Helpful? 2

How to properly insulate a poured slab with PEX?


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.

Asked by Ted White
Posted Sun, 02/23/2014 - 12:58
Edited Mon, 02/24/2014 - 06:44


13 Answers

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Helpful? 0

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.

Answered by John H. Stehman
Posted Sun, 02/23/2014 - 13:14

Helpful? 2

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.

Answered by Malcolm Taylor
Posted Sun, 02/23/2014 - 14:51

Helpful? 1

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.

Answered by David Meiland
Posted Sun, 02/23/2014 - 15:02

Helpful? -1

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: and there it sounds like you have 12" on center parallel rows

Answered by Ted White
Posted Sun, 02/23/2014 - 17:49
Edited Sun, 02/23/2014 - 18:29.

Helpful? 0

Ted, If you are going to use rebar it has to be in a grid.

Answered by Malcolm Taylor
Posted Mon, 02/24/2014 - 12:29

Helpful? 1

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!!

Answered by Ted White
Posted Mon, 02/24/2014 - 12:50

Helpful? 2

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.

Answered by Dana Dorsett
Posted Tue, 02/25/2014 - 17:32

Helpful? 0

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.

Answered by Malcolm Taylor
Posted Tue, 02/25/2014 - 23:33

Helpful? 1

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.

Answered by Ted White
Posted Wed, 02/26/2014 - 07:41

Helpful? 0

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.

Answered by Scott McCullough
Posted Wed, 02/26/2014 - 10:09
Edited Wed, 02/26/2014 - 10:11.

Helpful? 1

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.

Answered by Ted White
Posted Wed, 02/26/2014 - 10:39

Helpful? 0

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.

Answered by Malcolm Taylor
Posted Wed, 02/26/2014 - 12:35

Helpful? 1

That was such a relief for me to learn. Thank you again, Malcolm.

Answered by Ted White
Posted Wed, 02/26/2014 - 14:05

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