Ice Damming

Winter Maintainence: Ice Damming

Frequent freeze-thaw cycles in Southern Ontario's wintry weather create an ideal environment for ice damming — a problem that costs tens of millions of dollars in insurance claims and associated repair costs every year. Your property does not have to become one of those casualties, however; ice damming in lowrise housing is nearly always preventable.

Dangers Caused by Ice Damming

Ice damming is usually first noticed when large icicles grow over the edge of the roof, over the gutters and hang down over pedestrian areas. Underneath the ice is a backup of water, which was snow before it melted. The water and melted snow is trapped between the ice and the surface of the roof (see diagram below).

Any heavy, sharp object hanging over the edge of a roof is dangerous. Icicles can fall at any time, with sometimes fatal results. Liability costs are just part of the financial impact of ice damming. Structural damage to buildings can be significant and costly. Of $50 million worth of claims received by insurers in Southwestern Ontario during a January 1999 ice storm, much of the property damage involved ice damming and seepage, according to the Insurance Bureau of Canada.

Ice is Not the Only Danger

The dangers from ice damming do not stop at bodily and visible property damage. Water, or melted snow, is very much part of the package. Water often gets into the attic when you have an ice dam meaning moisture is present where it does not belong. That in turn can lead to serious mildew and mould problems. Asthma levels in Canada have increased three-fold in the last 20 years, attributed largely to the increase in bacteria, mould and chemicals in the atmosphere. There are currently more than 11,000 lawsuits being pursued in the United States by people who claim mould is damaging their homes and their health. One of the challenges that property owners, occupants, insurers and contracting tradespeople all face is the myth that adding ventilation to the attic will solve these problems. Though ventilation is important, it cannot prevent ice-damming disasters.

Canada Mortgage and Housing Corporation has published a document to put attic ventilation in proper perspective. CMHC indicates that ventilation is necessary for the drying of attics in the spring, but in a growing number of cases, it cannot cope with the moisture load coming from the inside of the house. CMHC is not alone. The American Society of Heating, Refrigerating and Air-Conditioning Engineers and William B. Rose of the Building Research Council, at the University of Illinois, agree that the top priority in coping with attic moisture problems, including ice damming and condensation, is to stop the air in the attic from becoming warm and moist from internal sources.

What Causes Ice Damming?

In winter, there will always be snow on the roof. Ideally, the surface of a roof should be cold since a properly designed building should have a cold attic. Unfortunately, attics are often warm and cause roofs to be warm. This heat melts the bottom layer of snow on the roof and the resulting water runs down the slope under the snow, refreezing when it hits a cold surface, such as a metal eaves trough or gutter. This is where icicles begin to form. Solar radiation can contribute to the amount of melting snow, even on the coldest days, adding to the problem.

Big icicles are unfortunately not the only problem associated with ice damming. The water and melted snow backs up under the shingles. Very soon, it finds its way through the roof and into the attic. Moisture problems in attics, which can lead to mildew and mould, are often caused by the same situations that result in ice damming. As a result, the pathway, it can be conducted through an uninsulated attic floor. Builders and occupants sometimes unknowingly worsen the problem by exhausting an appliance vent into the attic instead of outdoors.

Typically, warmth can seep into the attic through:

  • Gaps under baseboards and above cold air returns
  • Pot lights, skylights, dropped ceilings, bulkheads, stacks, top plates, plumbing, electrical, cable and other penetrations
  • Pocket doors, gaps between light fixtures, exhaust grilles and attic hatches.
  • In condominium townhouses, the masonry party wall between units and the gaps at either end often act as one huge chimney running the length of the house. This provides the easiest route for warm, moist air to travel from the basement to the attic. Heat can also travel through the ceiling - glass fibre on an attic floor does not always mean it is insulated. One look at how dirty the glass fibre is will tell you instantly how much warm air is getting through to the attic - the preventive and mitigating measures you take to deal with one can also alleviate the other.

Fixing and Preventing Problems

One of the few favourable things that can be said about ice damming and its associated problems is that they are visible. Unfortunately, you cannot always see the causes - except when melting patches on a roof show you the hot spots. Remedial contractors address the problems by first looking for the causes of the warm or wet attic, since the best way to prevent problems is to keep the attic cold and dry.
Warm moist air has a tendency to rise up through the house into the attic; this is known as a stack or chimney effect. Heat rises through every possible crack, gap, hole, leak and pathway, carrying moisture from a damp basement or an over-used bathroom and shower, through the attic hatch or the concrete blocks of a party wall, straight into the attic. Even if warm air cannot get through a vertical dirtier it is, the more heat is being transferred.

Sealing these pathways is the first task, followed by insulating the floor of the attic, heating ducts and other direct sources of warmth to the attic. With party walls, the usual approach is to drill holes into the block and inject foam in order to block the chimney effect. The same technique is used behind the strapped-on drywall found at the edges of the party wall. Materials used include two-component polyurethane foam insulating air seal kits, one-component foam sealants and blown cellulose.
To combat the moisture problems at their source, the basement or crawl space and-perimeter are sealed and insulated using spray polyurethane foam. If there is an earth floor, it is covered with a moisture barrier. The settings and operability of humidifiers need to be checked as well. They are often set too high for extremely cold conditions, with dampers open wider than the recommended one inch.

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