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Community and Q&A

Frost-Protected Crawl Space Construction

jsfura | Posted in General Questions on

I am planning to construct a frost-protected footing/stemwall for a crawlspace in the northern Illinois region. Based on IRC guidelines, I only need to provide vertical insulation on the foundation wall to a minimum depth of 14 in. (depth to top of footing). Based on this, I have two general questions:

1. If I only need to provide vertical insulation, can I install it on the inside of the foundation wall instead of the outside? The commentary for ASCE 32-01 states that “vertical insulation may be placed within a perimeter wood foundation or … on the interior and exterior of a perimeter grade beam (ICS) provided there are no cold bridges between the interior of the building and exterior of foundation wall.” It would seem that the insulation could be entirely on the inside if there are no cold bridges as noted.

2. With such a shallow depth to the footing drain, I was considering extending gravel all the way up to grade and creating a perimeter gravel bed around the foundation. This would not allow drainage away from the foundation wall, but would that matter if any water could drain right down to the footing drain?


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  1. GBA Editor
    Martin Holladay | | #1

    It's very common for crawl spaces to include interior insulation. A GBA detail for this type of crawl space is shown below.

    If you backfill with gravel or crushed stone, I advise that you stop about 6 inches before your final grade. Install a layer of landscape fabric over the crushed stone, and then install 6 inches of clay-rich soil, sloped to drain.

  2. GBA Editor
    Martin Holladay | | #2

    Here's another detail -- this one from Building Science Corp. The good thing about the BSC detail is that it shows that the crawl space floor is above grade, which is the best way to detail a crawl space.

  3. user-741168 | | #3


    Exterior insulation keeps the footing warm, so it cannot heave from frost. Only with exterior insulation can you consider a footing shallower than frost depth for your area. A shallow footing that is cold--and wet--is a prescription for heave.

    Bill Rose

  4. GBA Editor
    Martin Holladay | | #4

    Thanks. I hadn't realized that Jon intended to install his footings at a shallower depth than usual.

  5. jsfura | | #5

    Martin - Yes, the top of my footings will be approx 16 in. below grade which is why I considered extending the gravel up to grade. I understand the benefit of the clay-rich soil for drainage away from the foundation wall. But would there be any benefits of a gravel perimeter around the house, such as to help with termites or other critters? Thanks.

    Bill - Does it matter whether the footings themselves are warm or cold? I thought the point of the insulation was to keep the soil warm. Wouldn't insulation on the inside of the stem wall still keep the soil warm (above freezing)? Thanks.

  6. GBA Editor
    Martin Holladay | | #6

    If frost gets under the footings, the footings can heave. The insulation should be located between the footing and the cold soil. The coldest soil is the soil near the surface.

  7. Expert Member
    Dana Dorsett | | #7

    Frost heaves propagate in the direction of heat loss. For a foundation footing to frost heave requires three things:

    1: sufficient moisture in the soil to grow long frost crystals (no water== no frost== no frost heave)

    2: sustained temperatures in the soil under the footing below freezing followed by freeze thaw cycling (the better to grow long ice crystals)

    3: the temperature the frozen soil under the footing to be substantially warmer on average than the frozen soil above the footing. (To propagate the crystal growth from the soil below the footing, upward)

    Fix any one of them and you've fixed the problem. A common solution is to put the footing below the design frost depth for the region, guaranteeing that the soil under the footing won't freeze. Another is to insulate the stemwalls on the exterior and bury "wing" insulation around the perimeter extending as far out from the foundation as the design frost depth, effectively earth-coupling the soil under the insulation to the deep subsoil temps, which is also sufficient.

    But with sufficient drainage the problem is also solved (that's how they do it on highways, after all.)

    Uninsulated foundations and slabs in unheated buildings are far more prone to frost heaving in heated buildings. In unheated buildings the heat loss in winter is always from the ground-up. But in heated buildings, even those with insulated slabs, the heat loss (and frost crystal growth) at the foundation is mostly lateral, and away from the foundation from the dirt under the slab toward the dirt under outside Only in very shallow foundations would you get sufficient turn to change that to an upward direction at the footing, even with insulation on the interior side of the stem wall.

    Bottom line, controlling the moisture content of the adjacent soils is primary, the rest of it matters only if you screw up the drainage. Clay traps & wicks moisture, sand & gravel not so much. Compacted pea gravel doesn't wick water well at all. Sand can, but nothing like clay & loam will. With better draining back fill and even a modest amount of wing insulation you can get there, but it's your house, your risk if you cut too many corners. Personally I'd move the insulation to the exterior, AND beef up the drainage. There's quite a bit of online material detailing "frost protected shallow foundations", and it's worth reading up on before finalizing any decisions. eg:

  8. user-741168 | | #8

    There may be several mechanisms of “frost heave” in buildings. I’m really familiar with only one: An outlying part of a building (for example a garage slab corner away from the house) receives no vagrant heat and is a dump site for roof rainwater. The soil at the site gets saturated, and the temperature turns cold. The ice at the surface attaches strongly to the building element. Then, because of expanding soil beneath or a growing ice thickness at the attachment, the attached surface ice layer rises and takes the building element up with it. There is a diagonal crack in the garage slab about 2' back from the corner. I've seen it dozens of times.

    Use of the term “frost heave” is curious. I was told by my forebears that the concept of a frost depth comes from gravediggers, who knew the depth at which soil went from frozen to semi-fluid. Soil in cemeteries is usually well-drained, so the solid soil was still presumably porous. Such soil will look frosty rather than like a cake of ice. I usually use the term frost, as distinguished from ice, to refer to crystal formation where the source of the frost is vapor in air. I doubt that such crystal formation can lift buildings when the crystals are surrounded by air. Porous soil would compress before the building would rise.

    I’ve heard the “ice lens” theory, and I’m not (yet) convinced. Water that is bound in a porous soil matrix will not escape that matrix on its own.

    If my experience holds for some, many or most building heaves, then the idea of digging a foundation to “frost depth” doesn’t achieve much since the heaving is done at the surface. The fact that it works pretty well may perhaps be attributed to the resistance to rise that a deeper foundation provides (provided there is some tension tie in the foundation elements).

    Admittedly, my experience is limited. Central Illinois is not very cold. This is probably a bigger issue than this post alone.

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