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Cold floor over crawlspace

jburkard | Posted in General Questions on

Problem:  My house is uncomfortably cold during the winter, especially the floors.  Running the heat leads to excessive dryness.

House details:

  • Located in Charlotte, NC (Zone 3)

  • Built in the 1970’s, on a sloping lot.

  • Sits on a crawlspace that  varies from 3’ to 8’ high.

  • Has a large number of casement windows

  • Has a cathedral ceiling in the living room, which is by far the coldest in the winter.

  • Walls are insulated with fiberglass batts, sheathed with particleboard and vertical cedar siding.  No housewrap.

  • Floor is insulated with fiberglass batts that aren’t in great shape

  • We’ve already laid down plastic in the crawlspace, plugged the vents, and added a dehumidifier.  This was done to prevent mold.


I’m open to other suggestions, but I think the most economical solution would be to deal with the cold floor.  The crawspace gets cold due to the tall uninsulated walls. I think I have two options for fixing this:


Option 1:  Spray foam the crawlspace walls.  Rigid foam probably won’t work, because the walls aren’t flat (CMU pilasters with brick infill).   Unfortunately the high side of the crawlspace has some drainage issues, and causing water to travel beneath the poly which covers the dirt floor.  I’d probably have to rip out some mature bushes and install a french drain to fix this. Also, spray foaming the band sill is risky, because my deck ledger is not properly flashed.


Option 2:  Attach 1” polyiso to the bottom of the floor joists.  The advantage is that i could probably do the work myself.  One disadvantage is that there will still be heat loss through the ducts.  Also, it could get tedious taping all the joints, notching around footings, and dealing with Romex and piping that is attached to the floor joists.


Any comments on my best option are greatly appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    Put these three things together:

    1. "My house is uncomfortably cold during the winter."
    2. "Running the heat leads to excessive dryness."
    3. "The room with a cathedral ceiling is by far the coldest in the winter."

    ...and the evidence screams one conclusion: "This house has air leaks."

    The leaks aren't just in your floor system. They are also in your roof assembly.

    Here is a link to an article that explains what you need to do: "Blower-Door-Directed Air Sealing."

  2. GBA Editor
    Peter Yost | | #2

    Hi J -

    Your options describe an unvented (insulate/air seal exterior walls) and vented configuration (insulate/air seal the first floor assembly, even though in your case since you say you are running a dehumidifier in the crawl, you--hopefully--are not currently venting.

    In my experience, it is harder to get continuous insulation and air sealing at the first floor assembly than the crawlspace exterior walls. Recommend Option 1.

    But regardless of what approach you take, the rule is manage bulk water first, before you air seal.


  3. jburkard | | #3


    I'll see if anyone offers that service in my area. You are right that I probably have leaks, regardless of how poorly my home is insulated. Things were a lot worse before I sealed off my whole-house fan opening, but i'm sure there are other less obvious leaks.


    You're probably right that getting a good seal with rigid foam below the joists would be difficult. Cutting around every plumbing, HVAC, and electrical penetration, as well as the foundation piers would be a nightmare. I just need to bite the bullet and install a swale or french drain at the front of my house to deal with the water. For my unflashed deck ledger, I can just caulk the band sill instead of spray foam so that it can still dry toward the inside.

    Thanks again!

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