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Thread: Townhouse 2-hour rated firewall (3.17.2 Exception)

  1. #1

    Default Townhouse 2-hour rated firewall (3.17.2 Exception)


    This is my first post here, so please bear with me if I'm not quite up to speed on technical know-how or on this forum's etiquette. I'm still learning.

    I have lived in my townhouse for close to two years. We are the 3rd owners, and it was built in 2001.

    My main concern is sound transmission through the common walls with the neighbors on each side. A secondary concern is whether the common walls are built to code for their fire rating.

    Footfall on the floors and stairs, closet doors closing, baby crying, and chairs being slid over hardwood floor are all clearly audible. When one neighbor closes her front door, I can feel a thud move through the structure, and the common wall creaks. None of this seemed right to me, especially since when we bought the place I assumed each unit was separate and independent, with it's own "exterior" walls between adjacent homes.

    I was very disappointed when I discovered that my townhouse shared a single common wall with the neighbors on each side. Prior to that, I had always thought of townhomes as independent structures. Needless to say, the IRC code exception under 3.17.2 for a 2-hour fire-resistance wall instead of two independent structures is NOT my friend.

    So now I'm trying to figure out if the walls I have are actually approved 2-hour rated assemblies. From the walls in the unpainted attic, I know that the outermost sheet of drywall is 1/2" (not sure if it's "X type" fire rated). The wall with one neighbor has 3 electrical penetrations; the other common wall has no penetrations. The penetrations do not appear to have any fire stop behind them (no putty pad or other visible stop), and I can clearly see the drywall sheet on the other side while looking through the gap next to the electrical box with the plates removed. The cavity is not insulated, at least near the electrical penetrations.

    I have tried to measure the inside of the wall cavity by sliding a thin ruler between the electrical box and the sheetrock, and the measurement I get to the outside of the drywall is 4-1/2". That leads me to conclude that I have two sheets of 1/2" drywall on my side, plus a standard 3-1/2" depth from the stud.

    So, assuming I've measured correctly, and that the drywall on my neighbor's side of the wall is the same is mine, does anyone know of a 2-hour rated assembly that uses two sheets of 1/2" drywall (either X type or non) and an uninsulated cavity? This seems to be closer to a 1-hour rated assembly, which from my reading of 3.17.2 exception, may not be to code.

    Any and all help is appreciated. And again, my ultimate concern is to address the sound transmission issues, which are certainly related to the thickness of the common wall, lack of insulation, and unprotected penetrations. I have been told that my area did not have any sound transmission code in place when my townhouse was built, so the fire resistance rating of the wall assembly may be my only protection under code. I also hope it doesn't matter that our townhouse is equipped with a sprinkler system - I didn't see that mentioned under the townhouse section of the IRC.

  2. #2
    Join Date
    Dec 2003
    Welland Ontario


    Seeing the drywall sheet on the other side is not a good thing. I don't know what the code says about it but there should definitely be some soundproofing insulation in the wall if you want the noise cut down.

  3. #3
    Join Date
    Oct 2003
    Living in, not from North Carolina


    I think the double layers of 1/2" drywall on both sides of the studs gives you your 2 hour fire rating, non the less the sprinkler system would most certainly give leeway to this. The lack of insulation between the walls and fire stop material around outlet boxes might be an issue though, allowing a fire tunnel.

  4. #4


    Thank you for the replies.

    pushkins - is there are reason you think having sprinklers eases the requirement for a 2-hour fire wall? The exception under IRC 3.17.2 seems pretty precise, and doesn't mention allowance for anything less than a "2-hour fire-resistance-rated wall," irrespective of a sprinkler system.

    Also, can you link me to any records online that shows a double-layer of 1/2" drywall over 2x4 studs rates as a 2-hour assembly? I haven't been able to find anything that shows that.

    This book (I hope the link works) shows several examples of 1-hour and 2-hour wall assemblies. Building Codes illustrated (Google Books)

    The guide shows wall assemblies with standard 2x4 studs as only 1-hour walls. Each of the 2-hour walls has either metal studs, 2x6 studs, a sandwiched panel of heavier drywall, or blocks/concrete.

  5. #5
    Join Date
    Dec 2003
    Kent, WA


    Which IRC code book are you reading (year)? Which one, if any, was in effect at the time your place was built? Were any local amendments adopted?
    Kent, WA

  6. #6


    Sorry to hear about these problems. The IBC (Building Code) does not require much sound isolation at all. Much less than in the EU, for example.

    In my experience, when someone is checking up on code compliant construction because of noise, generally everything is compliant. The problem is that code accommodates noise walls and ceilings.

    In your case there could certainly be non-compliant construction, and you can check with the local building inspector's office. I'm simply saying that the construction was presumably checked by local building inspectors, and is likely (not absolutely) compliant.
    Ted White

    Soundproofing Company

  7. #7



    Thank you - I was quoting from IRC 2004. When the townhouse was built, I believe the 2000 code was in effect, though I don't have access to that book. I haven't been able to find any local amendments pertaining to townhouse common walls as of yet.

    Whether or not there are local amendments, however, I would still like to know if the wall assembly I've described can even qualify as a 2-hour fire resistance wall.

  8. #8



    Thank you for your feedback from a soundproofing perspective. One thing I do know from the city permits office is that something changed between when the plans were submitted and approved, and when the place was actually built.

    The plans on file with the city for my townhouse show what I would call "traditional" townhouse construction - each unit having it's own insulated stud walls, separated by a small air gap.

  9. #9


    That's what we would consider a "double stud wall," which is the gold standard for decoupled wall framing. As long as the insulation isn't over-compacted, and there's no piece of drywall in that 1" gap.

    That doesn't mean that such a wall frame can't be severely compromised by some means.
    Ted White

    Soundproofing Company

  10. #10


    Thanks again Ted.

    The reason I believe that my place is built with a single common wall rather than a double wall as you are describing is the interior vs. exterior measurements.

    The townhouse is 20' wide, so the interior width should be that, minus the thickness of the walls. The highest possible measurement on the inside should be 19' 2" if each unit has its own wall. The measurement I get it greater than that, though I forget by how much. I'll measure again this afternoon. This is how I'm calculating what the width should be with double walls:

    <-- neighbor || 1/2" sheet || 3-1/2" stud || 1/2" sheet || ---interior width--- || 1/2" sheet || 3-1/2" stud || 1/2" sheet || neighbor-->

    Thicker drywall, more sheets, and/or an air gap between each units' walls would make the interior measurement less. The greater interior width tells me that there are not double walls between the townhouses, but a single common wall.

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