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Green Building Blog

(At Least) Six Things Are Wrong With This Crawl Space

The answers to last week’s ‘What’s Wrong With This Picture?’ game

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Last week, GBA published a photo of a crawl space in an old house under the headline, “What’s Wrong With This Picture?”

The photo showed an unvented crawl space in a cold climate. The home was built in 1885. This crawl space is attached to an adjacent concrete-floored basement. The foundation walls are made of mortared limestone.

Even in the small area captured in the photo, there are a number of problems that compromise energy efficiency, building durability, and life safety. Here are our answers to the question posed last week.

Issue 1: Electrical cable isn’t securely stapled

At some point in the past, modern electrical cable was installed to serve a floor receptacle. The cable was not properly fastened to the structure, and drapes freely across a five-foot span. This is a code violation.

Required Corrections: The cable should be rerouted and supported with staples meeting building code requirements. If necessary, replace the cable so that it can be correctly routed and supported.

Issue 2: Asbestos insulation

A hydronic heat pipe is wrapped with asbestos insulation. The asbestos lacks encapsulation to prevent contact with asbestos fibers. If this insulation is abraded, asbestos fibers will be released into the air. If these fibers are inhaled, they can lodge deep in the lungs, eventually leading to lung disease.

Required Corrections: Hire a certified asbestos abatement contractor to remove the asbestos or stabilize it in place.

Issue 3: Knob-and-tube wiring

There is some knob-and-tube wiring. Although the wire appears to be cut, it’s impossible to tell by looking at it whether the wire is still live.

Required Corrections: The wire should be removed back to a code-approved junction box. Consider eliminating all knob-and-tube wiring in the house.

Issue 4: No capillary break at the top of the foundation wall

There is untreated wood in contact with the masonry foundation, a common situation in houses of this age. There are no signs of a capillary break to prevent foundation moisture from wicking into the framing.

Required Corrections: If the foundation drainage is good and the assembly is exposed to warm interior air for drying, the risk of wood rot is relatively low. However, insulating the rim-joist area on the interior can increase the risk of rot, since added insulation would lower the drying potential of the sill. If the homeowner plans to insulate this area on the interior, it’s worth considering the installation of a capillary break. If the area will be insulated on the exterior, a capillary break may not be necessary.

First, verify that there is good site and soil drainage on the exterior side of the assembly. Lift the sill — a task that sounds more daunting than it is in practice — and install a capillary break (metal flashing or a rubber membrane). Then insulate on the interior using rigid foam or spray polyurethane foam.

Issue 5: No polyethylene covering the dirt floor

The crawl space has a bare dirt floor. The crawl space air is in communication with the interior of the home. Because there is no vapor barrier on the dirt floor, ground moisture evaporates into the crawl space air, raising the interior relative humidity.

Required Corrections: To limit the entry of ground moisture and soil gas, the dirt floor should be covered with a polyethylene vapor barrier, with seams lapped and sealed. The edges of the polyethylene should be sealed to the foundation wall. If radon testing shows elevated radon levels, then a sub-slab depressurization system should be installed to exhaust soil gases to the exterior.

Ideally, add a layer of horizontal rigid insulation to limit heat transfer to the ground. Protect the insulation with a concrete “rat slab.”

Issue 6: The crawl space walls are uninsulated

The foundation walls are uninsulated.

Required Corrections: The crawl space walls can be insulated with rigid foam or closed-cell spray polyurethane foam. Because there is no capillary break at the top of the masonry wall, exterior insulation is preferable to interior insulation.

Garrett Mosiman works as a Research Fellow for the Center for Sustainable Research at the University of Minnesota. He has a bachelor’s degree in architecture from Rice University and an M.S. in architecture, Sustainable Design Track, from the University of Minnesota. He is also a LEED Accredited Professional.


  1. user-980774 | | #1

    What about floor joist insulation?
    Or was that too obvious to put on your list?

  2. GBA Editor
    Martin Holladay | | #2

    Response to Richard Patterman
    Most building scientists recommend that it is better to insulate crawl space walls rather than insulating the floor joists above. To read about all of the reasons why this is so, see Building an Unvented Crawl Space.

    1. oldmainevictorian | | #6

      Is this still recommended for limestone foundations? Eventually you will need to repoint the foundation (while limestone mortar is self healing, eventually it wears away)- so, are homeowners to expect to remove and reinstall insulation every time? Also, for homes in northern climates (like mine in Maine), even with proper exterior grading, we still get minor flooding around the seasonal spring thaw (which is why we haven't installed a vapor barrier or insulated the walls). If this was the case in the picture, would a different system be recommended?

      1. GBA Editor
        Martin Holladay | | #7

        Old Maine,
        Absent a water entry problem, my recommendation to you would be to go ahead and insulate the interior of your crawlspace walls with rigid foam insulation or closed-cell spray foam insulation. But the fact that you have an annual problem with water entry changes everything. (It's the moisture, by the way, that contributes to the mortar deterioration.)

        The required steps to fix a wet crawlspace are similar to the required steps to fix a wet basement. To learn more about potential fixes, see this article: "Fixing a Wet Basement."

        If you live in an old house with seasonal flooding in the crawlspace, it may be hard to keep water out of your crawlspace -- so expensive to fix that it's not worth fixing. In that case, you may just need to learn to live with the problem. In such a house, it's acceptable to install insulation between the floor joists, instead of insulating the crawlspace walls.

        For more information, see "Building an Unvented Crawlspace."

  3. user-980774 | | #3

    Martin,That MIGHT be true if
    That MIGHT be true if there is duct work in the crawlspace.
    With no duct work in crawl space, insulate the floor before the crawl walls.

    PS: Insulating the floor would NOT involve fiberglass batts.

  4. GBA Editor
    Martin Holladay | | #4

    Second response to Richard Patterman
    In some areas of the country, your suggested approach can work. Here in Vermont, however, your approach would lead to frozen plumbing pipes.

    In any case, insulating the crawl space walls almost always requires less insulation than insulating the joists above -- because the area of insulation is less.

  5. 3XjWycNcQQ | | #5

    If it's not broke don't fix it (just my opinion).
    If this home was built in 1885 it's 127 years old. Unless there is something seriously wrong with it you're better off to leave it alone. Yes it is in efficient, that's the cost of living in a piece of history. I'm sure you all know, better than I do, that once you start changing air flow / moisure control etc., this home will become a house of cards, and you'll have to go through just about every system in the house. If someone was real concerned about the "problematic" ground gas that has been coming from the basement for the last 1-1/4 century, create a vent to the outside. But I think your better off to leave it be. Same Idea with the asbestos. As long as it is in good shape, don't touch it, and it won't hurt anyone.

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