Frost protection

[mattradk]

I need to place an 8" stem wall for a proposed unheated garage immediately next to an existing stem wall for an existing garage that was converted to habitable space.

With the slope of the driveway being moderate, I'd like to start the new garage floor 6" below the existing floor to try to keep the slab as close as possible to the slab height at the garage door 24' away with an additional 6" slope.

The problem is that I need to maintain a 48" footing depth for frost protection. The building code says that the foundation depth can be less than the required depth if frost protection is designed by a "design professional". I have looked for guidance on this but I've been unable to determine what frost protection might be adequate to protect the new portion of the foundation.

The drawing I've uploaded shows two situations. The first shows what the wall might look like with a 3'6" depth to footing bottom, but with R10 rigid insulation added under the slab and against the new wall. The second situation has the new footing extending down to 48" (still with insulation).

My reservation with the second situation is the potential for undermining the existing footing, though I don't know that this is much of a problem extending only 6" below the existing footing.

The garage is 2x6 wall construction with trusses.

Are there any suggestions for the use of insulation to be able to implement the first situation in the drawing, or would am I over-concerned about the potential for undermining the foundation?

I had asked a similar question before, but I hadn't figured on dropping the beginning elevation of the new garage floor 6", so frost depth was not a question.

Thanks

https://files.engineering.com/getfi...2e-6b9be6bf82fd&file=Adjacent_Foundations.jpg

 

[BridgeSmith]

Considering the adjacent heated space, I wouldn't have any heartburn with going 3'6" depth for the footing on that side. The other sides presumably exposed on the outside, I'd go the 4'.

 

[oldestguy]

I'd use the shallower depth and indicate the distance to foundation and its support from the edge of the buried insulation meets the requirement of 4 feet of "depth" (distance) from the ground surface. Just use a four foot wide sheet of buried insulation. Your alternative to 4 feet depth raises concern of disturbing support for the house. Most buildi9ng inspectors I have run into will allow changes if explained properly to them. If you can't locate what they call "engineer", see if the municipality has or hires a consultant. Actually "frost depth" usually is an average of area conditions and you may have a shallower depth, depending on many factors. Also, if it was a separate building, could you use a shallower foundation, or even just a slab?

 

[BridgeSmith]

If you're still hesitant, perhaps consider JAE's proposal in this thread

"Without the heating system, you would then have to look at the potential temperature of the inside (vs. the outside) and then adjust the depth of footings or granular accordingly.

So if the exterior frost depth footings are at 36" below grade, your interior footings may be at 30", or 24" or something proportionate to the low temperature."

The temperature in an unheated indoor space won't get as cold for as long as the outdoors, especially if it's fairly well sealed and has windows.

 

[mattradk]

As far as making it a separate building is concerned, the owners want the existing dwelling and the new garage mated together. They needed a variance because the garage will be half the distance to the road than the setback allowed. The neighbors objected to the way it would look, so I'm trying to blend it together as much as possible.

 

[MTNClimber]

I'd go with Oldestguy's recommendation of installing horizontal insulation.

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http://msdssearch.dow.com/Published...foam/pdfs/noreg/178-04042.pdf&fromPage=GetDoc

https://www.homeinnovation.com/~/me...de-to-Frost-Protected-Shallow-Foundations.pdf

 

[OldBldgGuy]

In determining the thickness and location of insulation, draw a section and look at the path for the cold to travel. The depth of cover (4' in my area) must be replaced by equivalent insulation in the path of travel of the cold. The short version of what I just said is place the insulation horizontally to meet the requirement for frost protection just as oldestguy said.