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Community and Q&A

Do I need to insulate a shared wall in the attic of my condo?

Luke Savoie | Posted in Energy Efficiency and Durability on

Hi Everyone!

I’m in central Mississippi (Zone 3). I have an unconditioned attic.

I have finished air sealing my attic. I cleaned out the ridge/soffit vents then placed preformed channels so that the ventilation in my attic is appropriate. I have also repaired, sealed and insulated the HVAC ducts in my unconditioned attic (yes I know). I am getting prepared to bring the insulation up to its proper level with loose fill blown in. Currently I have 4 inches of rock wool (put in back in 1973).

THE ISSUE: I have shared walls in the attic on two sides. The material appears to be some sort of board material. It says “Georgia Pacific intermediate sheathing.” This is all I could make out. It looks like a solid black board. This is all that separates my attic from my neighbor’s attics. I have sealed around the edges of the board material, etc.

1. Will this dividing wall be enough to keep my neighbors’ hot attic air out of my attic?” If not, then what do you recommend I do? Insulate on my side of this board material? Something else?

2. I know I need a firewall. Could I do a firewall and cover my insulation concerns as well?


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  1. D Dorsett | | #1

    If you're insulating at the attic floor there is no need to insulate the partition walls between your space and the neighboring attic.

    If you are insulating at the roof instead of the attic floor, insulate the partition walls.

    If the partition walls are asphalted fiberboard on both sides of a studwall you can probably blow cellulose in the cavity and call it a day. If you want a fire-rated assembly 2" (R13) fire-rated Thermax glued to the fiberboard with foam-board construction adhesive &/or atteched to the studs with cap-screws will probably be adequate.

    The original fiberboard SHOULD be air-tight (but probably isn't.) It's probably worth air-sealing it and covering it with wallboard to better control fire spread issues even if you're insulating at the attic floor.

    Any ducts or air handlers in the attic?

  2. GBA Editor
    Martin Holladay | | #2

    If by "firewall" you mean a code-mandated separation between attached units in a multi-family building, talk to your local code official or fire department. What you describe is clearly not enough in almost all U.S. jurisdictions for such an assembly, and I'm not convinced that Dana's recommendation to use Thermax is correct.

    I've seen CMUs used for this type of firewall, and two layers of 5/8" drywall, but never Thermax. Needless to say, the firewall separation has to start at the foundation, and sometimes has to continue above the roofline.

  3. Luke Savoie | | #3

    Thanks for the responses. Yes, the Plenum is in the attic with the air ducts. However, everything else in located in conditioned space in closet.

    I spoke with the local building inspector. I've got the requirements for the firewall. Just as you stated Martin. For some reason (probably age of the condo) there is a firewall between units just not in the attic. I will make sure that this is up to code.

    Honestly, my main concern was having all of my repair and insulation work go to waste if my neighbors' attic air was coming into my attic.

  4. GBA Editor
    Martin Holladay | | #4

    It's a shame that your ducts and forced-air plenum are in the unconditioned attic. But it sounds like you realize that this situation isn't ideal, and that you have decided (perhaps reluctantly) to live with the ducts the way they are.

    Dana is right: since your insulation layer is on the attic floor, you don't have to insulate the wall that separates your attic from your neighbor's attic. Everyone's attic is going to be hot in the summer and cold in the winter -- your attic and your neighbor's.

  5. D Dorsett | | #5

    Martin is right that Thermax on it's own would not meet the fire ratings required for a firewall between apartments, but would only be sufficient to not need a thermal barrier against the foam itself. (Mea culpa- was being a bit sloppy!).

    If the ducts & plenum are not perfectly air sealed the attic insulation won't do quite as much for you as you might have hoped, but it will still do a lot. The ducts themselves need to be insulated in an air-impermeable fashion if you intend to put fiber insulation over them, or condensation may end up wetting the fiber insulation during the cooling season.

    It's quite a bit more money, but insulating at the roof to bring the ducts inside of conditioned space is more effective, and would eliminate the duct condensation issue, since the humidity of the attic air would then track that of the house, not the outdoors (as it currently does.)

  6. Jeremy M | | #6

    If you're still not convinced from an insulation standpoint about the walls think about it: you're insulating the floor so that in the summer the attic is hot but, on the other side of the insulation, your condo below is cool.

    What will the neighbors have in the summer? Hot attics as well, so it doesn't matter.

    If you were northern climate and needed to keep the attic cold in winter to prevent ice dams that might be a different story.

    All of this of course my semi-educated and still learning opinion:)

  7. Luke Savoie | | #7

    It all makes perfect sense now.

    I will consider bringing the ducts into conditioned space. I'll post an update later on.

    I cannot thank you all enough.

  8. Expert Member
    Malcolm Taylor | | #8

    Luke, the problem with trying to make your fire separation continuous through the attic is that it needs the rated drywall on both sides, not just yours. Some multi-unit buildings have a shared attic with no fire separation because they have fire rated ceilings instead. It might be worth seeing if that is the case with yours too.

  9. Aj Builder, Upstate NY Zone 6a | | #9

    Nothing is built worse... than a condo.

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