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.
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 (see
photo) 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 our photo, 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.
Prevention of Ice Dams
Installing new shingles will not prevent ice dams. Prevention only occurs when the entire
roofing system is addressed, including attic insulation and roof ventilation. Ideally, the temperature in the attic
space should be close to the same temperature as the outside air.
- Attic Insulation – adding insulation to your attic will minimize the heat that escapes from the living space into the attic. Additionally,
electric outlets, plumbing vents, furnace stacks, and bathroom vents can allow heat to escape into the attic.
- Roof Ventilation – inevitably, heat will get into your attic during winter months. Proper roof ventilation allows that heat to
escape and prevents your roof deck from warming. The freeze-thaw process that results in ice damming is unlikely to
occur on a roof that is properly ventilated.
- Underlayments, such as installation of Ice and Water barriers
protect the roof deck from ice dams by creating a water-tight barrier between the shingles and the roof deck, but
do not prevent ice damming from occurring.