Ice damming is a major source of water damage to many homes….
ROOF ICE BUILD-UP
Sometimes known as ice dams, these giant icicles hanging from the eaves are not only dangerous to people walking underneath, they are often a sign of serious problems with the roof and/or in the attic.
In a well designed and constructed home, the attic has its own dry and cool environment, quite separate from the living space. If your attic is not isolated from the rest of the house with proper air sealing and insulation, it can become warm and moist. Warm attics can help melt the bottom layer of snow on the roof and the water runs down to the gutter where it refreezes. This is the beginning of an ice dam.
HOW DOES MOISTURE GET INTO THE ATTIC?
There are several sources, both exterior and interior. If the roof is old and leaking, obviously that is an exterior source. But lets talk about interior sources that are often overlooked, but are not too difficult to correct.
Interior sources include air leaks from the house into the attic. Moisture is carried by air movement from the warm home into the cold attic, where the moisture in the air condenses and wets cold surfaces, such as the roof sheathing and rafters. Over time, chronic moisture causes mold and mildew.
PLUG THE AIR LEAKS TO STOP THE MOISTURE.
The largest “holes” leading directly from the house to the attic are the attic access and the whole house fan.
When an attic access is a fold-down stairway, it was difficult to control moisture and heat loss. The Battic Door Attic Stair Cover is a low-cost, easy-to-install solution to this common problem.
When a whole house fan is installed in the home, it was difficult to control moisture and heat loss. The Battic Door Whole House Fan Cover and Battic Door Whole House Fan Shutter Seal provide two low-cost, easy-to-install solutions to this common problem.
From the experts at University of Minnesota:
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 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.
Nonuniform 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 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 (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.
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 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.
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 kneewall spaces.
Photograph 1 shows a single story house with an ice dam. The points of heat loss can be clearly seen as those areas with no snow. The ceiling below this area needs to be examined for air leakage (often through an attic stair and/or whole house fan), missing or inadequate insulation, leaky or poorly insulated ductwork, and the termination of a kitchen or bathroom exhaust into the attic space.
Photograph 2 illustrates unusually high heat loss from the roof. There is very little snow left on the roof and at its edge is both an ice dam and a “beautiful” row of icicles.
So it is primarily heat flowing from the house that is causing the nonuniform temperatures of the roof surface leading to ice dams.