even 24. The rebar is there to keep any cracks tight. It won't do half as much for the strength of the slab as a properly compacted subsurface will.
Looking at Crete-Heat. Holds the PEX, has an R-10, acts as the Vapor Barrier.
In my mind if you're doing a vapor barrier, it should be a barrier not a sieve.
We went through the same questions when we poured our basement slab in 2011. We installed the vapor barrier over the foam as suggested and used chairs to hold up the wire mesh. Using rebar costs more than mesh and seems to be overkill. Pulling the wire up with rakes seems very unreliable. The chairs get the wire where it's supposed to be. I'm not sure how they could get crushed, you'd really have to stomp on them to do any damage. And it wasn't much trouble walking around with the mesh on the chairs, just have to be a little careful. Cutting for expansion joints is one option of many. To avoid cutting our Pex we used the inexpensive and easy to install plastic 'T' shaped joints that get buried in the pour. 2 years later the only cracking we have are the tight and straight lines along the plastic joints. We got the Pex laid out and tied within a few hours. http://eaglepondhouse.wordpress.com/2011/10/24/basement-slab-preparation/
Not even worth trying to keep the PEX up into the slab. For practical purposes, PEX at the bottom of the slab vs in the middle of the slab works equally well.
I spent a LOT of time spinning my wheels until this was pointed out.
Ted, you are right. The discussion around raising it is simply because it is attached to the mesh or rebar, and there isn't any point in having either of those unless they are raised up into the slab.
That was such a relief for me to learn. Thank you again, Malcolm.
Ted has is right.
We installed over 100,000 feet of PEX in concrete slabs last year. All PEX was stapled directly to rigid insulation and reinforcing steel, mostly 6,6-10-10 was place over the PEX in the middle third of the slab where it belongs.
This includes over 25,000 thousand feet of heated driveways. No chairs, no re-bar, no cracking, curling, cupping or abnormal growth in nearby vegetation.
5 years later (but still a relevant topic!)... Hi Morgan, if you are till out there, x5 years later... what size PEX do you typically run, because 1/2" PEX would only put the mesh 3/4" up into the concrete, which is not in the middle 3rd... it's in the bottom 1/3. Additionally, my concern with stapling PEX directly on top of the foam is that, it's not embedded into the concrete, and in essence makes "notches" in the back side of the slab, which weakens it... basically making a 3.5" thick concrete slab the equivalent of 2.75" thick concrete. Whereas, if you were to suspend the PEX up into the concrete at least a 1/2" to 3/4" or so, you would eliminate these "notches" on the backside. Very similar to how wooden studs retain their strength when you drill holes into them, but NOT when you notch their edges. My thought would be to pin little 3/4" thick pieces of foam to the foam board where you want to staple attach your PEX. Then, staple the PEX through into and through it, all the way into the foam beneath, in essence, raising your PEX up 3/4" off the foam board sheets. Then, lay and wire connect either your mesh or rebar over the top of this. This keeps everything in about the right place, helps protect the PEX from getting cut when adding control joints, and gets the PEX up and away from sitting directly on the foam, so it will be completely surrounded by concrete, aptly heating the slab more quickly than it would otherwise.