Derailing the Ice Dam: Moisture Management is Key in Colder Climates

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.

Beneath the Bark : Tree Rings Tell Many Tales

We’re all familiar with the obvious changes northern-latitude trees go through as winter approaches, but did you know that there’s more to a tree’s seasonal changes than autumn’s brightly-colored foliage?

Researchers at the Forest Products Laboratory (FPL) study the both the external and internal structure of trees, and FPL’s Center for Wood Anatomy Research notes in the Wood Handbook that changing temperatures affect far more than the crimson and orange hues of fall.

When a tree grows, the wood is produced one layer of cell divisions at a time — but we do know from experience that in many woods, large groups of cells are produced at the same time, and these groups act together to serve the tree.

Transverse sections of woods showing types of growth rings. Ring development in softwoods ranges from no transition (A) to an abrupt transition between earlywood and late wood (C). Hardwoods (D-F) exhibit a similar range. The arrows delimit growth periods when present.

These collections of cells produced over the same time interval are known as growth increments. Because of the tree’s internal biological processes, these increments are arranged into layers. More commonly, these layers are referred to as growth rings.

In temperate portions of the world (and anywhere else with distinct, regular seasonality) trees form their wood in annual growth increments. All of the wood produced in one growing season is organized together into the recognizable, functional entity of the growth ring. In many tropical woods however, growth rings are not evident, as their climate zones lack seasonality.

Woods that form distinct growth rings, and this includes most woods that are likely to be used as engineering materials in North America, show three fundamental patterns within each growth ring: no change in cell pattern across the ring; a gradual reduction of the inner diameter of conducting elements from the earlywood to the latewood; and a sudden and distinct change in the inner diameter of the conducting elements across the ring.

The orientation of these rings can effect the tensile strength and elasticity of a wood product, and industry professionals must take this into consideration when deciding how a tree should be used.

In addition, most know that by counting the annual rings, researchers can determine the age of the tree, but analyzing growth rings can also tell us about the environmental conditions present when they were forming, including moisture levels in the soil and air, temperature, and sunlight.

In larger trees, annual rings can represent decades, if not centuries, of growth.

Abnormal rings can also be linked to traumatic events in the tree’s past, like forest fires, disease, or climate events, and the rings become not only a record of the life of the individual tree, but of the forest and environment as a whole. Many other disciplines, like archaeology, can use this information (known as dendrochronology) to support their own research, making wood one of the best record keepers on the planet.

For more information, please see Chapter 3 of The Wood Handbook, Wood as an Engineering Material.