Rigid insulation placed under continuous concrete footings?

[tgengr]

I have a client (homeowner) who wants to insulate under the slab on grade and wants to continue the insulation down under the continuous concrete footings (turned down footings 12" embed. The location of the project is in the west and there is not frost depth it is a one story wood frame. The slab is going to be heated (reason for insulation). Any advice or recommendations from the cold weather structural engineers since this is a first for me and not our typical construction method and does not appear normal practice of running the insulation under the footings.

 

[ATSE]

For lateral loads, friction resistance (concrete to soil) is usually much greater than passive. Putting a layer of special insulation with unknown friction properties between the concrete foundation and the soil doesn't seem like a good idea for high wind or seismic.
Go deeper.

 

[Splitrings]

ASCE 32, "Design and Construction of Frost Protected Shallow Foundations", is a good reference.

My experience is that most install insulation under the slab and inside face of the wall. Most are actaully extending the wall insulation from the footing all the way to the top of the concrete wall. This also creates a thermal break between the slab and wall.

 

[HeavyCivil]

I am also skeptical that any real benefit will be realized here.

But if you're using 25psi rigid foam, that's 3600psf. Assuming that's an allowable load, that exceeds the performance of many soils we encounter around here... I don't see how 'disastrous settlement' could occur - although it might be smart to consider how the product breaks down over its service life. If you're providing 2" or 4" and over 50yrs it breaks down and is crushed to a small fraction of its original thickness then I suppose you would have disastrous settlement, albeit relatively even throughout the structure...

The only issue I see with a floating slab with a 1/2" fiberboard joint for thermal break is seeing the 1/2" fiberboard joint. Doesn't bother me if it's caulked cleanly, especially in a basement.