Ice Dam and Attic Condensation Details: 

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. Figure 1 shows a cross section of a home with an ice dam.

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 roofs outside surface must be above 0 Degrees Celsius while lower surfaces are below 0. For a portion of the roof to be below 0, outside temperatures must also be below 0. When we say temperatures above or below 0 Degrees Celsius, we are talking about average temperature over sustained periods of time.

The snow on a roof surface that is above 0 will melt. As water flows down the roof it reaches the portion of the roof that is below 0 and freezes. Voila! – an ice dam.

The dam grows as it is fed by the melting snow above, but it will limit itself to the portions of the roof that are on the average below 0. 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 resdience. In rare instances solar heat gain may cause these temperature differences.

Heat from the residence

Heat from the residence travels to the roof surface in three ways: conduction, convection, and radiation. Conduction is heat energy traveling through a solid. 

In a house, heat moves through the ceiling and insulation by conduction through the slanted portion of the ceiling (Figure 1). 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.

Heat loss by air leakage

There is another type of convection that transfers heat to the attic space and warms the roof. In Figure 1, the winding arrow beginning inside the house and going through the penetration in the ceiling, from the light 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.

So it is primarily heat flowing from the house that is causing the non-uniform temperatures of the roof surface leading to ice dams.

Preventing and dealing with ice dams

In all Alberta communities it is possible to find homes that do not have ice dams. Ice dams can be prevented by controlling the heat loss from the home.

Immediate action

• 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/roof insulation to cut down on heat loss by conduction.

Evaluate snow load

Both of these actions will increase the snow load that your roof has to carry because it will no longer melt. Can your roof carry the additional load? If it is built to current codes, there should not be a structural problem. Roofs, like the rest of the home, should have been designed to withstand expected snow loads.

In Alberta, 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 build-up during the summer.

Mechanical attic ventilation NOT recommended

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

WARNING!

• Any person on the roof during the winter or performing work on the roof from below is risking injury and risking damage to the roof and house. It is important to contact professionals to carry out this job.

Hire an energy professional

Weatherization contractors, (also “Energy Management and Conservation Consultants” or “Insulation Contractors“) are professionals who can deal with the heat transfer problem that creates ice dams. A blower door test should be used by the contractor you hire to evaluate the airtightness of your ceiling. In addition, they may have an infrared camera that can be used to find places in the ceiling where there is excessive heat loss.

Interior damage should not be repaired until ceilings and walls are dry. In addition, interior repair should be done together with correcting the heat loss problem that created the ice dam(s) or the damage will occur again.

Preventing ice dams in new homes

The proper new construction practices to prevent ice dams begin with following or exceeding the state code requirements for ceiling/roof insulation levels.

The second absolutely necessary practice is to construct 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, complicated roof designs, and heating ducts in the attic will all increase the risk of ice dam formation.

Effects of ice dams

Moisture entering the home from ice dams can lead to the growth of mold and mildew. These biologicals can cause respiratory problems. It is important that the growth of mold and mildew be prevented. This can be done by immediately drying out portions of the house that are wet or damp. Action needs to be taken to clean the home environment and maintain its air quality.