What Causes Ice Dams?
Part I is here.
What causes different roof surface temperatures?
Most ice dams form at the edge of the roof, where it is colder. Approaching the top of the roof, the temperatures rise, heated by the warm air rising through the house, into the attic.
In a house, heat moves through the ceiling and insulation by conduction (heat energy going through a solid) through 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.
Convection (rising air) heat contributes to ice dams in two ways. The first way is through insulation. The top surface of the insulation is warmer than the other surroundings in the attic, so the air above the insulation is heated and rises. This, by convection, carries heat to the roof.
A second way is through air leakage. Improperly sealed attic areas literally suck the hot air out of the house. 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.
Either way, the higher temperatures in the insulation's top surface compared to the roof sheathing transfers heat outward.
Dealing With Ice Dams
The first action is to make ceilings air tight, which will keep warm air from flowing into the attic space.
The second action is to insulate. No matter how the heat got there, heat transfer can be reduced by adding insulation. This will make the top surface temperature of the insulation closer to surrounding attic temperatures, which will directly affect convection loss.
Now be aware that these actions will increase the snow load that your roof has to carry (because it will no longer melt until outside temperatures warm up around March). Roofs built to recent Michigan Residential Codes shouldn't have any problems. Older homes might.
If you are dealing with an ice dam issue, have water pouring in, and need to do damage control right away, try these actions:
- Remove snow from the roof. with a roof rake (be aware this might damage roofing materials... also be aware that doing this from the roof has inherent slip & fall dangers, but working on the ground has its own "avalanche" dangers -- including icecicles that can act like spears).
- Breaking holes or knocking channels through the ice dam allows the water behind the dam to drain off the roof. This is, obviously, only a temporary solution as the dam will re-emerge in a day or two. Work upward from the lower edge of the dam.
Do not repair interior damage until ceilings and walls are dry. Make sure to correct the heat loss problem at the same time, or ice dam damage can occur again. If you have the option, construct a continuous, 100% effective air barrier through the ceiling. There should not be any air leakage from the house into the attic space.
The "F" Word
Moisture entering the home from ice dams can lead to the growth of fungi -- that is, mold and mildew.
To inhibit or prevent the growth of mold and mildew, immediately drying out portions of the house that are wet or damp. This takes more than borrowing your neighbor's wet/dry vac and extra fans in exchange for a six pack and a pizza. Don't be afraid to get help from a professional water extraction company.
Comments(4)