Ceiling insulation or lack of is not the problem. I have an unheated barn with a 12/12 pitch and I get ice damns. Even though you will get heat loss through the roof a considerable amount of melting is from the sun especially with a roof with little pitch and with a southern exposure.
Interesting, and that makes a lot of sense. I recall, however, seeing a much larger percentage of older, less well insulated homes that have large ice dams and icicles hanging off, while the newer homes do not.
Like many things, it's the combination of issues that really makes the deal.