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Unconditioned basement ceiling

user-6323668 | Posted in Green Building Techniques on

I am building a net zero house, 2000 SF, 2 stories in Mystic CT. It has 2 full baths, heating and cooling by 2 mini splits, ventilation with an HRV, range hood in kitchen. The walls are double stud with Zip/OSB sheathing, ventilated cladding and densepack cellulose and sheetrock on the inside.

The basement is unconditioned. The basement ceiling has 12” of densepack cellulose. The basement stair enclosure is well insulated and “air tight.” I think it will be cool and damp all the time in the basement. Do I need a vapor barrier (smart or otherwise) on the underside of the basement ceiling?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Do I need a vapor barrier (smart or otherwise) on the underside of the basement ceiling?"

    A. No. But you certainly need a good air barrier (for example, drywall).

  2. Expert Member
    Dana Dorsett | | #2

    If you are going to use the basement for ANYTHING it's prudent to insulate it to IRC 2015 code min performance (R15 continuous walls), as well as a couple inches of EPS under the slab. And that's whether you dense-pack the joists above or not.

    Summertime outdoor dew points are well above the deep subsoil temperatures, and the ventilation air for the basement will collect in the slab, walls, and the ceiling gypsum (if any) , and simply installing a vapor barrier in the ceiling won't fix that- it'll end up smelling like a basement unless you apply mechanical dehumidification, which is the opposite of where you want to be.

    If you use reclaimed EPS or XPS for under the slab and reclaimed roofing polyiso for insulating the walls it can be pretty cheap- possibly cheaper than dense packing the joists. If you have water heaters or laundry facilities down there insulating the walls & slab and NOT the ceiling won't change the annual energy use by very much, and it'll be a much more pleasant space, and dry enough to store things with even a modest amount of dehumidification. There are multiple vendors of reclaimed foam in southern New England- it's cheap stuff- often cheaper than box-store pricing on fiberglass batts in terms of R per square foot.

  3. user-6323668 | | #3

    If we do put an air barrier on the basement ceiling, can the floor above the joists and cellulose insulation act as the air barrier or does it have to be under the joists and insulation?

  4. user-2310254 | | #4

    If possible, you should follow Dana's advice. I don't think Martin would disagree.

  5. GBA Editor
    Martin Holladay | | #5

    In order to avoid the work involved with writing an essay in response to every question, I often limit myself to a brief answer, focusing on the actual question being posed instead of providing general advice.

    But I guess I have to dive in and provide advice here.

    1. As Dana suggested, it's always better to insulate basement walls instead of the basement ceiling. For one thing, basement wall insulation is required by code. For another, you need to bring the basement inside the home's conditioned space, for a variety of reasons (including the fact that most basements include plumbing pipes, ducts, and HVAC equipment). For more information on this issue, see How to Insulate a Basement Wall.

    2. If you don't want to follow the advice given by Dana and me, and you decide to insulate your basement ceiling instead of your basement walls, you may find yourself at odds with your local code enforcement official. That said, you can establish an air barrier on either side of the cellulose if you go that route, by either installing the subfloor with close attention to airtightness, or by installing an air barrier on the underside of the floor joists.

  6. Reid Baldwin | | #6

    Maybe I am missing something here. How would you dense pack cellulose between the joists without having something on the bottom of the joists?

  7. Dana1 | | #7

    Reid: Dense packing in mesh would be one way to do it without an air barrier.

    Foundation insulation vs. R30 between the basement ceiling joists is sometimes treated as an either/or situation by code officials. But from a performance & resilience point of view insulating and air sealing the foundation walls is (by far) a superior solution.

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