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Ideas needed: Sub-slab sewage ejector vs. airtight construction

jhrockwell | Posted in Green Building Techniques on

We are planning a new home (Zone 5) on Cape Ann, Massachusetts. Orientation, extensive shallow ledge and southward sloping topography suggest a south-facing lower level (walkout, finished basement) and the main living level (with master bedroom) above, grade access to which will be from the north.

To plan for a lower level bathroom that must be lower than the septic tank (thanks, ledge) and therefore must be pumped, I am wondering about details to preserve the integrity of my sub slab control layers and also accommodate the sewage ejector.

There just happens to be a drop in the ledge where the basement bathroom will go (thanks, ledge). I am kept up at night worrying if it’s feasible to pour some sort of box or pit (like an elevator pit), but working this out with subcontractors who aren’t familiar with air-tight construction is challenging.


John Rockwell

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  1. GBA Editor
    Martin Holladay | | #1

    It sounds like you are on the right track. Discuss what you need with your architect and concrete contractor.

    Concrete is an excellent air barrier, so you are probably worrying unnecessarily. If you have a cold joint in your concrete between two distinct pours (on different days), the joint can be caulked.

  2. jhrockwell | | #2

    I am the architect...I want to replicate Joe Lstiburek's "perfect" assembly, which is easy enough for a slab on grade, but when a downward bump-out occurs for the ejector pit, I'm not clear on how to really achieve air/water/vapor/thermal continuity...perhaps a closed cell foam bathtub? I wonder if a concrete distribution box (minus lid) is a pre-cast version of what I'm contemplating. It could be wrapped with insulation and 15-mil poly? Tying that assembly to the slab is the issue.
    Thank you, Martin.

  3. GBA Editor
    Martin Holladay | | #3

    The concrete pit you describe could be precast off-site and set into place, or cast in place before the slab is poured, or poured into a form on the same day the slab is poured. If you want, you could tie the pit to the slab with rebar pins, but that probably isn't necessary.

    In any case, it shouldn't be hard to install poly and rigid foam under the concrete pit and around its sides.

  4. jhrockwell | | #4

    Got it, presuming "off-set" means "off-site". Thanks so much.

  5. GBA Editor
    Martin Holladay | | #5

    "Off-set" was a typo, since corrected.

    Good luck.

  6. gusfhb | | #6

    At work we have a large trash can sized pump unit for our bathroom. It would not seem like a big deal to set that up so it is actually outside the envelope. Seems simpler to build a small airtight hatch for service than other options.

    Get a quote from granite busting guys of one type or another before spending too much.

  7. davidmeiland | | #7

    Why not put the ejector outside the building line?

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