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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. 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. The winding arrow( in diagram 1 above)
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. |
Points of Heat Loss |
This Photograph 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.
"The areas without snow are the points of heat
loss."
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High Heat Loss |
 "The unusually high heat loss on this roof has caused both an ice dam and icicles."
This Photograph illustrates unusally 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 "beuatiful row" of icicles. There is no telling how much these icicles cost the
homeowner here. Perhaps the husband should invest into diamond rings for his wife rather than letting his money slip away
through his roof. Both would be happier.
It is primarily heat flowing from the house that is causing the
nonuniform temperatures of the roof surface leading to ice dams. |
Mold and Mildew |
 Even though it's been around for
thousands of years, mold is a relatively new issue of concern in residential housing. Mold contamination has led to numerous
lawsuits across the country, and its attention has catapulted to the top of popular media such as Dateline NBC, Newsweek,
Time magazine and The Wall Street Journal. It has even been referred to as “the asbestos of the new
millennium,” inferring that mold is an issue of paramount concern among the real estate community. Moisture from
roof leaks, hidden plumbing leaks and basement water penetrations are the most obvious potential contributors to an interior
mold problem. Because the issue of mold has entered the mainstream so quickly, science has just begun to establish guidelines
for mold measurement and exposure. As the financial toll of mold continues to increase dramatically, measures need to be
implemented in order to minimize your financial risk and liability exposure. There is a newly published reference manual
available at http://www.safety-epa.com |
 Bypasses can allow enormous amounts of warm, moist air to leak into your attic. Mold often forms when this moisture
is allowed to stay in the attic. Once attic bypasses are sealed there is no longer a moisture source, therefore, the mold
will simply stop growing and die.
Condensation and Humidity
Proper humidity levels help save
your home from moisture damage. Excess moisture can migrate through walls, floors and ceilings to the home's exterior where
it can condense and freeze in the winter. This can cause extensive damage to the exterior of the home. Moisture can seep into
insulation and make it much less effective.
High indoor humidity can foster the growth of molds, fungi, and
mildew, which can damage home furnishings and cause allergic reactions and respiratory problems. |
 Moisture evaporates
into water vapor in your home. Relative humidity is a measure of how saturated the air is with water vapor. Whether your
trying to heat or cool your home, your comfort depends on humidity as well as temperature. Proper humidity levels help you
feel comfortable at cooler temperatures during winter and at warmer temperatures during the summer. That helps you save
energy.
When you seal and insulate your home for energy savings, large amounts of moisture from
bathing, cooking, washing clothes and other home activities can remain inside. Plumbing leaks, unvented clothes dryers, fish
tanks, humidifiers and basements can be additional sources of moisture in your home. High indoor moisture can be a hazard to
the home and its occupants, and drive up energy bills. |
Preventing Ice Dams |
Preventing ice dams
In all
Michigan 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
1) 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 if not
used correctly. Always consult with a professional if you are unsure of how to do something concerning a roof. More than
500,000 people per year are injured working with their roofs.
2) 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
1) 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.
2) Make the ceiling air tight so no warm air can flow from the house into the attic
space.
Note: 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.
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 buildup during the summer.
Mechanical attic ventilation IS NOT a
recommended solution to ice dams. 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.
- Whenever a
house is tightened up, ventilation systems, exhausting devices, and combustion devices must have enough air to operate safely
and effectively! |
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! |
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Stains on interior ceilings and walls
or mold and mildew growth
