An ice dam is caused by the warming of an attic space. And while attic ventilation and insulation contribute to the prevention of ice dams by keeping attics cold, they can be overpowered by other by other attic warming effects-such as air leakage from the house into the attic through ceiling bypasses, chases, open gaps, or un-insulated ducts placed in the attic. If significant conditioned air escapes into the attic throug bypasses, the attic ventilation will not be capable in preventing the warming of the roof decking and subsequent ice dams. Therefore sealing air leaks between the house and the vented attic is essential to making attic ventilation work.