It's recommended in my area (due to rocky soil) to install a sheath or channel under the slab for the pour, then slide the PEX into the sheathes afterwards during the rough. This also allows the PEX to be replaced later on.

What type of material should be used for this sleeve? Is it compatible with insulation if I wanted to insulate the hot and cold lines running under the slab?

Additional information: This will be inspected work, under IPC 2012 and IRC 2012. We are in climate zone 4.

  • 3" or 4" ABS pipe....
    – Michael Karas
    Mar 12, 2018 at 0:56
  • Flexible ABS? I thought the stuff they used was like a semi-stiff tubing, similar to what people put at the end of rain gutters. It's not rigid pipe, but not as pliable as a garden hose either. Similar to the flexibility of PEX.
    – Nick
    Mar 12, 2018 at 1:32
  • @JimStewart That's what they do in Florida, but I think it depends on your soil type. Georgia clay with rocks vs Florida sand.
    – Nick
    Mar 12, 2018 at 2:04
  • I have recently been observing the construction of a new house in my neighborhood. The PEX water pipes were just laid on the soil, then sand added with poly sheeting laid over that. The slab was then poured over that. Some heavy duty sheeting tape was wrapped around the PEX where it passed through the slab, and where it passed through what would be beams. I have wondered if some sort of conduit could be installed and then the PEX pushed and pulled in after the slab was poured. @Michael Karas can you give a link or reference to this method of installation? Mar 12, 2018 at 2:05
  • Without conduit for the PEX if one ever has to replace the PEX one has to tunnel under the slab. I cannot understand why direct burial is allowed. This is very short sighted. PEX has a service life which is shorter than the structure as a whole. Mar 12, 2018 at 2:09

2 Answers 2


You'd install PEX without sheath inside the slab for underfloor heating only. Otherwise it should be sheathed to allow it to comfortably expand depending on temperature variations, it also makes it possible to change the pipe if it bursts due to freezing, or bad quality pipe for example. Sheathing also protects against UV light which is bad for PEX.

Why not simply purchase pre-sheathed pipe?... (link to my favorite shop areound here). It is a little bit more expensive than naked pipe, but it saves a lot of effort. You can get pretty much any type of pipe with the sheath already on, you can even get them pre-insulated.

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I had (once) to shove about 10m of pipe through a sheath, and it was a lot of work, and we did it in the garden with the sheath held straight between two trees! The PEX pipe wants to curl back as it was when it was rolled, so it catches in the sheath corrugations, even with the end tapered and lubed it was a full body exercise...

If you want to do that... get a sheath that's a bit too large. For a 20mm pipe you'd get a sheath with 25mm ID, get 32mm ID instead, that will be a lot less work! And the larger ID allows you to stick a rounded end on the pipe (like a copper pipe endcap) which will make it slide a lot better...

  • This didn't answer what the sheathing material is though...
    – Nick
    Mar 13, 2018 at 18:28
  • I looked it up, got polyethylene or polypropylene, or polyolefin (which would most likely mean PP or PE). ABS is too hard, it would crack when bending it.
    – bobflux
    Mar 13, 2018 at 18:42
  • So is it the same kind used with rain gutters then like this: homedepot.com/p/…? That's polyethylene pipe, and 4" drainage pipe is readily available and would allow 2 runs of 1" PEX with insulation to be pulled through pretty easily. I do need to make sure that is code complaint though.
    – Nick
    Mar 14, 2018 at 7:04
  • Looks good. Also try google image search on "underground pex conduit" and "underground electrical conduit"
    – bobflux
    Mar 14, 2018 at 13:14

I do not believe that you would need to insulate the PEX hot water lines if they were in a conduit, PEX itself is good insulation.

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