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Musings of an Energy Nerd

Crawl Spaces vs. Skirts

If you have a house on piers, should you enclose the space under the house or leave it open?

In a warm climate, a mobile home skirt doesn't have to be airtight. This home is in a state where pipes are very unlikely to freeze, so there isn't any need to build an insulated skirt. In a colder climate, it would make sense to bury the bottom plate of the skirt, to install airtight skirt sheathing, and to include a layer of continuous rigid foam.
Image Credit: LanaiLens

Many older homes in rural areas have pier foundations. The piers may be made of wood (for example, creosoted posts or pressure-treated lumber), poured concrete, CMUs, or bricks. The space between the dirt and the underside of the floor framing may be enclosed or may be entirely open to the wind.

This type of home is more common down South than up North, because cold-climate builders have to find a way to keep the plumbing pipes from freezing. (It’s easier to keep pipes from freezing if a house has a crawl space or a basement than if the house is on piers.) That said, even up north, many rural homes (including mobile homes) have pier foundations.

Owners of homes on piers face several questions:

Should the area under the house be enclosed?

As long as there is no evidence that the insulation in the floor assembly is being disturbed, or that rodents are chewing holes to gain access to the house, there is no pressing need to enclose the space under a house on piers.

Leaving this space wide-open to the weather works better when piers are relatively high than when piers are relatively low, and works better when the floor assembly is airtight and well insulated than when the floor assembly is leaky and poorly insulated.

Problems occur when the spaces between the floor joists are insulated with poorly protected fiberglass batts. If these batts are held in place by chicken wire, it won’t take long for the mice, squirrels, and raccoons to move in. On the other hand, if the underside of the joist bays are sealed with a layer of carefully installed plywood or OSB, there are far fewer reasons for concern.

What’s better: a conventional crawl space foundation or a skirt?

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16 Comments

  1. Bob Manninen | | #1

    You realize of course, that your house will rot
    When you completely enclose the underside of the building on piers, the house will "rot" unless you can prevent the moisture from the ground from migrating into the building on piers. I have a camp in Maine, and this is what happened. I had to remove the foam enclosed skirting around the outside because there was so much moisture under the building (especially after the ground thawed) that was saturating the structure under the building. I then went with the "lattice" work skirting option (also impermeable foam panels covering the floor structure, as suggested by Joe Lstiburek) . Now, if the ground can be covered in polyethylene, and any runoff can be diverted to not flow under the building, the enclosed skirting might work, although I do not have any personal experience with this arrangement.
    So, I suppose in a dry climate where there isn't any moisture that can migrate into the building, the completely enclosed skirting technique can work, but in our "frosty" climates, I have to disagree.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Bob Manninen
    Bob,
    Thanks for you comments.

    As my article on sealed crawl spaces makes clear, this type of crawl space must always include polyethylene on the dirt, as well as proper grading around the perimeter of the house to direct surface water away from the building.

    These requirements are identical whether the crawl space walls are made of concrete or pressure-treated studs.

    Since some people might forget these facts, I have added a new paragraph to my article that underlines the importance of following all of the basic requirements of any sealed crawl space.

  3. User avater GBA Editor
    Martin Holladay | | #3

    Note to GBA readers
    I'll be on vacation from July 24 to August 7 -- back at my desk on August 8.

    So anyone who directs questions my way during the next two weeks will have to wait until August 8 for an answer.

  4. Malcolm Taylor | | #4

    Codes
    The building codes I am familiar with confirm Martin's practical and building science advice. They make no distinction in whether the area under the house is enclosed by a skirt or foundation walls. Once it is enclosed it is subject to all the requirements of a crawlspace, including access to services, minimum heights, vapour barriers, ventilation, radon mitigation etc.

  5. S.S. MacDonald | | #5

    Vapor barrier between joists and subfloor?
    I'm building a 4 season cabin on short piers (varying between 24" and 48" due to sloping terrain) in climate zone 8 near Yellowknife. Still not sure wether to skirt the perimeter or not, it could be difficult to install given the entire site is sloping, jagged bedrock. On the other hand, I'm worried about critters moving in. Also wondering if 6 mil poly should be installed between the floor joists (2"x10") and the 5/8" osb subfloor, connecting to the poly on the exterior walls? The floor will be insulated with fiberglass batts. The underside of the floor joists will be sheathed with 1/2" osb. thanks, Shawn

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to S.S. MacDonald
    Shawn,
    You don't need to install polyethylene between the floor joists and the subfloor, since OSB is a perfectly adequate vapor retarder. Just install the OSB with attention to airtightness (easily accomplished by the use of construction adhesive when installing the subfloor). For more information on this type of floor assembly, see How to Insulate a Cold Floor.

    In Yellowknife, which has bitterly cold winters, your biggest challenge with this type of foundation will be figuring out how to keep your plumbing pipes from freezing. Good luck.

  7. S.S. MacDonald | | #7

    Thanks!
    Thanks!

  8. Michael Bluejay | | #8

    How to build a skirt when there's not enough room for framing?
    The dirt level was perilously close to the bottom of my house. I excavated 6" below the bottom floor joist, but that still leaves precious little room to install a wood frame, especially if I want to maintain sufficient clearance between the ground and the wood.

