In snow climates, excess attic heat will lead to ice-damming and subsequent water damage.
Most people attribute the problem of ice damming to the roofer.
However in most cases ice damming and water leakage are not a result of unsatisfactory roofer workmanship, but rather are caused by one or more subtle sources of heat created by the electrician, insulation contractor, HVAC contractor, GC or architect. Poor attic ventilation can lead to moisture condensation causing wood rot, attic insulation failure, and the growth of mold and mildews that can lead to health problems for the resident. The rule of thumb is that properly vented attic air should never be more than 15 degrees hotter than the outside air. Ideally, the total of inlet air at the soffit will equal the outlet air at the ridge for a balanced and efficient airflow. The following are pictorial examples of some ways unwanted heat enters the attic space and some reasons why heat cannot exit the attic space.