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Community and Q&A

Slab Edge Insulation (or lack thereof)

Bruce Davis | Posted in Energy Efficiency and Durability on

Cold edge of Zone 4.  When I built my house a couple of years ago, I didn’t understand the purpose of slab edge insulation as explained by the sub doing the foundation waterproofing.  It was “optional” as the energy code is not enforced around here.  Fortunately, I came to understand it before the slab went in so, along the back walkout wall, I had R-10 board laid horizontally at a 2 ft width instead of vertically.

My neighbor built at the same time with the same custom builder (who never advised us about slab edge insulation).  As we are now just past a couple of weeks of record sub-zero temperatures, I was talking to my neighbor.  The base molding on the back drywall of their walk-out basement built up ice dams during this period.  I had no such problem with the same basic design and materials.  I also have a 15 mil vapor barrier compared to their 6 mil and better insulation.

Unlikely my neighbor has slab edge insulation.  Would this explain the situation?  What can be done at this point?  Would installing 2 inches of foamboard 2 feet deep on the outside of the foundation do the trick?  Thanks.

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  1. Expert Member
    Michael Maines | | #1

    Bruce, most of the heat loss through the slab edge is directly to the outdoor air, so I don't think laying foam horizontally below grade would do much for your neighbor. Concrete is a terrible insulator (~R-0.08/in) so at the baseboard it's probably close to the outdoor temperature. They should install foam insulation vertically on the exterior, preferably EPS or recycled XPS.

  2. Bruce Davis | | #2

    That makes sense. But, I only have 2 ft wide, R-10 horizontal foamboard under my slab; nothing protecting the slab edge from the exterior. I had no problem but my neighbor did. Is there some other issue involved? Thanks.

    1. Expert Member
      Michael Maines | | #3

      It could be several things: difference in interior humidity and/or temperature, orientation (direct sun vs no sun), rain or other exterior moisture source, air leaks (under wall plates often leak air), etc..

      1. Jason S. | | #4

        My money's on the air leaks, nothing bonding sheathing or housewrap to the concrete.

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