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In an unsinsulated service cavity (created by running horizontal 2x lumber) is fire blocking required? Or is it a good idea?

rocket190 | Posted in Building Code Questions on

My proposed wall assembly from outside to inside:

1. Ribbed steel siding
2. 3/6″ fanfold battens to create air space
3. 1/2″ cdx
4. 2×8 stud cavity filled with wet spray cellulose
5. 2″ eps nailed to studs (interior uninterrupted rigid foam)
6. 1.5″ uninsulated service cavity (created with horizontal 2x4s)
7. 3/8″ acx plywood (interior wall finish)

The building is a heated workshop with some pretty high electrical needs for the machinery. Since it’s not a residence, I could probably forgo the fire blocking. Leaving an open horizontal service cavity would allow for easy electrical changes in the future.

If the consensus is that the fire blocking is a good idea, I would probably fill the service cavity with blown insulation.

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  1. GBA Editor
    Martin Holladay | | #1

    The service cavity you describe needs fire blocking every 10 feet horizontally.

    For a good overview of fire blocking requirements, I recommend this JLC article: Fire Blocking Basics by Tim Uhler.

    The illustration reproduced below came from the JLC article. Although it doesn't depict your situation, it is comparable in some respects.

    For more information on service cavities, see Service Cavities for Wiring and Plumbing.


    1. stanfo3 | | #5

      Hi Martin,
      Would a piece of rockwool comfortboard sized to fit cavity and a few inches wide be good enough for fire blocking horizontal cavities?

  2. rocket190 | | #2

    Hmmmmm.....Well that pretty much negates the benefit of leaving this area un-insulated.

    Thanks for the answer. That's what I was concerned about. It's funny that in articles singing the praises of an open service cavity, the need for fire blocking was never addressed.

    Since (mechanically fastened) mineral fiber insulation can be used in lieu of wood blocking, do you feel it would be possible to snake a wire through it, or would this destroy the insulation and it's goal of fire blocking?

  3. GBA Editor
    Martin Holladay | | #3

    It's usually not too difficult to install a vertical furring strip every 10 feet. With a little planning, this vertical furring strip can be pre-drilled to allow for wiring at the relevant height.

    That's not much different from the way electricians usually operate. Electricians are used to drilling holes.

  4. stanfo3 | | #4

    Why not use foil faced polyiso instead of eps? Fire block the horizontal 2x material every 10 feet and you can benefit up to the equivalent to an r3 in radiant barrier effects from the reflective foil on the polyiso because of the 1.5 inch air space.

  5. Expert Member
    NICK KEENAN | | #6

    EPS is extremely flammable so fire safety is important.

    I would attach the surface layer of plywood with screws and put in the horizontal fireblock. When it's time to run new wiring take the whole panel off, drill a hole in the fireblock and run the wire.

    I would check if 3/8" ACX is acceptable over EPS, there needs to be a combustion barrier.

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