The answer
Summer: Trapped attic heat (140°F–160°F) bakes shingles from below, causing volatile oil loss and blistering. Winter: Indoor moisture condenses on sheathing, rotting wood and rusting fasteners; escaping heat causes roof snow to melt and refreeze into destructive ice dams. IRC R806.2 requires minimum 1:150 net free ventilating area (1:300 for balanced soffit/ridge systems). Inadequate ventilation voids shingle warranties.
Sources
This answer is drawn from the standards and guidance below. It is general information about how these systems work — not a diagnosis of your property.