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Community and Q&A

Insulated basement slab vapor barrier location – above or below the under-slab insulation?

Michael Tuso | Posted in Energy Efficiency and Durability on

We are planning the prep work for a basement radiant slab in our house addition. The proposed material stackup from bottom to top is:

1. Compacted gravel fill
2. 3″ XPS rigid foam under slab (also 2″ of vertical insulation isolating slab edge from basement wall)
3. Poly vapor barrier
4. 4″ slab with welded wire mesh reinforcement and PEX hydronic tubing

Everything I read on the Building Science Corporation web site says to put the vapor barrier ABOVE the XPS rigid foam. (between slab and rigid foam).

I reviewed recommendations on several radiant heat component and installation web sites. They specify the vapor barrier BELOW the XPS rigid foam (between rigid foam and compacted fill).

Does anyone have experience that says an above-insulation vapor barrier location is better than a below-insulation location ?



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  1. Expert Member
    Dana Dorsett | | #1

    The reason it's sometimes better to put it above the insulation is that it won't create reservoirs for liquid water to collect on top of the vapor barrier. If it's under the foam it starts out with air-gaps, that have potential to fill. A puddle of water under 3" of foam takes forever to dry to the interior, and it can negatively impact moisture-sensitive types of finish flooring, causing paint to blister, stains to discolor, or wood to warp/mold etc. The foam isn't much impacted by moisture, and with the vapor barrier on top of the foam there's a tight seal between the concrete & vapor barrier, and any moisture that MIGHT accumulate dries fairly rapidly through concrete.

    XPS has a fairly hefty global warming impact since it's blown with HFCs (primarily HFC134a, with a 100 year global warming impact about 1400x CO2). Over a handful of decades as the blowing agent escapes the performance of XPS slowly drops to that of EPS of equal density. EPS is blown with pentane, at about 7x CO2, which is mostly gone by the time it hits the distibutor's warehouse, and it's performance is labeled at it's fully depleted value. It does NOT lose performance over time. So, if you started out with 4" (R16) of EPS, it'll still be performing at R16 in 50 years, whereas if you start out with 3" of XPS (R15), it'll be performing at R13 or less in 50 years. (Cost-wise R16 EPS is typically 15-20% cheaper than R15 XPS too.)

    The compressive strength of 1.5lb density XPS is somewhat higher than 1.5lb "Type-II" EPS, but for this application it really doesn't matter. The slab distributes the dynamic loads over a wide area, and the static load is a tiny fraction of the compression rating. (In residential applications this is only important when the foam is supporting a real load over limited area, say, holding up the entire house with foam under the foundation footing, etc.)

    From a green-greener-greenest point of view XPS and closed cell spray polyurethane blown with HFC245fa are at the far end of the brown end of the spectrum, and if you can design it out without affecting the performance one arguably should. The greenest foam that you could put under a slab would be reclaimed/recycled EPS or XPS (but not polyiso, which would take on water in that application), and there are multiple vendors handling those goods in my neighborhood (central MA), and possibly yours. But if going with virgin stock foam in a sub-slab application, EPS is as good as it gets. (Nationwide Foam and Green Insulation Group are the bigger players near me, but there are smaller foam recyclers as well.)

  2. Expert Member
    Malcolm Taylor | | #2

    Along with your other good advice, you put a lot of effort into informing posters of the environmental downsides of XPS. I appreciate it.

  3. GBA Editor
    Martin Holladay | | #3

    Here is a link to an article that thoroughly discusses the question you raise: Polyethylene Under Concrete Slabs.

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