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Community and Q&A

Proper basement insulation – interior and exterior?

Hoops. | Posted in Green Building Techniques on

I own a home built in 2007 that has 1″ XPS insulation on the exterior of the basement walls below grade. Above grade there is no insulation on the exterior…about 6-8″ of the foundation is exposed above grade. Basement walls are 9′, poured concrete that used a textured brick form. The basement also has radiant heat in the concrete floor. It’s just waiting for my to finish it!

I’ve studied the building sciences page, and unfortunately it doesn’t show how to insulate the basement interior walls properly if there is minimal exterior insulation.

First off, do I need to insulate the above grade portion of the basement first? I’m thinking I would use 1″ XPS that has a stucco like finish.

On the interior, I was thinking of somehow attaching 1″ XPS to the concrete walls (that have the textured brick finish), then building a 9 foot tall 2×4 stud wall with unfaced batt insulation. Would this cause any condensation problems? Is it overkill? I wouldn’t mind making the walls as thin as possible, but I want it built right.

Basement has remained very dry since we moved in 2+ years ago. Though our house seems to be very “tight” as I had to have a HRV installed due to high humidity in winter/condensation on windows…the HRV solved that problem.

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  1. GBA Editor
    Martin Holladay | | #1

    Here is a link to an article that should answer your questions: How to Insulate a Basement Wall.

    I certainly urge you to finish installing the exterior insulation (the above-grade portion). All of the information you need on attachment details and finishing options are provided in the article I linked to.

    It's also important to install interior insulation. One inch of foam isn't much -- I would install at least 2 inches of interior foam if I were you, in addition to the 1 inch on the exterior of your wall. Again, all the information you need to do the work is in the article.

  2. Expert Member
    Dana Dorsett | | #2

    For the above grade exterior use something more vapor-permeable than XPS. Using EPS would be fine, and works with a number of purpose-made fiber reinforce cementitious stucco finish products designed for adhering to foam.

    By letting the above grade portion of the foundation dry at reasonable rates toward the exterior you limit the increase in moisture content of the concrete at the susceptible foundation sill that will occur once you insulate the interior. If you have a EPDM sill gasket it won't much matter- make it as vapor tight to the exterior as you like, but the pink & foamy sill gaskets doesn't really cut it as a moisture barrier to keep the concrete moisture from ending up in the wood.

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