What causes ice dams, and how do I stop them?

Short answer: Ice dams form when the roof deck is above freezing while it is below freezing outside and there is snow on the roof [1]. The fix is an airtight ceiling first, then more insulation, then ventilation under the roof deck [1].

Why they happen

Heat from the house warms the roof, the snow melts, and the water refreezes at the cold eaves. University of Minnesota Extension says air leaking from the house into the attic is, in many homes, the major mode of heat transfer that leads to the formation of ice dams [2].

The fix, in order

1. Seal the ceiling. First, make the ceiling air tight so no warm, moist air can flow from the house into the attic space [2].
2. Insulate. Then consider increasing the ceiling or roof insulation to cut down on heat loss by conduction [2].
3. Ventilate. Keep air moving under the roof deck so any heat that gets there is flushed out [1]. That is why we put baffles at the eaves.

Joe Lstiburek of Building Science Corporation puts it plainly: airtightness is more important than insulation levels. But both are important [1].

ENERGY STAR lists a history of ice dams as a sign of serious air leaks [3]. Attic insulation.

More questions

Will more insulation stop ice dams?

It helps, but air sealing comes first. Building Science Corporation says airtightness is more important than insulation levels, though both matter.