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Building Homes or Extensions/insulating a walkout basement

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Expert: Ted Barnhill (Principal, Design45 LLC) - 10/2/2010


Question

Walkout Basement
I am building a home in northern Wisconsin which has a north-facing walkout basement.  This wall is framed, has 2 windows and a door.  The other three walls of this lower level are concrete.  No backfilling has been done yet (don't know whether a retaining wall will be needed).  How would you suggest insulating this lower walkout level?  Should a retaining wall be used to ensure the concrete walls are entirely underground?  Currently the grade slopes to floor level.

Answer
Charles,
There are many ways of insulating and finishing basement walls, depending on your needs.  If you are considering adding a retaining wall to meet other goals, and can bury the entire concrete foundation wall, then a proven and recommended system is to insulate the outside of the foundation (e.g., 2"+ of XPS) and leave the interior walls uncovered.  This means the concrete is exposed to the interior, with at most a coating of plaster or breathable paint.  Of course you would fully waterproof and add drain tile on the exterior.  In this approach, your concrete acts as a beneficial thermal mass; also, there are no cavities for mold to grow in, as any minimal moisture intrusion can dry to the inside.

If you don't bury the whole basement, then you will need to provide more insulation (R-19, likely) and you will place it on the inside.  I would recommend using materials that will not trap moisture.  One of many possibilities is a 2x4 wall with closed cell spray foam, where the studs are spaced away from the wall.  Another method is a continuous layer of XPS boards with taped seams, behind a furred out wall.  Either way, avoid standard drywall and go with mold-free drywall, or better yet, magnesium oxide (MgO) board if you can get it.

Ted Barnhill
Design 45, LLC
    Questioner's Rating
    Rating(1-10)Knowledgeability = 10Clarity of Response = 10Politeness = 10
    CommentThank you, Ted. I value and appreciate your advice. Charles


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Ted Barnhill (Principal, Design45 LLC)

Expertise

I can answer questions about architectural design, how to work with a designer, and best construction practices for most trades. I specialize in cold-climate design (I'm in Minnesota), older home renovation, and energy-efficient/sustainable design.

Experience

As a Principal of Design45, I design and detail new homes and remodels. I am formally trained in architecture, but have also been a contractor and have experience with many trades. I strive to provide beautiful, durable, and efficient homes for clients and communities of all types and budgets.

Education/Credentials
Undergraduate work at Princeton University (Mathematics), Master's work at the University of Minnesota (Architecture)

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