Helpful? 0

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

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. 2x8 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.

Asked by Rick Van Handel
Posted Mon, 07/14/2014 - 07:57

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3 Answers

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1.
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Rick,
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.

.

Fire blocking image - JLC.jpg
Answered by Martin Holladay, GBA Advisor
Posted Mon, 07/14/2014 - 10:43

2.
Helpful? 0

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?

Answered by Rick Van Handel
Posted Mon, 07/14/2014 - 18:10

3.
Helpful? 0

Rick,
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.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 07/15/2014 - 06:59

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