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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 to exclude vermin?
  • What’s better: a conventional crawl space foundation or a skirt?
  • If the space under the house isn’t enclosed, what’s the best way to insulate and protect the floor assembly?

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.…

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  1. tenbob | | #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. GBA Editor
    Martin Holladay | | #2

    Response to Bob Manninen
    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.

    1. blueburaq | | #27

      Hi Martin,
      I'm a couple hours into my crawlspace research and I'm really caught on the idea/solution of enclosing the crawl space with some type of wall and backfilling it with proper grading. A foundation company I called out quoted me for encapsulation by building a plywood wall around skirt of the house, laying a polyethylene under the crawl space, and installing drain tiles + sump pump. No deep concrete wall between the piers and no grading or backfilling. I'm not 100% convinced and still concerned about surface water entering.
      The questions are:
      Where is that article on sealed crawl spaces? It says it's no longer available
      Do you think water will still get in?
      Treated and framed plywood a good enough crawl space foundation wall?

      1. Expert Member
        MALCOLM TAYLOR | | #28


        Here is a link to the article I think you are looking for.

  3. 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. Expert Member

    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. GBA Editor
    Martin Holladay | | #6

    Response to S.S. MacDonald
    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


  8. michaelbluejay | | #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. GBA Editor
    Martin Holladay | | #9

    Response to Michael Bluejay
    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. svalenziano | | #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. GBA Editor
      Martin Holladay | | #12

      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. Expert Member
    MALCOLM TAYLOR | | #11


    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. svalenziano | | #13

    All makes sense, thanks guys!

  13. fwsolar | | #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. GBA Editor
      Martin Holladay | | #15

      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. fwsolar | | #16

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

  14. Michelbasque | | #17

    Excellent article! My only concern with the airtight skirt is that the frost heave could pry up the skirt (I'm in zone 7). Maybe applying the same technique as a FPSF (frost-protected shallow foundation) would do the trick?

  15. GBA Editor
    Martin Holladay | | #18

    Frost heaves don't usually damage wood-framed skirts, for several reasons (one of which is that wood framing is more flexible than concrete). That said, frost heaves are more likely when the soil is saturated than when the soil is well-drained. Hopefully, your house location is well-drained -- if not, consider installing a swale or a French drain to keep the area as dry as possible.

    1. Michelbasque | | #19

      Hi Martin,

      Thank you for the quick reply. The skirt of my house is currently attached to the wall with a 4" J-trim allowing it to slide up & down. In winter, the soil swells 1-3 inches. I'd like to prevent the movement so I can insulate the skirt. I removed the batt insulation from the floor as I have mice problems.

      I'll definitely look into drainage solutions.

      Thank you!

  16. Smutrux | | #20

    Hello, I'm about to become the owner of a summer camp converted to year round house built in 1950 on piers. The piers look like they have solid footings, about 2'x4' poured concrete footers buried in the ground, with alternating cinder blocks and wooden shims that hold the house up. The crawl space is fairly high, about 5' at the access door, but reducing height to about 18" on ledge at the back of the space. I can tell that the previous owners tried to do some moisture mitigation, there are large sheets of black plastic on the dirt floor, the space has been skirted but poorly insulated with sheets of blue board. But at the corner of one of the piers, you can see that water still gets in and has started to erode one of the piers. There are also gutters all around the eaves, with roof-to-ground gutter pipes in an effort to control the water coming off the roof. I believe what needs to be done is that a drainage ditch needs to be dug around the perimeter of the house to prevent water running under the house, and then some moisture barriers put in under the house. It's important to know that there are some plumbing pipes under the house, and that there is a furnace under there that heats the house, and also tries to keep the space warm enough so that the pipes going to the septic don't freeze. The space needs to be able to stay above freezing, in addition to insulating those pipes and ducts for efficiency.

    I'm concerned about tightening up the space if water is still able to get under it. How does spray foam on the dirt floor hold up over time? How would I know if water is getting under the spray foam, and how would I be able to keep an eye on the integrity of the footings of the piers if it is all covered in spray foam?

  17. GBA Editor
    Martin Holladay | | #21

    You face two important tasks:
    1. Water management, which will involve regrading the soil around your house so that the grade slopes away from the house on all four sides. This may require the creation of swales or the installation of French drains.

    2. The conversion of your house from a house on piers to a house on a crawlspace foundation. The crawlspace foundation will be necessary to enclose your furnace and keep your pipes from freezing. (Knowing your climate zone or geographic location would help those who provide online advice.) You'll probably need to talk to a foundation contractor about building your home's new crawlspace walls.

  18. Expert Member
    PETER G ENGLE PE | | #22

    The big challenge is going to be that ledge. Trying to stop water that's running over the surface of ledge is not easy. You can't easily regrade or dig a trench uphill from the house where the water is coming from because of the ledge. Letting the water flow might be the best option. Do the perimeter foundation work first. Pin and bond masonry infill walls directly to the ledge and install a waterproofing system. This will slow down bulk water, not prevent it completely. Run gravel and drains from the uphill side around the house to daylight to help manage the exterior sheet flow across the ledge. If you have time to wait and see the performance, I would be tempted to stop there for a year or so, and change interior plans as necessary.

    Inside, even though it's a sloping floor, install a few inches of gravel, covered by a vapor barrier and a concrete slab. With the slope, installing the slab is going to be more like doing a stucco job than a concrete job. Shotcrete would probably be the best solution if there is a local vendor. Make sure that the gravel layer is free to drain through a pipe to daylight at the low point(s). Insulate and air seal the crawl space as mentioned in many articles here.

    Your specific conditions are going to control the exact installation methods, material choices, etc. You're going to need a foundation contractor who is very good with water management or an engineer to specify the details and make sure everything is done properly.

  19. TonyCavaliero | | #23

    I recently purchased a rural property in climate zone 2. Block and beam foundation with a foul smell in the home. The perimeter skirt is continuous metal and the crawl area is moist/wet. I intend to remove the metal to allow ventilation. The property is relatively flat during some recent heavy rains, fairly common in east Texas, the crawl area contained water. I am looking to regrade that area, with to slope from the center out and then insulate. I would like to use polyurethane, closed cell foam but don't see any mention of a spray foam system for floors in this article. Any advise on the use of Polyurethane foam on the sub floor in the crawl space?

  20. Joel471 | | #24

    I can't remember where I saw this detail for insulating the floor (it might've been this site; not sure). Looks like it works. Anyone have any concerns with it?

    1. GBA Editor
      Martin Holladay | | #25

      1. That method lowers the effective ceiling height -- so you'll need longer studs unless you like a low ceiling.

      2. That method provides about R-23, which is below minimum code requirements in Climate Zone 4 Marine and Climate Zones 5, 6, 7, and 8. However, you could get away with R-23 in a warmer climate zone (Zone 1, 2, 3, or 4 except Marine).

      3. If the floor assembly is exposed to cold outdoor air beneath the floor, be careful of that detail for insulating the drainpipe. Drainpipes can freeze solid in cold climates, as water slowly accumulates as ice and the pipe's effective diameter shrinks over the winter -- and the amount of insulation shown in the detail may not be enough.

      1. Joel471 | | #26


        Thank you for the reply.

        1. The plan is to set the sill plate on the plywood as shown on the attached. Hopefully the structural engineer won't object.

        2. This is for a residence in Climate Zone 2, with an R-13 requirement for the floor insulation. So I think I'm good there. I think I'll need about 3.5" of XPS.

        3. Copy on the pipes.

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