Hydronic heat and polyethylene – how to layer the materials?
So, I’ve read where Joseph Lstiburek implores us to never, ever put polyethylene sheeting beneath the XPS, beneath the concrete (see “Slab Happy” at BuildingScience.com). However, if I place it over the XPS sheeting, then attach my hydronic heat tubing with staples to the XPS as is typically done, what good it the polyethylene sheeting as it now is full of holes?
I live in climate zone 6, in an area where radon gas is an issue. I am employing hydronic heating, and insulating beneath and around the perimeter of my slab-on-grade with 2″ thick R10 XPS. I was planning on perforated piping beneath the polyethylene, vented through the building and out the roof to address the radon gas issue. But, I am not sure how to layer my materials if the polyethylene should not be first.
Do I:
1. 6 mil polyethylene sheeting -> 2″ XPS -> PEX secured with staples to XPS -> rebar ->concrete pour?
2. 2″ XPS -> 6 mil polyethylene sheeting -> PEX secured with staples to XPS -> rebar ->concrete pour?
3. skip the polyethylene and simply tape/mastic XPS seams?
4. find another means of securing the PEX tubing in place?
Guidance wanted! I’m sure I’m not the only one to have ever encountered this. My concrete guy says “Just put the poly down first. Always done it that way… no problem…”
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Replies
Kent,
Q. "if I place the polyethylene over the XPS sheeting, then attach my hydronic heat tubing with staples to the XPS as is typically done, what good it the polyethylene sheeting as it now is full of holes?"
A. It's an effective vapor retarder. After all, even if the poly consists of 2% holes -- that would be a lot -- it's still 98% as effective a vapor retarder as poly with no holes.
Remember, the concrete is your air barrier. If you are worried about radon, be sure to caulk the crack between the perimeter of your concrete slab and your walls, and be sure that your sump has an airtight lid. You should also include a layer of crushed stone under the poly, along with perforated pipe connected to a riser than can be hooked up to a fan if necessary.
Why not tie the pex to the rebar?
How thick is the slab going to be? You want the pex about 2" below the surface, not sitting on the bottom.
I second Cramer.
I thought about this - securing the PEX to the rebar. Concrete guy wants the PEX under the rebar... HVAC company advised the opposite. Concrete guy says he'd be OK with the rebar on chairs, and I think everything would fit in my 4 1/2" slab with ample coverage to slab surface. (1" chair, 1/2" rebar, 0.88" O.D. PEX)
Zip ties to secure the PEX to rebar, then?
Why are you using rebar and not wire mesh? We normally tie pex tube to the mesh...works great.
Total slab area over 1700 sq ft, 4.5" thick, garage floor in zone 6 with thermal swings. Concrete sub indicated anything short of rebar 2' o.c. grid is inviting trouble. Have to trust the people I hire to do what's best.
Bruce, how do you layer the identified materials? Do you place PEX on top of your mesh and tie it into shape? Below the mesh to hold PEX in place? Where do YOU put the polyethylene sheeting?