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Community and Q&A

Unheated building and frost heave in zone 5

mikeolder | Posted in General Questions on

I want to build a unheated building/garage in zone 5, and have been told footers below the front line should protect the stem walls.  But will I need to insulate under the garage slab if I have R30 walls and a R-60 roof? Will the building capture enough geothermal heat to prevent frost heave under the slab?
The building is located on a hill top with good drainage.


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  1. Expert Member
    Akos | | #1

    If you look at the prescriptive details for a frost protected shallow foundation, there is separate ones for a house vs unheated building. Generally an unheated building will have more sub slab insulation and larger wing insulation to capture more of the geothermal heat from the soil. The insulation of the structure above does not matter. R30/R60 for an unheated shop seems a bit overkill.

  2. mikeolder | | #2

    Thanks Akos.

    R30/R60 is overkill, but I may decide to build a 1000 sq' studio in 1/3 of the garage after removing the existing mobile home.
    Id like to leave the slab uninsulated to help cool the garage in the summer, and then travel south during the winter months but don't want to heat it while I'm gone, so this would be a summer house. I worked in a shop years ago with a uninsulated slab and it stayed cool because it was uninsulated.

    1. Expert Member
      Akos | | #4

      There no way around insulation if you don't want the slab to heave. With good drainage and soil prep, assuming the slab is not structural, this tends to be not an issue for garages.

      I find a bit of insulation is a good compromise. Even as little as R5 takes the edge off the cold slab in the winter time. This works best if above the slab with plywood above so when the heat turns on, the surface warms up much quicker. I rather have a more pleasant working surface 75% of the year than a slight bit of cooling for the two months in the summer. If you want cooling and cheap heat, can't beat a budget wall mount mini split heat pump.

  3. Expert Member
    Michael Maines | | #3

    If you don't insulate under the slab, you risk condensation forming in hot, humid weather. If the concrete surface is not sealed, the condensation will soak into the concrete, but it will be there and will support mold and microbial growth that leads to odors, and you might find that the elevated humidity causes your tools to rust. But if you use equipment to keep the humidity low, it may not be a problem.

    Frost walls that have footings that extend below the frost line will keep your walls from heaving, and will help keep the slab from heaving, but I've seen a lot of slabs that have cracked and heaved as a result of poor subslab preparation. The fill should be freely draining, preferably with an interior footing drain that connects with the exterior footing drain and leads to open air. It should be compacted in 6-8" lifts.

    If the area below the slab can't hold water then it can't freeze, and if doesn't freeze then it won't heave. If the area below the slab allows water to flow through and the soil is not compacted or if it contains fine material that can wash away, then you could get slab subsidence, sinking down into the ground.

  4. mikeolder | | #5

    Thanks again.

    I wont be able to use a FPSF because I'm building on a whale back hill top, so stepped footings and wall would be needed. I'm against foam because of insects, so I assume at this moment the best choice would be Comfortboard under the slab and on the inside of the stem walls? Looks like my local lumbar yards don't carry it. Are buried foam infestations rare?

    1. Expert Member
      Michael Maines | | #6

      Foam under a slab is usually considered fairly safe from insects, compared to insulation on the exterior of the foundation walls where they have easy access. If you're still concerned, you could use what I spec, borate-treated EPS. Or use Comfortboard if you can find a way to get it.

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