I'm looking for some ideas on how to retrofit a firewall in some duplex residential buildings.  They are rental properties, and the landlord is interested in taking advantage of utility incentives for envelope and HVAC upgrades.  However, to qualify for incentives there must be a firewall from the foundation to the roof deck.  These single story buildings have wide open attics and wide open crawlspaces.  So, the trick is to come up with a method that could provide a 1 hour rated assembly that could be done in manner that is both practical and not excessively expensive.  The incentives from the utility are good enough to make this step a consideration.

It appears that creating a separation between the attics would involve a double layer of drywall - tapped and mudded. That assumes there is enough of an opening into the attic to get drywall into the space.  The job would be a pain, but I think its fesible.

For the crawlspace, adding a footing in the middle and concrete block would be quite intensive.  However, the thought that is being proposed is installing a vertical "wall" to separate the crawlspace of each unit in the duples with foam board with some sort of structural support.  I'm not sure that we can get a 1-hour assembly in this method.

Any one have any experience with this situation?  Is it worth the trouble?

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  • Hey Todd,

    Ran into this as a Construction Manager for a school. 
    The method the sub-conctractor utilized and architect agreed and seemed the simplest, was to actually cut out the ceiling area betwen the roof joists and move the insulaiton and VB and your double layer of Type X 5/8 drywall on one side of the rafter or truss. Easier work space working the the bottom up, the area was a hall that was approximatley 10 feet in width.

    Regarding the crawl space, why do you need a footing? You're not installing a load bearing foundation wall?

    As Sean mentioned, go with a 6" wall complete with two layer of Type X 5/8 drywall and any penetraiotn via cables utilize fire rating caulking around it, and duct work can fire stopping damper, with sanitaiotn plumbing theire as fire clapse availible as well.
    Without pictures its a but harder, but some crawl spaces are moisture nightmares, but are manageable so choice of product for that should be considered.

    If you do the concrete block method, is usually the easiest and fastest method if your building new, but you wouldn't need a footing, because as staed before it is isn't load bearing.

  • Foam is a no - you basically have to do what you do for the attic OR as mentioned block it up. For the wall - pressure treated wood is a must & I would suggest a fiberglass encased drywall as long as it is fire rated. In the 2015 IRC you should review R302.3 Two-family dwellings - I think that also applies to 2018, but if not it is close to that area.

    One option you might consider with the AHJ & program is fire rated drywall at the "ceiling" level of said crawl space especially if there are no mechanicals down there 

    • The IRC is useful for knowing we need a 1-hour assembly, but of course is silent on the specific methodologies that could be used. 

      The crawlspace ceiling option is certainly an option to consider depending on how much ductwork, plumbing, and wiring need to be worked around.

      Another crawlspace option that came up is to make the vertical wall separation would of cement board.


      • Yes that is an option but remember it must be rated so you would need full specs & follow to a T

        With that I like that better than drywall especially if it gets wet, good catch.

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