Ice damming is one of those conditions that I encounter fairly often and is often misunderstood and improperly treated. So I thought I would do my best to explain the phenomenon and give my suggestions as to how it should be corrected.
Ice damming occurs when the temperature is below freezing but snow located over a warmer area of the roof of the house melts and runs down the roof line into a colder area, often out over the eaves or an unheated porch, and then refreezes. As this process continues through several freeze/thaw cycles, the area of refreezing becomes a raised ridge, similar to a dam in a river holding back a lake. The melt water pools behind the ridge of ice, and often backs up under the shingles causing a leak and water damage to the roof sheathing, structure and the interior of the home.
Often people mistakenly think that the presence of ice damming means they need new roof covering, but in fact, a brand new shingled roof can be susceptible to ice damming. The problem is not the covering but unwanted heat which is escaping into the attic space causing the snow to melt. If there is no house heat-driven melting, there will be no ice damming.
So why is there so much heat in my attic anyway?
Well, mainly because heat rises. Inside a house, a phenomenon known as the stack effect occurs where the warm air rises to the highest point of the home creating low pressure in the lower areas where replacement fresh air is drawn in perpetuating the cycle, similar to what occurs with hot gasses in a chimney. If there are any gaps or cracks in the ceiling below the attic, through light fixtures, the attic hatch, around plumbing stacks, electrical wiring, chimney chases, or gaps in the finishings, warm moist air is pushed into the attic space. Also, since the house air is humid and contains water vapour that will cool once in the attic space, the water vapour will condense into liquid water releasing even more unwanted heat (and moisture).
Since the warmest air in the house is against the highest ceiling surfaces, depending on the amount of attic insulation, heat conducts through the insulation and warms the attic air. There will always be heat conduction through the insulation but having a sufficient amount of properly installed insulation will reduce this portion of the attic heat load to an acceptable level.
Skylights are another source of attic heat because the chase around the skylight is often poorly insulated allowing heat into the attic and the glass surface at the roof loses a large amount of unavoidable heat energy to melt the roof snow built-up over the skylight.
Another possibility is that either a bathroom or kitchen exhaust fan or dryer exhaust improperly vent into an attic delivering a large amount of warm moist air into the attic space which can be an aggravating factor for causing ice damming.
To remove any heat that escapes into the attic, the space requires adequate ventilation. This is typically comprised of some combination of soffit, gable, roof or ridge vents. The Ontario Building Code prescribes a minimum total vent area of 1/300thof the roof area for sloped roofs, with 50% in the soffit and 50% near the top of the roof. Many homes have an insufficient amount of or improperly located roof vents which does not allow the excess heat and moisture to be vented out of the attic and thus the attic is kept above freezing and the snow above melts.
In some houses where blown-in insulation has been added, if precautions are not taken, often the soffit vents can become blocked with insulation. Also, roof vents can become blocked with snow, effectively disrupting the natural ventilation process.
Counter intuitively, the snow on the roof itself can actually insulate the roof surface, trapping the heat in the attic space allowing the air to slowly transfer its heat energy to the snow above turning it into water.
Ok, what can I do?
- Seal the attic space from the house to eliminate all air flow. A complete air barrier needs to be in place. All ceiling gaps must be sealed. Ensure that all ventilation fans and ducts exhaust out of the house and not into the attic.
- Improve the attic insulation by replacing or topping up to achieve the desired thermal resistance. Modern houses have R-40 insulation installed in the attic.
A very good solution is to have the insulation removed from the attic, have the attic side of the roof sprayed with a closed cell polyurethane foam to form a complete air barrier and then have loose insulation blown in over top to achieve the desired thermal resistance.
- Increase the amount or change the location of existing attic vents to ensure that the attic has adequate ventilation to shed any heat and moisture.
Now that measures have been taken to prevent the ice damming from occurring, consider installing some precautions to prevent it from causing damage if it still happens.
- Install a secondary water proof rubberized membrane under the shingles, which will protect the roof and keep the water out in the event that it backs up under the shingles.
- Consider installing a metal roofing system, which is resistant to ice damming.
- As a last resort, electrical cable ice melt systems can be installed on the roof at the eaves to melt any ice which accumulates, however this is a band aid solution and it is much better to solve the heat loss issues.
- Ensure that all ventilation fans and ducts exhaust out of the house and not into the attic.