Ice dams have plagued roofs in cold-climates for years. When accumulated snow melts and flows down a slanted roof, it remains liquid, insulated under a blanket of snow. When it reaches the eves, it encounters freezing air, and ice accumulates forming a dam. This ice dam impedes the proper drainage of melt water and precipitation from the roof, and may result in leaks, and damage to ceilings, walls, and insulation.
Weather conditions conducive to ice formation, limited ceiling insulation, and inadequate air leakage control have resulted in formation of ice dams on this home. Control of heat and moisture flows is an ongoing area of research at FPL.
But ice formations aren’t only a threat to the building — the dangers ice can pose to occupants and bystanders are real. In 2010, for example, falling ice killed five and injured 150 people in St. Petersburg, Russia, following a particularly cold winter.
Roof ventilation is the most commonly used strategy for the prevention of ice dams, but researchers aren’t convinced that ventilation is a silver-bullet to halt ice dam formation. A comprehensive review of studies concerning roof ventilation by various research organizations throughout North America was completed by former Forest Products Laboratory (FPL) researcher Anton TenWolde and Bill Rose of the University of Illinois. That review, which was published 13 years ago, still serves as a good guide for how to limit the likelihood or severity of ice damming.
The full article, published in ASHRAE journal, is available here. After a careful evaluation of the selected studies, TenWolde and Rose offer the following conclusions on how homeowners, designers, and builders can help derail the dams:
1. Indoor humidity control should be the primary means to limit moisture accumulation in attics in cold and mixed climates; we recommend attic ventilation as an additional safe-guard.
2. To minimize the danger of ice dam formation, heat sources in the attic and warm air leakage into the attic from below should be minimized. The need for venting to avoid icing depends on the climate and the amount of insulation in the ceiling. However, ventilation is necessary in climates with a lot of snow to prevent icing at eaves, regardless of insulation level.
3. We recommend venting of attics and cathedral ceilings in cold and mixed climates. However, if there are strong reasons why attic vents are undesirable, unvented roofs can perform well in cold and mixed climates if measures are taken to control indoor humidity, to minimize heat sources in the attic, and to minimize air leakage into the attic from below. However, ventilation is necessary in climates with a lot of snow to prevent icing.
4. Ventilation should be treated as a design option in cold, wet coastal climates and hot climates. Current technical information does not support a universal requirement for ventilation of attics or cathedral ceilings in these climates.
In summary, for each of the most commonly cited claims of benefits offered by attic ventilation (reducing moisture problems, minimizing ice dams, ensuring shingle service life, and reducing cooling load), other strategies have been shown to have a stronger and more direct influence. Consequently, the focus of regulation should be shifted away from attic ventilation. The performance consequences of other design and construction decisions should be given increased consideration.