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Community and Q&A

Zone 7A existing cabin crawl space headaches

dsEdwards | Posted in GBA Pro Help on

We recently purchased a existing seasonal lakefront cabin built in the 80s in northern MN. We are remodeling and converting to all-season occasional use and trying to address the vented crawl space. It is damp and there is some mold on floor joists, but I don’t believe there has been standing water and the soil composition is sand. The problem is access: there is very low clearance in most areas (only 6-8″ in some spots). Our contractor has proposed 1) adding rigid insulation to the perimeter block foundation, or 2) raising the cabin and adding two more rows of block. Option one seems not enough and option 2 is beyond our budget (and nerves).
All of the old flooring has been removed so subfloor is currently exposed. If excavation isn’t possible to allow crawl space encapsulation, are there other ways to insulate & add vapor barrier below the subfloor?
My head is spinning trying to figure out a reasonable approach that will improve the crawl space without costing tens of thousands of dollars.

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  1. GBA Editor
    Martin Holladay | | #1

    When the correct approach costs more than the budget, that puts the homeowner in a tough spot. The usual solution is a bank loan, but I know that isn't fun.

    If you can't afford to raise the house on a higher foundation, you'll need to excavate the crawl space (assuming that the footings under the foundation aren't undermined by this type of excavation). In same cases, it's possible to excavate a 4-foot-high trench down the center of the crawl space, from one end of the house down the other, to provide access, and to slope the walls of the trench on either side so that access tapers to only 2 feet on the right and left of the trench.

    In Victorian England, small children were employed to do nasty work in tight spaces. But child labor laws have made that approach illegal.

    For more information on the work that needs to be done, see Building an Unvented Crawl Space.

  2. GBA Editor
    Martin Holladay | | #2

    The trouble with your suggested approach is that, at the end of the job, you still end up with an inaccessible crawl space that is only 6 inches high -- one that wouldn't meet code in a new house. Future moisture problems are likely in this type of crawl space, and future repairs are difficult or impossible.

  3. Expert Member

    Surely the small children could be enticed to dig, rather than coerced? Hold out the promise of a new iPhone 6 or Playstation?

  4. charlie_sullivan | | #4

    Martin and Malcolm,

    I think I have a solution to both problems. This could easily reach between the joists, without exceeding their load rating, and what kid wouldn't want one?

  5. GBA Editor
    Martin Holladay | | #5

    Forgive the humor, Deb. We're still here to help you with your problem (which probably doesn't strike you as particularly funny).

    One factor that might drive the decision -- raise the house or excavate the crawl? -- is the exterior grade. Is the soil outside this cabin close to the lowest course of siding? Or is there a generous distance (at least 8 inches) between the soil and the lowest wooden components of your cabin (on the exterior, not in the crawl space)?

    If the exterior grade is already too close to the siding, that would bolster the case for adding a few courses of concrete blocks to your foundation.

  6. steveoneil | | #6

    We had the same problem with our house. Excavating down the center as Martin suggests is likely the most straightforward way to gain access to seal off and insulate the crawlspace. You may want to check if there are vacuum excavators in your area. These are big trucks that are usually used for trenching utilities by sucking up dirt. Depending on the size of the area, it could be just a couple of days of work. A structural engineer may have to get involved.

  7. charlie_sullivan | | #7

    How big is the cabin? It might be worth considering removing the sub floor or part of it and doing some work from above. That could include a plastic vapor barrier on top of the dirt. I am not sure whether that is a good approach but I like it better than the idea of coercing small children to excavate.

  8. charlie_sullivan | | #8


    Martin's suggestion of excavating a trench with sloping sides would certainly leave you in a better situation for future repairs than just putting down plastic would. Steven's suggestion of a vacuum excavator is a good one. Their speed, precision, and ability to access difficult spaces is amazing. Lower-tech approaches could also work, and I actually had that in mind when I suggested partial removal of the sub-floor.

    On the other hand, the plastic wouldn't cost much and also wouldn't make it any harder to, in the future, raise the cabin or excavate. So while it would be nice to take this opportunity to improve things towards future repairs, if would not be a tragic mistake to skip it if the budget doesn't allow doing much more now. I think you could get the plastic by pulling the corners with ropes, perhaps with some access cut through the sub-floor if needed.

  9. dsEdwards | | #9

    Almost all of the suggestions/comments are greatly appreciated ;) I did a bit of reading and followed Q&A on this site before posting. Martin, the grade is in contact with the wood on one side of the cabin and several vents are halfway below grade. I had also found some info on vacuum excavation which seemed like an option, but today we found that there is another foundation wall running midway between the exterior walls, so that may eliminate the trenching idea. Raising the cabin seems to be the best solution long term. Our contractor is in the process of getting bids. Thanks again to all for your professional opinions-it helps me to make a more informed decision.

  10. Expert Member
    MALCOLM TAYLOR | | #10

    Suitably chastened for my humorous aside :) , may I offer a more serous comment: Unless you are considering maintaining heat throughout the winter, you need to be careful about excavating too much fill from over the footings of your foundation without providing some other insulation for them. The fill may be what protects the footings from frost heave.

  11. dsEdwards | | #11

    Thanks, Malcolm. Good point regarding the footings and excavation.
    I really appreciate all of the timely input.

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