Air Sealing or Heat Cable: Which One Fixes Ice Dams Where You Live
Why the attic matters as much as the town
Most dam days come from heat leaking out of the house and melting the underside of the snow. On a typical house, about half of a typical city's dam days are attic-driven. Sealing the gaps in the attic floor and bringing insulation up to current levels cuts a median of 4.9 dam days a winter across the cities at risk. With a typical attic, 586 cities are high or severe; with a leaky one, 958; with a sealed and insulated one, 91.
Where cable still earns its place
Sunny freeze-thaw days melt snow without any help from the attic, and in the coldest, longest winters even a sealed house builds some dams. The 91 cities that stay high or severe after sealing are where this site suggests pricing heat cable along the eave and in the gutters. The three with the most dam days left on a sealed house are Truckee, CA (32.2 dam days), Steamboat Springs, CO (28.2 dam days), Jackson, WY (27.6 dam days). Elsewhere, cable treats a symptom that air sealing would remove. Why a snowy town can still come out low is covered in why snowfall misleads.
What cable costs to run
Left on whenever snow lies in freezing weather, 60 feet of 5 W/ft cable at 17 cents a kWh would cost from $5 to $173 a winter in those 91 cities, with a middle of $67. A thermostat or moisture controller that switches it off in dry cold weather cuts that. Running cost is the small part; installation and the shingle damage from badly fastened cable are the larger ones.
The order that works
Seal and insulate the attic first, because it lowers both the dam days and the heating bill. Add an ice-and-water membrane under the shingles at the next reroof, which stops the leak when a dam does form. Consider cable only where the sealed-house figure on your city's page is still high, and keep a roof rake for the storms in between. See the method for how the attic settings are modelled.