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.  The diagram 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 surfaces must be above 32 degrees F while lower surfaces are below 32 degrees F.  For a portion of the roof to be below 32 degrees F, outside temperatures must also be below 32 degrees F.  When we say temperatures above or below 32 degrees F, we are talking about average temperatures over sustained periods of time.

 The snow on a roof surface that is above 32 degrees F will melt.  As water flows down the roof it reaches the portion of the roof that is below 32 degrees F and freezes and an ice dam has been created.

The dam grows as it is fed by melting snow above it, but it will limit itself to the portions of the roof that are, on the average, below 32 degrees 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.  

 Non-uniform roof surface temperatures lead to ice dams!

 
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 surfaces 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.  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.  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 the surrounding attic temperatures directly affecting convection and radiation from this surface.

Exhaust systems like those in the kitchen or bathrooms 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 and drifting.  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.

 Preventing Ice Dams!

In all Minnesota 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!

 Dealing with Ice Dams

 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 cause damage the roofing materials.  In an emergency situation where water is flowing into the house structure, making channels through the ice dams allows the water behind the ice 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 effective within days and is only a temporary solution to ice dam damage. 

 Long-term action:  Increase the ceiling/roof insulation to cut down on heat loss by conduction.  State code requires an R-value of 38 above the ceiling for new homes.  In narrow spaces, use insulation products with high R-value (6-7) per inch.  Make the ceiling air tight so no warm air can flow from the house into the attic space.  

 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 roof 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 a recommended solution to ice dams in Minnesota.  It can create other attic moisture problems and may cause undesirable negative pressure in the home.

 Insulation Contractors are professionals who can deal with the heat transfer problems that create ice dams.  A blower door test should be used by the contractor you hire to evaluate the air tightness 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 from ice dams 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.  UA-7303841-1