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Community and Q&A

Insulating with Foil-Faced Batts Behind Basement Tub

css1813 | Posted in General Questions on

I am just trying to clarify on how to properly insulate behind a one piece fiberglass bathtub unit in a basement, that will be up against the outside basement wall.  We are in zone 5 (Michigan).  This is new construction.  We have a poured wall in which we have 2″ of closed cell foam up against it and a framed wall in front of that.  Now, we are getting ready to set in a bathtub against this wall.  I am getting ready to install faced fiberglass in the framing but want to confirm if I should leave the facing on, or remove the facing.  I’m a little unclear on if this would trap moisture or not.
I appreciate the help, and thank you in advance.

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  1. GBA Editor
    Kiley Jacques | | #1

    The attached PDF answers your question in detail.

  2. css1813 | | #2

    Hello Kiley,
    Thank you for this excellent article. I do have one question. In our case, since we have closed cell cellulose installed against the concrete, am I assuming that is the vapor barrier and we should vent the insulation to the inside then? Thank you very much!

  3. creativedestruction | | #3

    Near a tub/shower I would omit the batts entirely assuming the 2" closed cell foam is continuous between the framed wall and concrete. Batt insulaton will only increase risk in this location by lowering the temp of the foam surface in winter, which can put it below the dew point particularly during showers when humidity increases.

    2" closed cell is sufficient on its own but if you insist on more R value and must have the batts then yes, remove the foil facing. Any moisture in the cavities can only dry inward, not outward through the foam.

    1. Expert Member
      MALCOLM TAYLOR | | #4


      Good point. If more insulation was desired, maybe add another 2" of foam in the stud bays?

  4. css1813 | | #5

    Perfect. Thank you very much for all of this info!

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