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Community and Q&A

Insulating an unfinished basement ceiling in a flood plain

McGrai37 | Posted in Energy Efficiency and Durability on

Hello, I tried to find an answer to this but couldn’t, apologies if I missed it. Our house in upstate NY was built out of an old carriage house (in the 40s). Our subfloor is thick, wide and uneven barn beams, with gaps varying up to 1/2inch in between them. We have an unfinished, full basement. It would get damp, but we have a humidifier that keeps it at 50 humidity. We also live in a flood plain. The house has gotten water into the first floor during a major flood, but in a 100 year flood scenario it will likely stay in the basement. The floor is very drafty, and I’ve assumed it is because of the relatively wide gaps in our subfloor. I’d like to insulate the ceiling, but am concerned with what would happen in a flood.

My initial thought was to put rigid foam board up. My concern there is that the subfloor (basement ceiling) is uneven, and that it would leave gaps in between the insulation and the ceiling where mold could grow. I thought I could spray foam the ceiling, but people (non-pros) I have mentioned that to think it would be very bad in a flood scenario since I couldn’t remove the insulation to help dry it out. I don’t know enough about it to know if that’s true or not.

Hopefully I gave all the relevant information, if not please ask me. Thank you in advance for your help.

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  1. walta100 | | #1

    Generally it makes more sense to air seal and insulate the walls. Because it will require 75% less materials and the exterior wall will have fewer penetrations than the ceiling. If somehow you did effectively air seal and insulate the ceiling now all the plumbing in the basement would be more likely to freeze.

    If your current foundation is leaky consider jacking up the house and installing a new foundation that would put your home above the record flood level.


  2. GBA Editor
    Martin Holladay | | #2

    If you have determined that there is a high likelihood that water will rise above your first floor during a flooding event, then it doesn't make much sense to invest in any remedy other than jacking up the house and raising the foundation, as Walter suggested.

    Many homeowners balk at the idea, but I've seen the work done on homes in my community. It isn't as intimidating as it sounds.

    If you don't want to jack up the house, I don't think it matters much what materials you install in your basement ceiling. Put something up and get ready to rip it out when your house floods.

  3. Expert Member
    Dana Dorsett | | #3

    Don't use interior side Thermax below the anticipated high water mark, as it will wick moisture and take years / decades / forever to dry. EPS will temporarily take on some amount of moisture in the interstitial spaces between the macroscopic beads, but drys reasonably quickly after the flood recedes.

  4. McGrai37 | | #4

    Thanks for the replies everyone. Our first floor is just above the 100 year level, so while we're hoping to not have to deal with it we're trying to plan as if we will. Unfortunately with what has been done around the house (garage on side, sun room off back, stone patio between the two) I don't think we can realistically dig up around it and install anything, which I'd love to do.

    The suggestion that sealing the ceiling would create pipe issues is a great point and not one I had considered. In my head the ceiling was less surface area than the walls, but wouldn't be the first time I've been wrong. I'll start looking into what my options are for that, and start with EPS.

  5. Expert Member
    Dana Dorsett | | #5

    See if you can't find a local supplier for reclaimed roofing EPS, which is usually Type-II (1.5lb per cubic foot density) or Type-VIII (1.25.lbs), either of which would be suitable. Reclaimed goods are typically

  6. Jon_R | | #6

    If you dug down and installed EPS on the exterior of the basement walls, you wouldn't need to do anything after a flood. Or maybe some moisture resistant interior side options (stucco/plaster over EPS or thermax?).

  7. user-1072251 | | #7

    You could install some framing under the floor in order to "level" the plane, and install your EPS foam under that. Use screws in installing the foam so it can be easily removed and reinstalled if necessary in the event (and after) another major flood,

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