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Placement of vapor retarder for radiant heat in slab

Martin_F | Posted in General Questions on

I’m building a small house (~ 1000 sq. ft.) on a full foundation and am having radiant heat tubing installed in the slab. The plumbing contractor plans to staple the tubing to the foam insulation under the slab. I had planned to put the foam over crushed stone then a 10 mil sheeting over then the slab on that as I’ve seen in numerous foundation diagrams. But if the plastic sheeting is over the foam the staples for the radiant tubing will poke many holes in it so that seems like a bad idea. Would it be OK to put the plastic sheeting on the crushed stone then the foam and pour the slab on top of the foam. That way the vapor barrier wouldn’t have all the staples going through it? Thanks for your thoughts on this.

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  1. Expert Member
    Dana Dorsett | | #1

    Vapor diffusion is a function of total area. The total cross section of those annular holes around the staple penetrations will probably be less than 2 square inches, and the foam itself is vapor retardent. The effect on moisture migration from the soil to the slab is so low that it would be unmeasurable.

    From a practical point of view it might be more difficult for them to staple the PEX to the foam, but it's still do-able.

    If you want to be nice to the planet, use Type IX EPS rather than XPS for the foam, or use Type-II EPS for most of the R with only 1" of XPS for the stapling layer. Staple retention to lower densities of EPS than Type IX (2.0lbs per cubic foot nominal) isn't very good, but Type-IX retention is as good or better than most XPS. The HFC134a blowing agents used for all XPS in North America has a global warming potential about 1400x that of CO2, whereas the pentane used for EPS runs about 7x CO2. There's some hit for going with ~35% more polymer per R with Type-IX EPS rather than 1.5lb XPS, but it's modest in comparison to the blowing agent difference.

    If you want to be even nicer to the planet, use reclaimed roofing EPS or XPS rather than virgin stock.

  2. Martin_F | | #2

    Thanks Dana - I'm reassured and will tell the P & H contractor to staple away. Thanks also for the details on eco-friendly foam options, I'll do my best to incorporate.

  3. charlie_sullivan | | #3

    I'm not sure about this, but I think that the story on blocking radon is different from the story on blocking water vapor, such that the holes might make the system less radon tight even if they don't allow significant moisture through.

    I second Dana's suggestion to use EPS.

  4. Martin_F | | #4

    Charlie - thanks for mentioning the radon aspect. We will have a radon collection system in the crushed stone venting through the slab to the roof, but it's a concern. It would be a lot of tape/taping but maybe taping all the joints between the foam sheets would be worth it for that purpose? I'm planning to use W R Grace Perminator 10 mil sheeting and tape for the vapor barrier, I could be sure to get enough tape to do all the foam joints as well as the sheeting, assuming it would stick to the foam, if not there must be something that will. What do you think?

  5. user-2890856 | | #5

    Martin .
    This product can save you alot of time and meets what Dana and Charlie recommended for material . You will have less joints to tape , in fact the tape is integral . As far as vapor barrier I have unfortunately witnessed this stuff hold more than 3" of water after a rain storm where we had installed it extending up the walls 4" . We had to pump the water out as it would not drain , this on top of a gravely , sandy soil with spectacular perc .
    You can visit their site for testing results on permeance and other factors . To clarify , I am referring to the 1 1/4 " X5 product . I might add tha you should insist that your heating contractor and concrete contractor coordinate what to use and how to install so that the tubing is mid depth in the slab . Embedded tubing is slow enough to respond without having to heat a full 4" before it can output to the reservoir . I recommend 2" chair supports and attaching the tubing to the mesh .

  6. GBA Editor
    Martin Holladay | | #6

    The polyethylene acts as a vapor barrier. Dana is correct that the holes don't significantly affect its performance as a vapor barrier.

    When it comes to radon, you want an air barrier. The concrete slab acts as the air barrier.

    For more information on the placement of the polyethylene, see Polyethylene Under Concrete Slabs.

    For more information on reducing radon levels, see All About Radon.

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