I am building a home in North Dakota with footings just poured.  My footings are generally very deep, but along the walkout side of the house they are at the local code depth (~5 ft).  Before we backfill, I am tempted to place 2 inches of XPS rigid foam over the footing and extending out 1.5 – 2 ft from the footing as an extra level of protection from frost.  I want to set the foam in the existing trench which is why I’m only proposing it to extend 1.5 – 2 ft from the footing.  I’ve attached a detail showing my plan.

Is this worth doing?  Is there any reason not to do it, such as subsurface drainage issues with the foam sitting above the footing drain?  Appreciate any opinions!