Crawl Spaces vs. Skirts

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Crawl Spaces vs. Skirts

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

Posted on Jul 21 2017 by Martin Holladay

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. 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?

There are two ways to enclose the space under a house on piers: you can either install a conventional crawl space foundation — one with concrete footings and poured concrete or CMU walls — or you can install a wood-framed skirt.

Of these two approaches, a crawl space with concrete or masonry walls is by far the best. (The only disadvantage to this approach is the high cost.) Such a wall will be more durable, more airtight, and easier to insulate than a skirt.

A concrete contractor or a mason should be able to provide you with an estimate for the cost to retrofit this type of foundation under a house on piers. The work won't be cheap, but it's certainly doable.

For information on insulating this type of crawl space, see Building an Unvented Crawl Space.

Building an airtight skirt

The space under the typical mobile home is usually enclosed with a wood-framed skirt, and the same techniques used for mobile-home skirts are sometimes used for homes on piers.

These skirts are either simple affairs made of pressure-treated lattice (a type of skirt that is decorative rather than functional) or more carefully designed airtight enclosures insulated with rigid foam. Green builders will prefer the latter approach.

If you examine typical skirts built around homes on piers, you’ll notice that most of them are neither airtight nor insulated. If you want to build a quality skirt, here’s what you need to do:

  • Use a plumb bob to establish the location of the bottom of the skirt.
  • Dig a shallow trench, about 6 inches wide by at least 4 inches deep, around the house. The trench location corresponds to the bottom of the skirt.
  • Frame the skirt with a 2x4 stud wall. The bottom plate and the studs should be made of pressure-treated lumber; the top plateIn wood-frame construction, the framing member that forms the top of a wall. In advanced framing, a single top plate is often used in place of the more typical double top plate. doesn’t have to be pressure-treated. Start by placing the bottom plate in the trench; adjust the position of the bottom plate as needed by referring to the plumb bob.
  • Once the wall is framed, sheathe the outside of the skirt with pressure-treated plywood. Caulk the seams between the plywood sheets, and seal the crack between the top of the plywood and the house with caulk or canned spray foam.
  • Backfill the shallow trench to improve airtightness.
  • Install a continuous layer of rigid foam on the interior side of the studs. (There is no need to install insulation between the studs; the stud bays can remain empty.) Seal the seams of the rigid foam with high-quality tape. (Foil-faced polyisocyanurate is the easiest type of rigid foam to tape.) For fire safety and code compliance, it may be necessary to protect this rigid foam with a layer of 1/2-inch drywall. (Don’t let the drywall touch the dirt.)
  • If the skirt includes an access door, made sure that the door includes weatherstripping and a good latch or latches.

If you follow the advice above, you are creating a sealed crawl space (albeit one with wood-framed walls instead of concrete walls). Like any sealed crawl space, this type of crawl space will only work if the dirt floor is covered with a polyethylene vapor barrier, and if the exterior grade at the perimeter of the foundation is sloped away from the building. For more information on keeping sealed crawl spaces dry, see Building an Unvented Crawl Space.

Insulating and protecting the floor assembly

If the area under a house on piers will be left open to the weather, the floor assembly needs to be well protected. Here are some guidelines:

  • Install some type of fluffy insulation like fiberglass, mineral wool, or cellulose between the floor joists. Make sure that this insulation is installed well, without voids.
  • To address thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. through the floor joists, install a continuous layer of rigid foam under the floor joists. Seams between the rigid foam sheets should be sealed with caulk, canned spray foam, or high-quality tape.
  • To protect the rigid foam from abuse, install a layer of OSB or plywood on the exterior side of the rigid foam.
  • Seal air leaks at all penetrations.

For more information on this topic, see How to Insulate a Cold Floor.

Either leave it open or seal it up

Summing up, there are two basic approaches to handling the space under a house on piers:

  • Leave the area entirely open to the weather. This approach can make sense, especially if the piers are high and the floor assembly is airtight and well-protected with plywood or OSB. The main advantage to this approach is that the wind passing beneath the house reduces the chance of moisture buildup.
  • Enclose the area with airtight insulated walls. This approach protects pipes from freezing and excludes rodents. If you can’t afford to install a conventional crawl space foundation, you can achieve a similar result at a lower cost with a high-quality skirt.

Martin Holladay’s previous blog: “Air Conditioner Performance In Extreme Heat.”

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Jul 21, 2017 11:11 AM ET

You realize of course, that your house will rot
by Bob Manninen

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.

Jul 21, 2017 11:53 AM ET

Edited Jul 21, 2017 12:04 PM ET.

Response to Bob Manninen
by Martin Holladay

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.

Jul 21, 2017 2:35 PM ET

Edited Jul 21, 2017 2:42 PM ET.

Note to GBA readers
by Martin Holladay

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.

Jul 21, 2017 8:06 PM ET

by Malcolm Taylor

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.

Aug 10, 2017 10:51 AM ET

Edited Aug 10, 2017 11:04 AM ET.

Vapor barrier between joists and subfloor?
by S.S. MacDonald

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

Aug 10, 2017 11:04 AM ET

Response to S.S. MacDonald
by Martin Holladay

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.

Aug 10, 2017 11:23 AM ET

by S.S. MacDonald


Dec 31, 2017 6:07 PM ET

Edited Dec 31, 2017 7:37 PM ET.

How to build a skirt when there's not enough room for framing?
by Michael Bluejay

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?

Jan 1, 2018 6:06 AM ET

Response to Michael Bluejay
by Martin Holladay

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.

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