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Community and Q&A

Rigid foam for finished basement

Nathan Linder | Posted in Building Code Questions on


I have a relatively new built house (less than 1.5 years) and I installed a capillary break between the footing and concrete wall, then the contractor sealed the wall and applied 2” r10 foam to the outside of the foundation

My question is, being in zone 5, do I need to have r15 on the inside of the foundation for code/finishing purposes or can I have a TOTAL of r15, including exterior foundation and interior foam board? That would be way more cost effective and buy my more space in my rooms and office downstairs 



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  1. GBA Editor
    Martin Holladay | | #1

    As with all code questions, the person to ask is your local code authority. That's whose code interpretation matters.

    I certainly hope that your local inspector agrees that all you need to do is add R-5 insulation to the interior of your wall -- because that's the right answer.

    By the way, how did this non-code-compliant basement wall make it through the plan review process in the first place?

    1. Jon A | | #2

      I agree with Martin, this is code compliant for continuous wall insulation in a basement. My questions would be: Does the R10 exterior insulation extend above grade to the top of your foundation wall or did the builder terminate it at or below grade? If so you will need to add more than the R5 to the interior at the above grade locations. Was the R10 exterior insulation shown on your initial house plans? Was it noted or verified by the inspector when they did the foundation inspection? These may be questions your local building inspector wants answers too.


      If the basement wasn't originally permitted to be finished and the builder insulated the basement ceiling to code than this is a perfectly code compliant basement wall. It's actually better than code since the foundation wouldn't require any insulation.

  2. Nathan Linder | | #3

    Hi Martin and Jon,

    I believe the builder terminated it at grade, leaving about a foot to the sill plate bare (but waterproofed). I believe the product was called horizon? They said they used to use the above grade insulation a few years prior to when we built, but weren’t required to any more, so they didn’t offer the above product, just installed it below grade. Makes sense about needing to now have 15 total for that last foot, which is a bummer. I guess I could always add my own above grade to match the r value so inside is the same floor to ceiling thickness/plane? The insulation is noted and then was inspected as well by the county.

    This was an unfinished basement when it was built, I knew I could do it later and wasn’t in the mortgage at the time.

    Thanks again for your insight!


  3. Expert Member
    Dana Dorsett | | #4

    >"I believe the builder terminated it at grade, leaving about a foot to the sill plate bare ..."

    As long as the insulation is continuous up to the foundation sill and adds up to R15, any combination of interior + exterior would satisfy the IRC. If there is a foot of exposed exterior foundation it does not.

    With a continuous R5 foam on the the interior a batt insulated 2x4 wall tight to the foam it would meet the IRC code on a U-factor basis. In Zone 5 R5 is sufficient for dew point control at the foam/fiber boundary above grade, and it wouldn't need an interior side vapor retarder tigher than latex paint on wallboard. Unfaced batts would be preferred, since in the event of a minor flood kraft facers would wick moisture up several feet above the water line, and foil facers would create a moisture trap between the foam & facer.

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