What is an Ice Dam?

What is an ice dam? Chances are that if you are looking for information on ice dams, you already know what one is. It is a large bulge of ice that is frozen to the eave of your house, generally accompanied with large icicles that hang over your soffit or gutter–wrecking your Christmas lights, bowing your soffit as it stresses the eaves, and knocking your gutter off. The icicles continue to grow especially as the temperature hovers near the freezing point at around the 20-30 degree range. What you may not realize is that these large icicles while somewhat pretty are an indication of a common, yet serious and costly north land plague. The reason they are called a dam is because the bulge of frozen ice is causing a backup of melted snow (water) in which it can begin to leak inside your home or wall.

Why do ice dams form?

The short answer to the question “why do ice dams form?” is because of temperature variations on the roof of your house. Your home consists of two enclosures called envelopes and sometimes these envelopes overlap and are one and the same, and in other locations they may be in different places. The first is called the “building envelope;” the building envelope is the skin of the house and is always the furthest most surface plain towards the exterior, it is the house in its entirety. The second is called the “thermal envelope,” and this is the part of the house that is conditioned or heated. Therefore, any part of the house that is outside the thermal insulation separating the warm part of the house from the cold, is only a part of the building envelope not the thermal. Heat escaping from the thermal envelope in the form of air leaks and surface conduction is the primary cause of snow melt, which then refreezes on the cold eave.

Two Sources of Heat Loss

When defining the solutions to fixing the ice dam problem it should be stated that the desired outcome is the same, to get the roof deck as cold as the outside from the ridge all the way to the gutter at the eave. There are two basic problems to define before the appropriate solution can be given. The two primary problems are related to warm air leaking from the thermal envelope, or the warming of area outside thermal area through heat conduction. In the average house, there are chimneys, pipes, wires, fans, attic accesses, and etc, that are not “air sealed” that is the air can pass through causing a warm air geyser where it should have been contained. The vapor barrier (whether 4-6 mm plastic or kraft faced fiberglass batt insulation) is also intended to reduce the areas that leak, though it alone does not seal well around obstructions. This warm air bleeding of your home into the areas beyond the thermal envelope is further compounded by the fact that warm air particles are further spaced and thus contains higher pressure than cold air. Your home is like a hot air balloon forcing the warm air out in every direction.
The second issue often accompanied by ice dam development is through surface heat conduction. Heat conduction is when heat transfer happens because a surface plan is warmer than the air or material around it. The first area to assess this problem is the quantity and quality of the attic insulation. Insulation requirements for both Minnesota and Wisconsin are (depending on roof assembly type and other table subsets) between an R-value of 38 to 49. It is often difficult to achieve this thickness in slant ceilings that follow the cavity of the roof rafters. A second culprit of heat conduction are ceiling recessed lights, and chimney pipes that get noticeably hot to the touch in the attic cavity. The third common area where heat conducts through are pinch points where the roof slopes down to meet the outside wall.

Solutions in Common Roof Assemblies

Now that the two primary sources of heat loss have been explained, I will now discuss the solutions available to the homeowner to eliminate the formation of an ice dam. Solutions vary from the different types of roof assemblies the homeowner may have, however, there are three basic solutions or a combination of them that can be employed to combat ice dam formation: sealing, venting, sufficient insulation, and a metal roof. Of these solutions there are many different specific applications some are “in the attic” and some are “rooftop.” Every situation is different and an onsite evaluation is always advised.


Venting is often the cheapest and most effective means to eliminate ice dams. I once heard it said -though I cannot remember what I was reading- that the vented attic is the most under appreciated component in the building envelope, that is unless it does not work. The idea behind venting is that this system “washes” the warm moist air out of the attic. The warm air also contains more moisture than the cold outside air and the induction and exhaustion of fresh outside air also eliminate the potential for mold buildup. Venting is often misunderstood, a roof will have some vent planted near the ridge and hope for the best. The problem is that there is nothing for these vents to “draw” from. The venting system works by drawing in cold air at the bottom of the soffits and exhausting the warmer air out the top of the ridge vent. Warm air naturally rises forcing it out the vent and necessitating the replenishing of that warm air with fresh cold air that is drawn in.
That is the theory but how this works out on your home where some retrofitting may need to take place is a different story. If your house contains an accessible unheated attic above the dwelling, the most favorable solution is to make sure the bottom of the soffits have sufficient vents, chutes or baffles are installed anywhere insulation is close to contacting the bottom of the roof deck to allow an uninhibited one inch of clearance to channel air, and a ridge vent installed on top to exhaust. I have seen square vents installed low on the roof plane for the higher vents to draw from, rarely does it work and if it doesn’t there is now a low hole in the roof for the dam to back up into!
Venting becomes far more difficult when there is insulation in the cavity of the roof rafter assembly, or there is a room in the attic where there are knee walls. I have found that in retrofitting these homes getting them to vent is a challenge due to accessibility issues to install the system. There is hope, I have found the most cost effective and sufficient system to be installed on the roof, we call it a “deck vent.” The system works by installing 1 x 3’s up the roof on top of the old roof deck , and 1/2″ roof sheathing is installed over the 1 x 3’s, creating a vent channel.  We then install either vented drip edge or a product like cor-a-vent behind the drip edge to induce air flow. The underlayments and roofing is installed -whether asphalt or metal- and a ridge vent is installed to exhaust the warmer air. The new roof deck stays cold by having the cold air channel underneath the new roof deck.

