Ice Dams
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.
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.
Photo 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, missing or inadequate insulation,
leaky or poorly insulated ductwork, and the
termination of a kitchen or bathroom exhaust
into the attic space.
Photo 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.
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 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.
- Increase the ceiling/roof insulation to cut down on heat loss by conduction.
- Make the ceiling air tight so no warm air can flow from the house into the attic
space.
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.
ICE & WATER SHIELD PROTECTION
Ice-guard should be installed in all critical area’s like
gutter edges, valleys, skylights and chimneys
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