What is an ice dam? An ice dam is an ice formation created at the roof line and in the gutters that prevents water from the melting snow from being able to freely draining off the roof. These dams usually form at the gutters or soffit. This damming up of the snow melt makes it seem like water will flow up-hill against the water proofing of a house.
The result of this backed up water behind the dam of ice is to create leaks into a home and cause damage to walls, ceilings, insulation, trim, painting and other areas below the ice dam inside the structure since the water proofing gets by passed.
How do ice dams form
A series of conditions need to be present for ice dams to form. Basically heat rising from the inside the house melts the snow it runs down the roof under the snow then refreezes at the gutter line. It is a combined interaction between the amount of heat loss from a house, amount and type of snow cover, and a low outside temperatures that leads to ice dam formation when the water cools at the edge.
With the right amount of heat loss from the house, the heat persisting over time and cold exterior temperatures the dam grows as it is fed by the melting snow above it, but it will limit itself to the portions of the roof that are on the average below 32°F. So the water above backs up behind the ice dam and remains a liquid as it is heated from below. This water finds cracks and openings in the exterior roof covering and flows into the attic space. From the attic it could flow into exterior walls or through the ceiling insulation and stain the ceiling finishes.
What causes different roof surface temperatures?
Since most ice dams form at the edge of the roof, there is obviously a heat source warming the roof elsewhere. This heat is primarily coming from various parts of the house going around or through the insulation. Snow is an insulator so a deep snow cover or layer of ice on top of the snow can trap the heat lost from the house. Then this water reaches a cold section and refreezes creating the start of an ice dam.
Heat from the house travels to the roof surface in three ways: conduction, convection, and radiation. Conduction is heat energy traveling through a solid. A metal skillet conducts heat from the bottom of the pan out to the handle by conduction. Above the skillet, heat will rise by convection and radiation. The air right above the skillet is heated and rises. This is heat transfer by convection. Heat is also transferred from the skillet upwards by electromagnetic waves and that is called radiation. Sitting in front of a fire place you also get the affect of radiant heat
In a house, heat moves through the ceiling and insulation by conduction through the slanted portion of the ceiling. In many homes, there is little space in regions like this for insulation, so it is important to use insulations with high R-value per inch to reduce heat loss by conduction.
The top surface of the insulation is warmer than the other surroundings in the attic. Therefore, the air just above the insulation is heated and rises, carrying heat by convection to the roof. The higher temperatures in the insulation's top surface compared to the roof sheathing transfers heat outward by radiation. These two modes of heat transfer can be reduced by adding insulation. This will make the top surface temperature of the insulation closer to surrounding attic temperatures directly affecting convection and radiation from this surface.
There is another type of convection that transfers heat to the attic space and warms the roof. Any ceiling penetration presents the possibility of air flow from the living spaces into the attic spaces creating warm spots that melts the snow. The circular line by the light in the illustration shows this. In many homes this is the major mode of heat transfer that leads to the formation of ice dams.
Direct warm air transfere can be from Exhaust systems like those in the kitchen or bathroom that terminate just above the roof may also contribute to snow melting. Other sources of heat in the attic space include chimneys. Frequent use of wood stoves and fireplaces allow heat to be transferred from the chimney into the attic space. Inadequately insulated or leaky duct work in the attic space will also be a source of heat. The same can be said about knee wall spaces.