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Community and Q&A

Cold climate frost-protected shallow foundation

RICHARD EVANS | Posted in Green Building Techniques on

Hi Folks, 

Im in Climate Zone 6 (60″ Frost Depth) and am building an unheated garage myself.  I’d like to use a Frost Protected Slab approach to save on money and for the experience. 

I have a good understanding of this foundation style for heated, high-performance homes. But a garage version has me curious over a couple of details.  I read this great JLC article from Andy Engel but still have some questions:

1.) I realize I will need ‘skirt’/ horizontal insulation extending out 5’.  But do I need vertical insulation around the thickened slab edges?  It seems as though it would be destroyed on the front of the garage as cars would be driving over it. Also, it seems pointless given that the slab will always be cold anyways?

2.) Is 2.5″ of Type IX EPS under ‘footings’ and 2.5″ of Type II  EPS under the non-load bearing areas sufficient thickness for preventing freezing temps under the slab in my climate? I chose 2.5″ because I can cut it easily with my circular saw… I’ll also have a bed of 3/4″ washed stone under the sub-slab insulation and drains to daylight.

3.) Should I add a drain or slope slab to garage door? Would a drain invite freezing temps below the slab? If I slope, should I use steel shims under walls?

Thank you!

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  1. Expert Member
    RICHARD EVANS | | #1

    Bumping this hoping for some feedback... Thanks!

    1. Expert Member
      MALCOLM TAYLOR | | #2


      Have you read this? It gives guidance for both heated and unheated buildings:

      1. Expert Member
        RICHARD EVANS | | #3

        Thanks Malcolm- this is exactly what I needed. Looks like I will need more like 6" of EPS! I was way off.... Thanks again.

  2. Expert Member
    Michael Maines | | #4

    Rick, if you are required to follow the IRC, be aware that they no longer includes prescriptive guidance for unheated FPSFs, so if you are pulling a permit you may need to engage an engineer.

    R309.1 requires that if the garage will be used for vehicles, the floor surface needs to be sloped for drainage to the exterior or a floor drain. There are pros and cons to both but a floor drain is small enough that it should not create frost heaves. If you slope the floor for drainage--my usual approach is a 2" slope from back to front--you can either custom-frame the walls or build up a level curb from cast concrete or concrete blocks. But with a mono-pour it might be easier to do the floor drain, I'm not sure.

    1. Expert Member
      RICHARD EVANS | | #5

      Thanks so much Michael! (Was hoping you would chime in :-)

      No inspector here but I'll play it safe with my formulas using the document Malcom provided.

      Thanks too for the info regarding drainage. The floor drain sounds dramatically easier than sloping.

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