Insufficient Insulation and Sealing

The north land contains many older homes that are typically under insulated. Depending on the roof assembly this could mean a couple of different options. If you have an unheated attic that is accessible, it might mean adding more insulation to achieve a total r-value rating of at least 49. This does not seal the thermal envelope and it is best to also use a small can of spray foam to seal around attic penetrations. Recessed lighting puts off heat in the attic, and if you have recessed lighting it is necessary to make sure that it is IC-(insulation contact) rated, and cover it with appropriate insulating materials. Lastly, if your attic does not have spray foam to “air seal” your home, a working ventilation system is a must in addition to adequate insulation or ice dams are sure to form.
Spray foam or SPF has taken a large part of newer home construction because of its ability to “air seal” a home so the attic assembly does not leak. At a 6-8 r-value rating per inch it is among the highest rated of any insulating material. It is also very costly and sometimes it is used in conjunction with other less expensive insulating materials to achieve the desired r-value. The system known as “flash” and “batt” uses a thinner layer of spray foam to air seal the thermal envelope and other insulation (typically fiberglass) is used to give the area a higher rating. SPF can be sprayed on the floor of unheated attics and blown in insulation added to the top, or it can be sprayed directly to the bottom of the roof deck of cathedral and room in attic assemblies. This “in attic” system can be costly as the foam is more expensive and may require demolition in your home to get at the necessary areas. The requirements for ventilation used in conjunction with SPF is dependent upon your roof assembly and the applicable building codes, an onsite evaluation is important.
Like most older homes if the insulation is inadequate and the ability to get to the areas needed is difficult, a homeowner may decide on a “hot roof.” It is called a hot roof though the roof deck is actually cold. A hot roof is a double layer with staggered seams of 1 1/2″ or 2″ foam board. The entire roof deck is covered from the ridge to the eave eliminating the hot and cold spots. A nailable roof deck is installed over the system with long screws, roofing underlayments are placed, and the finished roofing (asphalt or metal) is installed. The benefits of this system is that it keeps the mess out of your home, adds insulation, eliminates ice dams, and provides an “air leak” layer on the roof deck.

Metal Roof

Metal roofs alone are believed to be the cure-all ice dam eliminator. If a homeowner has ice dams it is recommended that they have installed either the “deck vent” or a “hot roof” under the metal roof system. The idea behind a metal roof is that the snow should slide off and thus there is no snow to melt. While this may be the case on steeper pitch roofs it is not a given. It the roof collects dirt the friction is increased and it may not slide, also if the customer has a gutter it will catch the snow and the dam will form. If a customer wants a metal roof because of ice dams yet does not want to place a “deck vent” or “hot roof” under the system, we tell them they cannot have gutters. The gutters will fill and freeze and it can work up under the metal. The gutters should also have a cover that will let the snow slide over without ripping it off. Also, valleys are a place that are leak prone due to snow not sliding off and the dam forming and backing up the valley. I do believe that metal roofs are the choice material for north land homes because they do let snow slide off reducing weight, and last much longer.


Originally from Missouri I had never heard of an ice dam until I bought a home in Minnesota that had them. Now as I drive Minnesota and Wisconsin roads I see them everywhere. I would get jobs in the spring from ice dam leaks that would cost the homeowner thousands of dollars, yet if they did not leak inside they were still wrecking the roof shortening its useful life. I determined that for this to be the case homeowners did not have a knowledgeable contractor to turn to that understood the science of these creatures. For several years I read everything I could get my hands on to understand ice dams, insulation, ventilation, and air sealing. It seemed that most ice dam companies were more interested in temporary fixes and charging to remove them. There are options to the homeowner, you should be able to go on vacation during the winter without worrying about your ice buildup.

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