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Rooftop Ice Dams: Attic Insulation Indicators

By Judy Penz Sheluk

Winter and Canada go hand in hand, so it's not surprising that, every year, a number of our houses are affected by ice dams. These icicles are more than just a visually interesting formation of water. In addition to presenting a danger of falling ice, they can also cause water leakage inside the house.
To form, ice dams require periods of deep snow accumulation, with sustained temperatures moderately below freezing. In layman's terms, ice dams occur when there are difference points of melting and freezing of snow on a roof. This means if you live in Vancouver or Victoria, your roof is probably safe. If you live in Montreal, Edmonton, St. John's or Ottawa... not so much.
Ice dams will initially appear where there is inadequate insulation or major air leaks, so taking the time to look at a roof after a heavy frost or light snow can offer clues. For example, a common sight is a horizontal melt line across the roof of a storey-and-a-half house, where the short knee wall meets the ceiling. Other places are beneath a roof-ducted exhaust fan or over a leaky attic access hutch.
Is it possible to determine whether a roof is susceptible to ice dams simply by observing snow and frost melt patterns? In some cases, such as the aforementioned example of the sloped section on a half-storey roof, the answer is yes. However, other melt patterns can be much more difficult to analyze. In fact, a research study undertaken by Canada Mortgage and Housing Corporation (CMHC) offered inconclusive findings.
"Our objective was to determine whether snow or frost melt patterns, observed when a roof had a heavy frost or light layer of snow, could serve as a good indicator of ice damming during the middle of winter," says CMHC project manager Don Fugler. "We also knew that early morning - before the sun melted away our 'evidence' - would be best for viewing."
The CMHC research program seemed simple at the outset, with contractors in three cities - Toronto, Peterborough and Winnipeg - photographing interesting melt patterns on roofs in the fall/early winter of 2001. Unfortunately for the study (although not for residents of these cities), it was not until the winter of 2003-2004 that ice-damming conditions occurred in Winnipeg. Even then, the research was somewhat flawed.
"In hindsight, it would have been more efficient to photograph houses with ice dams, in a winter prone to ice dams, and then to record the roof and melt patterns the following spring or fall," notes Fugler. "Secondly, it would have been best to match each house with ice damming, and then compare the roof melt patterns to a similar house without ice damming."
So why does it occur? "If the attic temperature is above freezing, it warms the roof sheathing which melts the snow lying on the shingles," Fugler explains. "This water runs down the roof until it meets the roof overhang, which is not warmed by the attic and will be at the temperature of the surrounding air. If the air and the attic are below freezing, then the water will freeze on the roof surface and start an ice damn."
While not very energy efficient, an attic without insulation will generally not have a problem with ice dams. That's because the home's heat will come through the attic wall and melt the snow as it lands. Equally free of risk is the well-sealed and insulated attic. However, it's important to note that roofs complicated by many valleys and dormers and large roof overhangs are susceptible.
The "cure" for ice dams is to seal all the attic leaks and insulate thoroughly. Unfortunately, many attics, including those under low-sloped roofs, do not have enough space for adequate insulation at the edge of the attic floor. If soffit insulation requires a baffle to keep a ventilation opening against the sheathing, there will often be just four inches of space for insulation. This will melt the snow off just above the overhang, which actually promotes ice damming. In these instances, blown foam insulation works best.
Another solution is to put a self-sealing membrane on the entire roof, thus preventing roof leakage even if the ice damming occurs. This goes well beyond new housing building codes, which require that such membranes are installed on the lower part of the roof.

New product on the market
Introduced to the US market in 2006, Guardian Building Products Distribution began the Canadian distribution of DuPont(tm) Tyvek(r) AtticWrap(tm) in early 2007. When properly installed over the top of rafters or trusses, Tyvek AtticWrap acts as a vapour-permeable air and water barrier that stops airflow, while still allowing attic moisture to escape. At the same time, it forms vent channels under the sheathing that generates air flow from soffit to ridge. Venting in this manner keeps the roof deck cooler, so the remaining air in the attic stays cooler in the summer and warmer in the winter. Since a cooler roof deck minimizes the thaw-freeze cycle, the chances of ice dams in the winter are reduced.
Basically, AtticWrap is a softer version of DuPont(tm) Tyvek(r) HomeWrap(tm), and allows builders to completely wrap the house envelope. Aluminium coating on one side adds a low-E quality to the product.
Each 4'11" wide by 164' long roll has an integrated adhesive strip along the top edge, for adhesion of one sheet to the next for an airtight seal. Estimated installed cost is about $1 per square foot of roof. However, since the R-value of the attic insulation is protected, additional costs for product and labour should be recouped within three to four years.

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