    I do have some HDPE 2x4s left over from another project that I'm thinking I could use to build my frame, because I can run them to the ground, along with an HDPE bottom plate, and they won't rot. They're not structural, but I don't think they need to be for this application, especially because the lengths will be so short. On the outside I could screw corrugated plastic paneling to the HDPE frame and on the inside I could screw foil-faced rigid foamboard to it. Does this sound like a good plan?

  9. User avater GBA Editor
    Martin Holladay | | #9

    Response to Michael Bluejay
    Michael,
    There are several possible ways to address your foundation problems. What you end up doing depends on your budget.

    The correct answer to your dilemma is to jack up your house before improving the foundation.

    If you don't want to do that, the second-best alternative is to lower the grade around your house, so that you have a higher crawl space, and so that rain will flow away from the house in all directions. With most homes -- homes with walkways, stairs, and landscape plantings -- this option is unrealistic, so it's usually easier to jack up the house than to lower the grade.

    Anything short of these two options should be considered a temporary compromise. You can rig something up with materials left over from another project, but that approach may still leave your joists at risk of rot.

    So here's my answer: Jack up the house and install a conventional crawl space foundation with poured concrete walls. If you can't afford that, jack up the house and install a new pier foundation.

  10. Steven Valenziano | | #10

    I'm hoping Martin or someone else can offer some clarification on the following statement:

    "Leaving this space wide-open to the weather works better when piers are relatively high than when piers are relatively low"

    Would 2'0" be plenty? How about 3'0"?

    Some context, if you like: I'm planning a new build in North Carolina with a pier foundation in order to minimize impact on existing trees. I've ruled out slabs for their high impact on the root zones, and crawlspaces for the the same reason + air quality issues. I'll most likely insulate the floor using 2" exterior foam (protected with OSB) and blown-in cellulose as outlined in Building Science Corporation's BSI-064 brief and a couple GBA articles.

    Many thanks, Steven

    1. User avater GBA Editor
      Martin Holladay | | #12

      Steven,
      My own preference is for a crawl space height that allows a person to access the space and perform repairs. (Remember, you are planning to attach OSB to the underside of your floor joists -- think through how you plan to do that.) Three feet is obviously better than 2 feet, and 3'6" would be even better.

  11. Malcolm Taylor | | #11

    Steven,

    It's a trade off between a few competing things. From a moisture control standpoint, the higher the better. From a structural one, if you go too high you will need to brace the piers. Then there is also the relationship to grade you want to achieve between the house and its surroundings.

    For me two feet is a starting point. High enough to avoid splash-back and promote good airflow, provides good access to work, but not poking out of the ground like a tree-house.

    Consider including an insulated core about three feet square, preferably built of concrete, that extends from the main floor down to the frost level, and using this to run utilities.

  12. Steven Valenziano | | #13

    All makes sense, thanks guys!

  13. Brian Bailey | | #14

    Greetings all,

    I am preparing to insulate a 40-year-old, double-wide mobile home. CMUs support the structure, with a nice, level poured concrete slab under the CMUs. Skirting was previously done with wood framing with a thin layer of concrete (!) around the framing, I guess to give a "foundation" look to the place.

    The place was in very rough shape when we got it, with extensive rodent damage and a thoroughly foul smell throughout. We are now replacing all the interior surfaces, including subfloors and wallboards. New siding will be installed eventually though the existing siding is basically functional. There are no significant mold or rot issues in any of the framing. Floor joists are mere 2x4s on 16" centers.

    My question: I would like to frame and enclose the skirting area, as described in this article. Because I have a good concrete foundation to work with, I'm planning to insulate directly above the concrete (using EPS rigid foam), such that the entire crawl space would be insulated. If cold floors turn out to be a problem, I may install batts between the 2x4 floor joists; but for now I'm hoping the crawl will be warm enough to avoid the need for the batts.

    My thought is that this approach would make it easy to implement, since laying the foam on the concrete will be much easier than fastening it to the underside of the floor joists. And it would make it easy to keep the plumbing above freezing throughout the Northern Indiana winter. This all goes double if I keep the forced-air ductwork in place,which also located in the crawl (I may yet decide to switch to a ductless minisplit).

    Is there any reason not to insulate against the concrete directly? Any tips on how to do so? I do not plan to do a polyethylene layer under the foam, but I will tape the foam seams and caulk or foam around joints with the CMUs and at the interface between the horizontal and vertical foam sections.

    1. User avater GBA Editor
      Martin Holladay | | #15

      Brian,
      You can certainly install a horizontal layer of rigid foam above the slab. If you want, you can also install a layer of OSB or plywood above the rigid foam -- to cover either the entire crawl space, or some portion of the crawlspace (as a kind of catwalk).

      If you do this, make sure that you also insulate the skirt -- and pay attention to airtightness when you build the skirt.

      For more information, see "Installing Rigid Foam Above a Rigid Slab." (Note that you don't need to do as careful a job for a crawlspace floor as would be necessary for a basement floor -- the OSB and fasteners are optional.)

      1. Brian Bailey | | #16

        Very helpful, Martin. Nice to know I'm on the right track. Thank you!

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