Torpedo the dams: insulation is key

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This article was published 23/12/2022 (636 days ago), so information in it may no longer be current.

Question: We have a one-and-three-quarter-storey 1920s house with the ends of the roof abruptly terminating at the lower walls, with no overhang. It’s very prone to ice dams, and we manage them as best we can with a combination of heat cables and cleaning the snow off the roof. It’s been a big challenge this year! When I walk in the neighbourhood, I spend a lot of time looking at other peoples’ houses for ice dams. There are lots of houses with this design in the neighbourhood, and, indeed, a lot of them have ice dams visible from the street. We’ve had the understanding that the reason such a design is prone to ice dams is because it’s difficult to get the attic insulation all the way out to the very edges of the eaves.

However, there are a few houses I can see with this design that simply don’t have any evidence of ice dams. Also, their roofs are not particularly clean of snow. They might be on the same street with a row of similar houses all oriented the same way towards the sun, and yet, this one house is free of ice dams. This leads me to wonder if this is indeed an insulation-related solution? Have we overlooked a permanent solution to this problem, or is it just a matter of particularly good insulation installers?

Thanks so much. — Karen.

Answer: Certain older house designs may be prone to issues like ice damming for a number of reasons, most often being related to attic issues. Solutions may vary, but the common goal is to reduce warm air and heat intrusion into the attic. An experienced insulation contractor or building enclosure specialist should be able to help you air seal the necessary areas to minimize or prevent ice dams in the future.

As I have written many times in this column, most problems within homes are related to moisture and attic moisture issues are typically caused by warm air intrusion from the living space. This does include ice damming, which is the result of melting and refreezing of snow on the roof. This will occur if the roof deck becomes too warm, transferring some of that heat energy to the snow above. Particularly on milder, sunny winter days, the bottom layers of snow may warm up and remain warm due to a combination of solar energy, heat transfer through the roof deck, and the insulating properties of the snow above. When this happens, typically during daytime hours, the frozen moisture in this area will melt, turn to water, and trickle down the roofing under the snow pack. When the sun sets, and the melted snow hits the cooler decking near the bottom of the roof, it will refreeze. The corresponding ice may build up day after day, preventing further melted snow from draining away, and leaving behind a curb of frozen water, often called an ice dam.

The implications of the ice dams on your roof may range from a minor inconvenience to major moisture intrusion and damage, depending on their size and severity. Some ice dams may melt daily, along with the snow higher up on the roof, and leave behind long icicles hanging from the eavestroughs. These may be a safety hazard for anyone passing by underneath, but can otherwise be relatively harmless. Knocking them off with a long pole or snow rake may help minimize the danger. On the flip side, if the ice constantly melts and refreezes it can damage the shingles, allowing leakage through the roof and into the ceilings and walls of the home. In that scenario, the roofing will have to be replaced much more frequently and the damage to the drywall or plaster repaired after it has sufficiently dried.

Regardless of the severity of the problem on your home, taking proper measures to minimize the ice damming should be attempted to improve the situation. This normally will require attempts to prevent warm air intrusion and heat loss into the attic. In a standard attic, with a flat floor, this may be achieved by adding more air-resistant insulation such as cellulose fibre. Air sealing the attic access hatch, light fixture boxes, and any other items penetrating the ceiling below the attic will also help. The problem with your house design is that the ceilings slope slightly as they approach the eaves, making simple insulation installation impossible. You may be correct that this is the underlying problem leading to the ice dams, but it is a bit more complex than just the lack of insulation in those areas.

To prevent warm air intrusion into your attic and the ceiling near the eaves, these areas may have to be properly air sealed, rather than just insulated. That will require partial removal of the plaster ceilings, at least around the inside of the exterior walls, to access the sloped sections. Once exposed, this area, and possibly the entire top side of the upper floor ceiling, could be covered with spray-on high density polyurethane foam. With a minimum five centimeters of this product an excellent air barrier can be achieved. Filling the entire cavity at the sloped edges of your ceilings should prevent any air leakage and provide reasonably good thermal resistance, as well. Adding or reinstalling conventional insulation over top of the rest of the attic floor, once sealed, and ensuring good roof ventilation, should be the final pieces of the puzzle.

The key to an ice dam free roof on a home of your design is to air seal the upper floor ceilings, especially at the perimeter sloped sections, to keep warm air from leaking in from the living space. This may require some ceiling removal and replacement, but will be most achievable with the installation of high density spray foam, by a reputable insulation contractor knowledgeable about building enclose issues.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and a Registered Home Inspector (RHI)(cahpi.ca). Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at trainedeye.ca.

trainedeye@iname.com