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Insulating stud wall crawlspace with rigid foam

debump | Posted in Energy Efficiency and Durability on

Apologies if this has been answered already, but I haven’t been able to find an answer here or elsewhere. There are lots of great articles on rigid foam over concrete, but apparently a stud-wall crawlspace is uncommon–I haven’t found any recommendations for that configuration.

I have a 4′ crawlspace that’s composed of a stud wall on top of a footer (or buried foundation–not sure which is the proper term). Inside the crawlspace, there is just an inch or two of exposed concrete; the rest of the 4′ is studs. Currently the stud bays are filled with paper faced fiberglass, circa mid-90s (as are the joist bays). I’d like to either replace or supplement the wall insulation with rigid foam board. Is that foolish?

This is near Leadville, CO, at 10,000 feet. Humidity is really not an issue here, especially not in the winter. I suspect that in a more humid location, placing foam over fiber on an crawlspace interior would be asking for a mold habitat in those cavities–here, I’m not sure. The crawlspace is un-vented, but has an exposed (and powder dry) dirt floor. Access is via a conditioned, attached garage.

If I do remove the fiberglass, should I cut the foam and fit it into the stud bays, or attach it directly over the studs like wallboard? It seems like it would be a lot easier to attach it like sheathing, and there wouldn’t be thermal bridging from exposed studs to crawlspace air.

I realize that spray foam is an option, but that’s a little more daunting as a DIY solution.

Thanks in advance for any suggestions or info!


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  1. user-2310254 | | #1

    David. How much clearance do you have between the framing and the ground? You want to avoid any contact between the ground and the framing. What is your objective for the crawlspace? Are you having any issues in the conditioned parts of the house?

    Also, Leadville is supposed to have especially high radon levels. Have you tested your crawlspace? The results may influence your decision making process.

  2. GBA Editor
    Martin Holladay | | #2

    I'm going to ignore the radon issue for the time being, but you should definitely test your house for radon.

    The basic problem with these stud walls is that the bottom plate of the wall is too close to grade. Over time, the studs begin to rot.

    If you must have this type of stud wall, the studs should be pressure-treated, and the exterior side of the stud wall should be sheathed with pressure-treated plywood.

    Even better: Remove the stud walls and replace them with a CMU wall or poured concrete wall. (I know -- that's a lot of work, and it's expensive. But it's the best solution.)

    If your budget is limited and you need to go ahead with your plan, you need to pay attention to airtightness. After removing the fiberglass insulation, seal up any obvious leaks in the wall with caulk and canned spray foam. Then install one or more layers of rigid foam on the interior side of the wall -- installed as continuous layers over the studs. Don't cut the rigid foam into strips and try to insert the strips between the studs.

    Again, pay attention to airtightness. Seal the seams with high quality tape, caulk, or canned spray foam.

    Ideally, the last layer of rigid foam will be covered by a layer of OSB, drywall, or cement board underlayment (for fire safety).

  3. debump | | #3

    Thanks for the replies and info!
    For most of the perimeter, the exterior of the stud wall is one or two feet below grade. The sheathing is plywood, and that is covered with painted metal, which is the only thing between the soil and the plywood. I have not excavated to know if there is something between the metal and the plywood. All of that said, the house was built in 1996, and the framing and plywood, show no signs of moisture or deterioration from the inside.
    My objective is to improve the air sealing and winter performance of the entire house; the attic and the crawlspace are the easiest to improve on, so they seemed like a good place to start.
    We did test for radon, 6 months apart, and both were below the threshold.
    I will take a closer look at the materials--I'm pretty sure the studs are not pressure treated, but I don't know about the plywood. As I mentioned above, the timber is all in excellent condition, so there must be some decent water barrier on the exterior that I'm not seeing. It's all glacial moraine, so the soil drainage is incredibly efficient as well. This is soil that makes a gardener weep.
    Thanks again; I'll plan on removing the existing fluff, sealing, and running continuous rigid foam. Local code does allow exposed foam in an unoccupied crawlspace, but I'm considering covering XPS with foil-faced polyiso, staggering the joints.

  4. user-2310254 | | #4

    David. Do you know if the crawlspace was built as a permanent wood foundation? It would be good to know if the design was intentional. If it is a PWF, I would have concerns about not being able to inspect the subgrade wood. Maybe someone with experience in this area could comment.

  5. debump | | #5

    Steve, I don't--I hadn't heard of PWF until now. Looks like I have some homework! Based on the designs from Southern Pine, it certainly sounds like it is though. Thanks for bringing that concept up! There are a couple of interesting threads on this board about insulating PWS, though I'm not seeing a consensus.

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