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Community and Q&A

Insulation for a basement with an internal perimeter drain

ins2000 | Posted in General Questions on

We’re in a Zone 5 location and are finishing a conditioned basement that has an internal perimeter drain and a plumbed-in dehumidifier (set at 55%) with sealed sump pump. After originally planning to use roxul, I now understand that rigid foam insulation would be best for below grade interior wall cavities. Am considering R-5.78 Rigid Foam. However, I’m concerned that the foam will block the perimeter drain. The base plate has been set approximately 3/4 inch away from concrete block wall to prevent any blockage of said drain. Should the insulation be raised to leave a gap above the drain and/or set away from the wall (would be sealed to the studs with foam/tape)? Haven’t been able to a straight answer from anybody regarding basement insulation. Very frustrating. Any help will be much appreciated.

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  1. NormanWB | | #1

    Just my initial thoughts. I am sure others with more experience will chime in...

    Does the basement wall leak? If so, then the insulation will get wet (every foam absorbs water to some degree), so you will need to separate the insulation from the wall or find a way to shield the insulation from the water.

    If the perimeter drain is to capture water coming from below and the walls are not a problem, then set the foam board up above the base you mentioned.

    Whatever you do, do not use poly in the wall as it will keep the wall from drying and promote mold and mildew.

    Also, i don't think R-5 is enough for CZ 5.

  2. user-2310254 | | #2

    The 2012 IRC requires R 15 in Zone 5. But Norman is right that you have to do more than insulate if water is entering the basement. See this article for details.

  3. GBA Editor
    Martin Holladay | | #3

    First of all, can you tell us your name?

    The standard solution in this type of situation is to install a dimple mat on the wall before you insulate. The dimple mat allows water to trickle down the wall and find the drain at the base of the wall.

    Next, install the code-required amount of wall insulation -- probably a minimum of R-15 (check with your local code authority) -- in the form of rigid foam with taped seams. You can use two or more layers of rigid foam to achieve the required R-value, or you can install a 2x4 wall on the interior side of the rigid foam and install some of the R-value between the studs if that's easier.

    For more information, see How to Insulate a Basement Wall.

  4. Expert Member
    Dana Dorsett | | #4

    When taking the hybrid foam + insulated studwall approach, it's important to install sufficient foam-R for dew point control on the fiber-R, to avoid wintertime moisture accumulation at the foam/fiber boundary on the above-grade portion. In zone 5 that would be R5 minimum foam-R for a 2x4 wall, just as it would be for fully above grade walls.


  5. JC72 | | #5

    Perhaps a variation of this using a dimple drainage matt?

  6. GBA Editor
    Martin Holladay | | #6

    Yes, the Hammer & Hand crew also recommends a dimple mat, as I did. Once the dimple mat is installed, either rigid foam or spray foam can be used for insulation.

  7. JC72 | | #7

    @ Martin,

    Sincerest apologies. I didn't catch that.

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