I run into this all the time.
In Colorado where the winters are cold, the floor above the crawlspace is uninsulated. The foundation walls are leaky and the home loses a lot of heat into the crawlspace… that’s what keeps the plumbing pipes hanging from the floor joists from freezing since there’s no means of heat distribution through the crawlspace.
Clients want to know if they should insulate the floor to save on heating costs. Well, saving on heating costs is always a good idea.
“How do I keep the pipes from freezing?” they want to know. Heat tape can help, but it’s electrical and in the mountains, when it gets real cold, sometimes we lose the electricity for a while. Sometimes for days. If you happen to be gone and your electric heat tape is all that stands between you and a big bill for a flooded home… you’ve got a potential problem.
I ask local plumbers about this and they just roll their eyes. Any suggestions?