What Causes Ice Dams?
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 attic warming effects - such as air leakage from the house into the attic through ceiling bypasses, chases, open gaps, or un-insulated ducts place in the attic. If significant conditioned air escapes into the attic through 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 the attic ventilation work. Air leakage from the interior into the attic also introduces moisture. If significant interior leaks into an attic, attic ventilation may not be sufficient to prevent attic moisture and condensation problems.
Check out Section 2.5.5 of InterNACHI’s free, online “How to Inspect Moisture” course for information on inspecting methods and practices of preventing air leakage into attics.
Air Leakage and Air Sealing: A Homeowner’s Guide. This guide provides descriptions and illustrations of the 19 most common areas of the home most likely to have air leakage.