Typically, ice dams form due to poor insulation or not enough ventilation. Warmth from the interior of the home rises, as it tends to do. That warm air heats the attic, which heats the roof sheathing from below—causing the snow to melt and refreeze, and forming the dreaded ice dam.
This means you want your attic to allow some air from the outside to come inside, therefore keeping the attic “cool enough” so that it doesn’t warm up the sheeting and start that whole process.