Dealing with and preventing ice dams

It is primarily heat flowing from the house that causes the nonuniform temperatures of the roof surface leading to ice dams.

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.
• Heat also transfers 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 transfers from the sun to you by radiation.

Adding insulation

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 insulation 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.
• 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.

Convection and radiation 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.

Heat loss by air leakage

There is another type of convection that transfers heat to the attic space and warms the roof. In the diagram, the winding arrow beginning inside the house and going through the penetration in the ceiling, from the light fixture to the attic space, illustrates heat loss by air leakage. In many homes this is the major mode of heat transfer that leads to the formation of ice dams.

Exhaust systems

Exhaust systems like those in the kitchen or bathroom that terminate just above the roof may also contribute to snow melting. These exhaust systems may have to be moved or extended in areas of high snow fall.

Chimneys

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.

• Remove snow from the roof. This eliminates one of the ingredients necessary for the formation of an ice dam.
• A "roof rake" and push broom can be used to remove snow, but may damage the roofing materials.
• In an emergency situation where water is flowing into the house structure, making channels through the ice dam allows the water behind the dam to drain off the roof.
  • Hosing with tap water on a warm day will do this job.
  • Work upward from the lower edge of the dam.
  • The channel will become ineffective within days and is only a temporary solution to ice dam damage.

Long-term action

• First, make the ceiling air tight so no warm, moist air can flow from the house into the attic space.
• After sealing air leakage paths between the house and attic space, consider increasing the ceiling or roof insulation to cut down on heat loss by conduction.

Immediate and long-term actions will increase the snow load that your roof has to carry because the snow will no longer melt.

If your house is built to current codes, there should not be a structural problem. Roofs, like the rest of the home, should be designed to withstand expected snow loads.

In Minnesota, plans showing design details to meet expected snow loads are usually required to receive a building permit. The plans for your home may be on file at your local building inspection office.

To help you understand the plans, or if you cannot find plans for your home, you may want to contact an architectural engineering firm. A professional engineer should be able to evaluate the structure of your home and answer your questions about the strength of your roof.

Natural roof ventilation

Natural roof ventilation can help maintain uniform roof temperatures, but if the long-term actions described here are done effectively, then only small amounts of roof ventilation are needed to maintain uniform roof surface temperatures.

If heat transfer has been reduced substantially, then snow will build up on the roof and cover natural roof ventilation systems, reducing attic ventilation rates.

Natural attic ventilation systems are needed to dry the attic space and remove heat buildup during the summer.

Mechanical attic ventilation is NOT recommended

Mechanical attic ventilation is not a recommended solution to ice dams in Minnesota. It can create other attic moisture problems and may cause undesirable negative pressure in the home.

• New construction practices should follow or exceed the state code requirements for ceiling and roof insulation levels to prevent ice dams.
• New construction should have a continuous, 100% effective air barrier through the ceiling. There should not be any air leakage from the house into the attic space.
• Recessed lights, skylights, complex roof designs, and heating ducts in the attic will all increase the risk of ice dam formation.