I find this interesting. The rules between different parts of the country are so varied. I naturally assumed the stricter requirements would be in Buffalo rather than in Michigan.
Here, for residential work, the minimum footer ground depth is 42" . It used to be 36" and if you wanted to be extra sure on a given build, you went to 48". The idea being that those were the max frost depths and anything buried at least that deep would never freeze. I can only assume the addition of insulation in some way enhances that.
Ricky do you know the min frost depth where you are?