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What Conditions Cause the Snow To Melt?

Next, I created a heat transfer model to predict the roof’s tendency to melt snow. I had to consider not only the roof’s thermal resistance but also the insulating effects of the blanket of snow on top. I wanted to figure out what outside air temperature and snow depth would cause sufficient snow to melt to create the ice dams.

I used a common winter scenario for the area, assuming light wind, cloud cover (no solar loading), an uncompressed snow layer, and an inside temperature of 74°F. Based on those assumptions, I modeled the roof’s heat transfer behavior as a function of ambient outside temperature and the thickness of the snow cover. I was specifically interested in pinpointing the conditions in which the roof surface — the interface between the snow and the cathedral ceiling assembly — would have a temperature of 32°F. With an interface temperature equal to the melting temperature of snow, I could assume that melting would take place and that the liquid water would run down to the colder eaves, where freezing — and ice dams — would occur. (Note that for simplicity I ignored solar loading in this model: It’s uncommon in the area during winter, and white snow reflects most of the solar radiation.)

The results of the model are summarized in (Figure 4). The graph shows that the existing roof assembly would in fact melt snow, given my assumed indoor and outdoor conditions, which are common in Toivola during the winter. For example, with 4 inches of snow on the roof, melting would occur when the outside temperature climbed above 10°F. The eaves would of course be at the outside ambient temperature, and the liquid water running down the roof surface would refreeze before dripping from the edge, generating ice dams.

By contrast, if the roof had been properly vented so that the underside of the sheathing remained at the outside temperature, the roof would theoretically generate ice dams only when it was exactly 32°F outside. When it was warmer outside, the snow would still melt, but the water would drip from the roof edge because the eaves would also be above freezing.

Comparing With Actual Weather Data

Now that I had the model, the final step was to compare its predictions with the actual weather data from the period when the ice dams occurred. From Weather Underground (wunderground.com), I collected weather observations for the week before January 7, 2006, when the photographs of the ice dams were taken. There was no data posted for Toivola, so I used the data for Houghton, Mich., the nearest town with data available.

I then created a bar representing a range of snow depths from 2 to 12 inches, based on an average of the recorded temperatures during the week before the photographs were taken. I superimposed this on the modeled conditions in Figure 4, demonstrating that weather conditions during that week were conducive to ice damming on this roof, as predicted by the heat transfer model.

Industry Recommendations

In addition to this evidence, I provided the builder with information from the low-density insulation manufacturer recommending that fully vented sheathing be used for climates like Toivola’s (Zone 7 as defined by the International Energy Conservation Code). The manufacturer’s recommendation specifically includes soffit and ridge vents, as well as installation of polystyrene vent chutes under the sheathing before the foam is sprayed.

Last, I pointed out section R806.1 of the 2003 Michigan Residential Code — currently enforced in Toivola — which dictates that “enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow.” Installing the foam directly against the roof sheathing without venting was in fact a code violation.

In my report, I concluded that had the builder stuck to the original plans for the house, which showed a fully vented cathedral ceiling, severe ice dams would have been unlikely. In fact, in our region, where it is not uncommon to get 200 inches of snow in the course of a winter, a vented roof assembly is always the safest precaution. As the graph shows, even if the levels of insulation in the ceiling assembly were increased, normal weather conditions for the region would likely lead to ice dams on an unvented roof.

As it turned out, my client — the builder — settled out of court, and the owner and insulation subcontractor agreed to split the cost of a new roof. They planned to strip the shingles and then install 2x4 sleepers to create a vent channel, followed by new sheathing, new shingles, and soffit and ridge vents.

Jeffrey Hoffman is a partner in Hoffman Engineered Solutions in Marquette, Mich. The firm specializes in mechanical, chemical, and environmental engineering services.