Could spray foaming my crawlspace floor joists cause frost heave??
You say that your foundation was "built on a rock foundation". I guess I don't know what you mean. Are you saying that your foundation was built on rock (ledge)? Or are you saying that your foundation is fieldstone? Then you say your foundation is a foot deep. Does that mean the bottom of your foundation is one foot below grade?
My guess is that if you have not had frost heave since 1906, you are not likely to develop it just because you insulate the joists, since I doubt the amount of heat lost through the bottom of your floor would have been sufficient to prevent ground freezing.