Ice dam prevention
As winter progresses through cycles of freezing and thawing, buildings experience ice buildup and roof damage. In general, ice dams are formed when attic heat moves up to warm the roof and melts snow at or near the ridge. Snow melts, runs downward and hits the edge of the cold roof, resulting in re-freezing snow. These cycles cause ice to accumulate and back up under shingles. Damage appears in the form of soaked insulation, stained, cracked and damaged sheet rock, damp, smelly, rotting wall cavities and stained, blistered and peeling paint.
The fundamental problem is that the attic space and roof are warm. The solution, then, is to try to maintain a cold roof. Minnesota State code requires insulation that has an R-value of 38 or above. In addition, the ceiling must be airtight. Areas around the chimney, plumbing vents, wiring, or thin spots in the insulation are escape routes for warm air coming from living spaces.
Attic ventilation plays a relatively small part in controlling ice dams. There are a number of reasons for this. The first and most important is that winter air (cold air) just does NOT have the capacity to "pick up" moisture entering the attic space from the home and remove it. Where ventilation does play a role is that it helps maintain a cold roof deck.
Two forces move air through the attic. One is wind. The other is temperature difference between attic air and air outside. As attic air warms up it rises and exits the ridge vents and as it leaves, it is replaced by cold outside air entering through the soffit vents (passive ventilation). This process helps keep the roof deck cold, which is the objective in preventing ice dams. Usually, mechanical ventilation which exhausts air from attic space only causes problems. Systems which positively pressurize attic space to help control ice dams must be designed by persons with experience!
Passive attic ventilation helps maintain dry attic space and remove heat build-up during summer months. The greater the vertical distance between the outlet and inlet of air flow the more effective these systems are. Using a combination of ridge vents and soffit vents is based on this principal.
Weatherization contractors may be listed under Energy Management and Conservation Consultants or Insulation Contractors in the Yellow Pages. The contractors could use a blower door test and an infrared camera to help detect places where heat and moisture are escaping up into the attic space.
Revised: Richard Stone, 2006
Reviewer: Richard Stone, 2010, 2014
Related publication: Ice Dams, Timothy Larson, Lewis Hendricks, and Patrick Huelman. Revised: Richard Stone, 2006.