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Community and Q&A

Insulating Basement with Poly Over Sill Plate

zac2944 | Posted in General Questions on

I’m in the process of finishing my basement, and trying to figure out if I should remove poly sheeting before adding insulation.  I currently have 6mill poly sheeting running from sill plate down to a drain tile in the floor around the entire basement, similar to the attached photo except no stud wall built yet. Purpose of the poly is to channel water down wall and into drain tile.  I’m planning to insulate according to the article, but it makes clear that “No interior vapor barrier” should be used to allow for drying to the inside.

I’ve got 2″ xps that’s ready to install over the poly, but thinking I should remove the poly sheeting first.  Will I have issues if I leave the poly up?  Seems like the interior surface of the XPS should be warm enough to avoid condensation.   I’m in NY climate zone 5.

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  1. Expert Member
    BILL WICHERS | | #1

    That poly between the block and the back of your foam insulation isn’t an “interior vapor barrier” in your application. What would be a problem here is if your wall was block, foam, batts, poly, drywall, in that order. The poly is now on the interior side of the stack up, and could potentially trap moisture in the wall.

    There is no need for the masonry foundation wall to dry. Your rigid foam would count as a vapor barrier there anyway. I would leave the poly in place, but keep in mind that with it pressed against the wall, it won’t do much to help with drainage. You would need a dimple mat here to help with drainage if that’s important.


  2. Expert Member
    KOHTA UENO | | #2

    Short answer: assuming that the polyethylene on the wall is doing its job correctly, and draining water into the below-slab gravel, leave it in place, cover it with closed-cell spray foam (or XPS, and don't forget to air seal and tape the seams), and call it done.

    Longer answer: I just responded to comments saying that "we can't use a vapor barrier in foundation insulation; keep it 1 perm or higher"--check out that discussion here:

    But what you have is basically an interior drained system: it intrinsically involves a Class I (0.1 perm or less) drainage layer, to direct bulk water into the below-slab gravel. These types are assemblies are shown in the links below:

    BSI-041: Rubble Foundations

    BA-1108: Hybrid Foundation Insulation Retrofits: Measure Guidelines

    Also, what you're showing is similar to the AVID system shown in BA-1108 (see 3.3 Building Envelope Services, Inc. (2008) AVID System, page 19).

  3. zac2944 | | #3

    Thanks for your insightful replies! Your comments and the linked documents support my design and give me some assurance I'll be successful. In the 10 years I've lived in this house, I've only had small amounts water coming through the walls (weeping) twice and both were due to clogged gutters in a downpour. I'm now better maintain the gutters, but as a precaution I installed a drain tile and the poly as part of finishing the space since I was replacing the basement floor anyway.

    I've attached my wall assembly plan below and some in-process build pics in case others are looking for a similar solution. All XPS joints will get taped or caulked, and the interior wall cavities will not be insulated.

  4. Expert Member
    BILL WICHERS | | #4

    You could save some money here and use the cheaper foamular 150 version of XPS. The higher compressive strength of the 250 version is not needed here.


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