Improper installation can lead to air leaks and issues—like ice dams.
Why would I get ice dams?
If your attic is not conditioned, the temperature of the whole roof should be largely consistent all over and you wouldn’t have snow melting in one area and then re-freezing in another. However, if heat is finding its way from your conditioned living area into the attic you can get warm areas and cold areas, which may cause ice dams. Remember, both heat flow and warm air travel from areas of higher temperature to areas of lower temperature. Proper air sealing and ceiling/attic insulation can prevent warmth from escaping and causing ice dam issues.