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Basement insulation…pull the XPS?

wgolden | Posted in Building Code Questions on

Long story short, between work and family obligations I’ve been busting my tail to get my basement walls insulated and studded in preparation for finishing it off. Had an inspector come through only to hear that he considers the 2″ xps I installed behind the stud walls to be a prohibited vapor barrier. Mind you I took extra caution to put in nice tight seams, tape the seams with that special red foam tape, and caulk around the perimeters AND I put up and secured the stud walls in front of the xps. He’s now advising I rip it all out ($600 + worth of materials) and fill the stud cavities with batt insulation. Called around to two other local community inspectors and was given contradictory information, most all of the websites recommend rigid foam board as a means to improve the heat retention and discourage batt insulation on below grade stud walls. Any suggestions? I don’t want to get on this guy’s bad side, but I don’t have a very strong grasp on the fine print of the uniform dwelling code (I live in southern WI) to call him out. Help!

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  1. GBA Editor
    Martin Holladay | | #1

    I'm sorry to hear about your problem. This inspector is wrong. He is urging you to remove excellent insulation and to replace it with insulation that will cause problems.

    As far as I know, there is no provision in the code the justify his ruling. The first step is to politely ask him to tell you the code reference (section number) that justifies his ruling, so that you can look it up. (As far as I know, it doesn't exist.)

    The next step would be to find out the name of his supervisor, and see if you can get a more reasonable, educated person involved.

    If anyone in your local building department seems reasonable, you can refer them to this article: How to Insulate a Basement Wall, or to similar articles on the Building Science Corporation web site.

    Good luck.

  2. GBA Editor
    Martin Holladay | | #2

    I have found a code reference that might help you:

    According to Wisconsin State Code:.

    "SPS 322.38  Vapor retarders. ...
    (4) Concrete or masonry basement walls. A non-rigid sheet vapor retarder with a perm rating of 0.1 or less is prohibited in all of the following locations:
    (a) On a concrete or masonry wall which is below grade to any extent...."

    The key element of this code provision is the phrase "non-rigid." The intent of the code is to prevent the installation of polyethylene. Your XPS is rigid, so this code prohibition does not apply.

    When you point out to your local code official that he is wrong, be gentle.

    This code provision was quoted in another GBA thread. Here is the link:

  3. Dana1 | | #3

    Also note, at only 2" the vapor permeance of XPS is about 0.5-0.7 perms, which is WAY above the prohibited 0.1 perms.

    If you pull up and print the specifications for the product you can prove it. The permeance is typically specified as the ASTM E96 performance at a thickness of 1". At 2" the permeance will be half that.


    "Water Vapor Permeance7 , maximum perm (ng/Pa•s•m2) ASTM E96 1.5(86)"

    The max at 1" is 1.5 perms, so the max at 2" would be 1"/2" x 1.5= 0.75 perms They don't specify a typcial or min, but if the maximum is nearly a full order of magnitude higher than the prohibited 0.1 perms, there's no way it's even close to being a violation.

  4. wgolden | | #4

    Thanks everyone, its good to know I'm not off in left field. Will let you know how things pan out, I'm planning on taking your advice to politely ask him to provide the code to support his ruling. Still not sure if it's a ruling or a suggestion, will have to get a more solid answer. If it's simply his opinion, I'm not very concerned.

  5. Expert Member
    Dana Dorsett | | #5

    Print out the relevant section of code and the specs for the foam, and present it as a question:

    Does the fact that it's rigid rather than non-rigid, and the fact that it's way above 0.1 perms mean that it's actually in compliance? If not, why not?

  6. wgolden | | #6

    Typed up and sent my very carefully worded email to the inspector referencing the code language and material specs you all suggested above, got back a single sentence answer that "it's fine to use then." Glad he's changed his mind, although I've lost a lot of faith given how easily he flip-flopped. Glad to have found this website, will surely be posting in the future for input/advice!

  7. user-4053553 | | #7

    Lost faith, you should have renewed faith, most enforcers will not listen to logic, written words or anything else because their pigheadedness gets in the way. You didn't bamboozle the guy, you showed him the actual code and he agreed. Of course he should have known this before you met him, but at least he is willing to admit he is wrong and not hold his own mistake against you.

  8. wgolden | | #8

    Good point Alan, I do appreciate his willingness to reconsider. Its resolved now so I just need to move on.

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