The Truth:

We all agree ventilation is beneficial for a home, however, experts say it is often highly over-rated as a solution for ice dams. Many factors when designing a properly ventilated roof are not given adequate consideration or are missed altogether. Roof layout (dormers, valleys, etc.), slope, air leaks, true insulation values and airway passage sizing must all be carefully calculated and constructed, and still may not eliminate ice dam formations.

This roof to the right was constructed with R-40 insulation and continuous eave and ridge ventilation.

More Truth:

Here’s a thought about which you don’t hear among ventilation proponents (but real-world observations show): when the primary element of flow-through ventilation–continuous ridge venting–is covered with even a few inches of snow, flow-through ventilation is impeded and ice dams begin forming. Even a well-designed ventilation system stops venting.