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insulated shallow frost protected slab with no building on it

this_page_left_blank | Posted in General Questions on

I’ve fallen far enough behind in my workshop project that I’m doubtful I can get the building closed in and heated before winter. I’m still hoping to get the foundation done, however my understanding is that this kind of slab requires some heat from the building for its frost protection. Would leaving it barren over the winter be bad? Is there some way to mitigate it? What about burying some heating mats under the slab, near the perimeter and connecting them to a thermostat set to just above freezing?

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  1. Expert Member
    Michael Maines | | #1

    The recommended details for FPSFs assume that the building is enclosed and heated to 64°F. In those buildings, the earth's heat from below and the building's interior heat are both working against cold air trying to freeze the soil. When that cold air is also working over the entire slab, the soil is more vulnerable to freezing.

    The IRC does not provide for frost protection of unheated buildings but the source documents do; in my area (Maine) the difference is having no frost wings--the horizontal insulation around the building perimeter--to having them 5' out from the building.

    I have seen new foundations heave in the frost in similar situations, so I would be careful. Ideally, just put off the foundation until the following spring. If you are just pouring a perimeter foundation wall, you could cover it with heat blankets. If you pour a slab as well, you'd want to keep that warm too.

    1. buildzilla | | #4

      i'm considering pouring a basement foundation in cz-6 this late-summer as well, but it's not clear that i would be able to dry-in a frame before winter.

      would footings and walls with no slabs need some kind of special treatment over the winter to avoid damage?

      what about a basement slab with r-15 eps under?

      or worst yet, a garage-slab with no insulation?

      1. tdbaugha | | #5

        I was planning the same. Originally I thought if I didn’t back fill the walls I’d be good too. Now that I think about it more, I’m not sure that it’s a good idea to leave the foundation in that condition over the winter.

        What level of insulation is needed to keep the frost out of the ground under the footings and slab?

        1. buildzilla | | #7

          your footings would be "under-the-frost-line" (eg 60"), but in order for that to be true, i guess your foundation would need to be back-filled to that depth.

          slabs would be above the frost-line, and should have good drainage underneath, but still unclear if they can withstand months of sub-zero temperatures at their surface.

          an example could be an unheated, uninsulated detached garage, or even an open carport on a slab...

          1. tdbaugha | | #10

            You can’t backfill the walls though. Frost heaves in the direction of heat loss. If you backfill, the frost will heave horizontally into the wall. A client of mine had this happen to him, not good!

          2. buildzilla | | #15

            @tdbauga good info.

            so if u back-fill the entire height of a foundation wall, there will be warmer dirt on the outside and freezing cold air on the inside, and u r thinking that would create a pressure from the outside->in that could cause damage.

            maybe just back-fill to frost depth on both the outside and inside evenly.

            would want people with experience to chime in tho, i'm not gonna risk the foundation on a theory :)

  2. Expert Member


    As Michael said, the guide does show details for unheated buildings. You can see the difference on page 17:

    Like Michael I'm a bit wary. I'm unsure of how differently an unheated building acts as opposed to just a foundation.

  3. Expert Member


    I keep forgetting to ask: You are Trevor aren't you?

    1. this_page_left_blank | | #11

      Yes. I've complained multiple times that the site won't let me change the display name, but they don't care.

  4. walta100 | | #6

    Seems to me it just doesn’t matter. If it does freeze it will lift the slab maybe a inch and it will settle back in the spring. Could the slab crack? Maybe but assuming there is some steel in the concrete any crack will be small and the slab would still be usable. Look at your driveway it freezes and thaws year after year without cracking.

    Given that you have so little time for this hobby that completing the shop will not happen before winter it seems likely you will tired if paying the cost to heat the shop before long and the slab is going the learn to surf with the freeze thaw cycles and it will likely do just fine so long as it is not attached to something that does not move like your house.

    The other option is to cover the slab with 2 inches of foam and cover it with a truck load of dirt.


