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Community and Q&A

Basement wall fire blocking material?

mackstann | Posted in General Questions on

Martin’s basement wall insulation article is a classic that I constantly refer to when planning my basement renovation. The main thing missing from the equation, however, is fire blocking. As I understand it, fire blocking generally needs to penetrate through any foam insulation all the way to the concrete. This presents issues with thermal bridging as well as compromising the air barrier and vapor retarder layers that the foam provides. What’s the least bad choice? Mineral wool? Pressure treated lumber? Paperless gypsum board? Perhaps with the latter two, their “buried” portion could be painted with vapor barrier paint to keep them a bit more dry?

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  1. Expert Member
    Dana Dorsett | | #1

    Foam does not need fireblocking, since it is not air permeable, and is thus not a "concealed draft opening" the way air-permeable fiber insulation is.

  2. mackstann | | #2

    Dana, I would love that to be true, but from a lot of googling I've found that since foam is generally flammable to some degree, at least some inspectors require the blocking to go all the way back to the concrete. I guess I'll have to ask my building department to see what they require.

  3. Expert Member
    Dana Dorsett | | #3

    Foam requires a thermal barrier between the foam and conditioned space, but until your post I've never heard of any local code requirement for fire-blocking it.

  4. user-1030561 | | #4

    I found this Q&A while researching to prepare a code appeal for one of my projects. Nick is right that building inspectors are likely to ask for a series of fireblocks every 10 feet, which will interrupt the continuous layer of interior-side foam.

    On our project this was pointed out by the inspector after the foam and wall framing were installed, so adding a fireblock would have been a big expense. Plus we are using dense-packed cellulose as cavity insulation, and thus were counting on a layer of foam to keep the cellulose physically separated from the potentially wet concrete wall.

    I am pleased to report that we successfully appealed this requirement. You may view details of this appeal at this link:

  5. GBA Editor
    Martin Holladay | | #5

    Here is a link to a good article on fire blocking:
    Fire-Blocking Basics

  6. jay443 | | #6

    I know this is old but I have this exact same question. Has anyone else had trouble with an inspector requiring a fireblocking that cuts through the layer of foam to the concrete? I can't read Martin's linked article as I'm not a subscriber so I've been looking all over. Seems like most examples of fireblocking are when there is a gap between the wall and foundation. In my case, I have 2" of foam and a 2x2 wall planned to be framed right against it, as I can't afford a gap due to space issues.

  7. GBA Editor
    Martin Holladay | | #7

    I'm attaching an image from the JLC article. This seems to be a matter of code interpretation. Most inspectors don't consider a continuous layer of rigid foam to be a "concealed draft opening," but some do.

    Here's the code language: Section R602.8 of the IRC states, “Fire blocking shall be provided to cut off all concealed draft openings (both vertical and horizontal) and to form an effective fire barrier between stories, and between a top story and the roof space.” If your local code inspector is interpreting the code in a way that makes framing difficult, it's worth having a discussion to see how much leeway there is.


  8. jay443 | | #8

    Thanks Martin. What I'm also having trouble with is the fireblocking at the top plate. I have a continuous layer of foam from the rim joist, to spray foam over the mudsill/top of concrete block, to the wall foam. Do I have to interrupt this continuous layer to install blocking between the vertical wall bays and the horizontal floor bays above it? Or is it enough to just use a doubled-up top plate so that there is no air gap?

  9. GBA Editor
    Martin Holladay | | #9

    Here is the fireblocking detail for the top of a basement wall.


  10. jay443 | | #10

    Hmmm. I guess I'm hosed, then, because I can't attach a fireblocking piece in that way due to the spray foam that's covering the mudsill, sill gasket, and top of block.

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