Understanding Required Frost Wall Depth
Emel
| Posted in General Questions on
Zone 6, the frost wall requirement in our area is 4’. Until now, I always thought that meant 4’ of the stem wall & footing was required below grade but you would also add the above grade requirement (for example 12” above grade to prevent water intrusion). If done this way, the wall with footings would total 5’. However, our contractor said that only 4’ of insulated coverage is required, meaning not an entire 4’ would be below grade level. Does that sound more accurate than my original understanding?
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Replies
Emil,
Codes require that the bottom of your footings has to be "below the depth of frost penetration", or similar language. If that is 4 ft where you are, that's where the excavation needs to go down to.
That’s what I thought and said but they said because it’s a slab on grade and the interior of the stem wall is insulated and under the slab itself is insulated, it only had to be 4’ total and not go below the full 48”. I had never heard that.
Emil,
By his logic if you have all of the four foot stem-walls exposed you could set your footings at grade.
Time for a new contractor
Your excavator is wrong, but he might be conflating the rules for frost protected shallow foundations. The exact requirements vary more finely than by climate zone, but I'm in CZ6 and in many cases just 12" of vertical foam insulation on the exterior of a slab-on-grade is enough to satisfy the requirements here. I have used the same approach on stem-wall foundations (aka frost walls), incorporating a frost wing--sloped, horizontal insulation above the footing.
There have been a bazillion Q+As and a few articles on the topic here; this a recent, good one: https://www.greenbuildingadvisor.com/article/frost-protected-shallow-foundations.
Here is the IRC code reference:
https://codes.iccsafe.org/content/IRC2018/chapter-4-foundations#IRC2018_Pt03_Ch04_SecR403.3