    1. Expert Member
      Michael Maines | | #9

      It might settle down to its original position, but it might not--it's pretty risky to take the chance. The frost-heaved foundations I've seen settled partway but not all the way. Garage driveways often end up higher than the garage slab because each year they heave a little but only come partway back down.

      I like your idea of covering everything with insulation and dirt.

      1. this_page_left_blank | | #12

        Wouldn't 2 extra inches below the slab accomplish exactly the same thing as 2 inches on top? The concrete is not going to frost heave. And if that's true, if I'm already exceeding the minimum recommendation by 2 inches, I should be fine.

        1. Expert Member
          Michael Maines | | #20

          Frost can get under the edge of horizontal insulation when the building isn't heated, so you're right--if there is enough insulation under the slab for the warm ground to win the fight against cold air--but the foundation perimeter is still vulnerable. That's why when I've done the calculations I get 5' frost wings at building corners with 4' frost wings in the field, away from building corners.

  5. gusfhb | | #8

    Piles of hay?

  6. tdbaugha | | #13

    This article may be useful:

    This paragraph is important:

    “ Soil has an insulating value of about R3 per foot, where foam insulation will be between R3 and R5 per inch. That’s why you can build in an area where the frost depth is 4 feet in winter by using 4 inches of insulation instead of 4 ft of dirt.”

    Another useful source of info, Joe Lstiburek. “Building science meets mountain climate” skip ahead 1:23 for foundations.

    With regard to my concern further up, pouring a basement foundation in the fall and leaving it exposed over the winter, what should be the strategy to protect it from frost heave? How much wall insulation is needed to keep the frost heave direction vertical instead of horizontal on the walls. And how much slab insulation should we use to protect the horizontal concrete surfaces. According to the article above, 4” should do the trick unless I’m missing something.

    1. tdbaugha | | #14

      I can’t think of a good enough strategy worth trying that’s better than either A) getting a framer to start framing in the fall or B) postponing the foundation until the spring.

  7. buildzilla | | #16

    @tdbaugha i'm leaning with u. unless there are tried-n-true strategies that don't involve a lot of effort, too much effort/risk to get a head start in the spring.

  8. maine_tyler | | #17

    If you build to the methods Michael mentioned in post #1, you would pretty much be good to go, and you would have the added benefit of being able to not heat the shop at a later date if desired (this includes new owners, who may decide not to heat). Seems an overall more robust strategy to begin with and accomplishes the goal of allowing a fall pour.

    Alternative may be to lay temporary horizontal insulation out beyond the foundation (wings) and then repurpose for elsewhere in the build later. Probably would want a good even grade and some ballast in this case so it doesn't all get ruined.

    1. charlie_sullivan | | #19

      Excellent advice.

      An option for temporary insulation is straw. It can be repurposed as mulch and erosion control.

  9. seabornman | | #18

    Just insulate under the entire slab.

  10. rocket190 | | #21

    You’re also overthinking it compared to us northerners that work in the cold all the time. Pour the slab, and if you don’t get the shell done, put down a layer of plastic, then a nice thick layer of straw or hay, and then another layer of plastic. You technically don’t need the lower plastic, but hat that starts to rot can leave a black stain on an unsealed slab. The plastic on top keeps it from blowing. The hay works much better than concrete blankets or foam sheets.

    If you are doing a basement wall that needs to sit over winter you CAREFULLY backfill the exterior of the walls with frost chunks. The frozen chunks won’t expand and thus put undue vertical pressure on your walls. The wall trenches do settle significantly in spring, but that’s about the only downfall.

  11. vetitude | | #22

    For what its worth, I followed the protocol outlined in this article last fall. The slab is in Maine, zone 6A near the coast.
    Then due to unanticipated events, the garage was not built last fall. Again, due to the same unforeseen events, no insulation or any attempt to protect the slab was attempted. 8 months later, there are zero cracks, and the slab is level. So, in essence, I am agreeing with those who say insulate under the slab, provide a drainage system, and then insulate 4 ft out around the slab.

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