What is an ice dam?
An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow (water) from draining off the roof. The water that backs up behind the dam can leak into a home and cause damage to walls, ceilings, insulation, and other areas.
What causes ice dams?
There is a complex interaction among the amount of heat loss from a house, snow cover, and outside temperatures that leads to ice dam formation. For ice dams to form there must be snow on the roof, and, at the same time, higher portions of the roof’s outside surface must be above 32° F while lower surfaces are below 32° F. For a portion of the roof to be below 32° F, outside temperatures must also be below 32° F. When we say temperatures above or below 32° F, we are talking about average temperature over sustained periods of time.
The snow on a roof surface that is above 32° F will melt. As water flows down the roof it reaches the portion of the roof that is below 32° F and freezes. Voila!!- an ice dam.
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. 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 finish.
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 the house. In rare instances solar heat gain may cause these temperature differences.
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 good example of this is the heating of a cast iron frying pan. The heat moves from the bottom of the pan to the handle by conduction.
If you put your hand above the frying pan, heat will reach it by the other two methods. The air right above the frying pan is heated and rises. The rising air carries heat/energy to your hand. This is heat transfer by convection. In addition, heat is transferred from the hot pan to your hand by electromagnetic waves and this is called radiation. Another example of radiation is to stand outside on a bright sunny day and feel the heat from the sun. This heat is transferred from the sun to you by radiation.
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 fro insulation, so it is important to use insulation's 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.
Jamie Woodend
Sales Representative
Remax a-b Realty Ltd Brokerage
88 Wellington Street
Stratford, Ontario, N5A 2L2
519-273-2821 ext 229 Office
519-949-0345 Cell
Sales Representative
Remax a-b Realty Ltd Brokerage
88 Wellington Street
Stratford, Ontario, N5A 2L2
519-273-2821 ext 229 Office
519-949-0345 Cell
