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An Ice Dam Analyzed, Images 1-7

An Ice Dam Analyzed, Images 1-7

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    Ice dams on this new home in Michigan's Upper Peninsula formed above interior cathedral ceilings.

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    The cathedral ceilings were insulated with low-density open-cell spray foam and were unvented.

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    The roof sections with no ice are above unheated areas of the home.

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    By taking surface and air temperature readings on both sides of the foam-insulated garage walls, the author was able to estimate the R-value of the insulation in that location and — by extrapolation — in the cathedral ceiling.

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I am a partner in a small mechanical engineering firm in Michigan’s Upper Peninsula. We provide preconstruction design services and investigate design failures in existing buildings. In the case shown here, I was called in the fall of 2007 to investigate some large ice dams that had formed on the snow-covered roof of a new home in Toivola, Mich., the winter before. Construction of the house was completed in the fall of 2005, and the owner moved in soon after. By January 2006, substantial ice buildups were occurring along most of the roof edges, causing severe leaks inside as water from melted snow backed up behind the ice and made its way under the asphalt roofing shingles. The flooding was so bad the local building official asked the owner’s family to move out until repairs were completed or the leaks subsided. The homeowner, thinking he needed a new roof, sued the builder, who called me to get to the bottom of the problem.

I learned from the builder that the original plans for the house had called for a conventionally vented roof, with the usual ridge and soffit vents. According to the plans, there were to be insulation baffles between the 2x10 rafters to provide a vent space, with fiberglass insulation, a vapor retarder, and a cathedral ceiling below. However, during construction the plans changed: After talking with the insulation subcontractor, the homeowner decided to instead install a common low-density open-cell spray foam directly against the roof sheathing. The insulation contractor claimed that no vent space was necessary. The homeowner liked this option because, according to the insulation contractor, it would speed up the construction schedule.

I focused my investigation of the ice dams on three areas: the design of the roof, including the unvented cathedral ceiling; the performance of the spray-foam insulation; and the effect that a blanket of snow on top of the roof would have. What I discovered was that the unvented cathedral ceiling coupled with the heavy snowfall during the winter in question were the main factors causing the snow on the roof to melt and create ice dams.

Measuring Insulation Performance

I first visited the home in November 2007, ten months after the ice damming shown here. On the day I visited, there was no snow on the roof, which provided the ideal conditions for monitoring its thermal performance. Using a Flir Systems B400 infrared camera, I recorded thermal images of the pertinent roof surfaces. I also took ambient air temperature readings and measured surface temperatures at key locations inside and outside the house. Using this information, I was able to estimate the thermal conductivity of the foam insulation, as well as the overall effectiveness of the insulated roof.

Launch Slideshow

An Ice Dam Analyzed, Images 8-13

An Ice Dam Analyzed, Images 8-13

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    Infrared images of the areas that had experienced severe icing the previous winter showed that temperature variations across the surface were within a few degrees of each other, indicating that the roof assembly was uniformly insulated and not subject to "hot spots" that could have caused localized melting.

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    A second infrared image

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    Given the calculated R-value of the roof and an assumed interior temperature of 74°F, this graph plots melting on the roof surface as a function of snow cover and outside temperature. Below the sloped line, no melting occurs; above it, but below 32°F, melting will occur and ice dams will form at the eaves. Above 32°F (the horizontal line across the top), melting would take place without freezing.

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    The author collected historical weather data for the week before the ice dams were photographed.

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    The data was averaged and superimposed on the graph to make the point visually that conditions were ripe for icing at the eaves.

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    Even with ceiling R-values up to 60, the normal range of winter temperatures would still lead to ice dams, given a blanket of snow on the roof.

Insulation R-value. To measure the insulation’s R-value, I used an exterior garage wall. Like the roof, it was insulated with spray foam, and it provided easily accessible surfaces for taking temperature readings on both sides.

I then used heat transfer equations to calculate the R-value of the insulation in the garage wall, which I determined to be 3.2 per inch, with an uncertainty of ±0.45. My result was slightly less than the foam manufacturer’s published value of 3.6 per inch, but given the uncertainty, I concluded that the insulation itself was not defective and was performing satisfactorily.

Assessing the Overall Roof

The homeowner reported that ice dams had formed along all the roof edges and had several photographs to prove it. The photographs, taken in January 2006, also showed sections of roof that had no icicles. The areas without ice were all above unheated sections of the home, which suggested that in this situation the icicles were caused not by solar heat or above-freezing outside air temperatures but by heat transfer from within the structure.

I made a number of thermal images of the roof sections that had experienced icing. The infrared photos showed that the roof surface temperatures were uniform within 3 or 4°F. This suggests that the roof was uniformly insulated, with no major voids, and that the melting was not the result of local hot spots but was a surface phenomenon.

Using the building geometry and the estimated R-value for the insulation as calculated based on the garage wall, I estimated the overall effective R-value of the roof to be 30, ±4. While this is slightly lower than the Michigan code requirement of R-38, it’s worth noting that lack of insulation did not alone account for the ice dams, as I’ll show later.