PNNL | February 06, 2017

YouTube // Roofing, Insulation & Air Sealing

How Ice Dams Form, and How To Stop Them


Four ways to prevent ice dams in three types of roof, for two climates in one video. Boom.



Homes in snowy climates can sustain moisture damage from ice dams in the winter.

An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow from draining off the roof. Icicles are an indication that ice dams are forming.

Ice dams are caused by nonuniform temperatures on the surface of the roof. Snow melts over the warmer areas‚ perhaps where the insulation is thin, and heat is escaping from the home.

The melted snow runs down to the cold edge of the roof and freezes, forming icicles and ice dams. The ice dam on the edge of the roof can cause water from snow melt to back up and form pools on your roof under the snow.

Eventually this water can seep through your roof and cause damage to your insulation, ceilings, and walls.


Ice dam prevention begins at the ceiling

The best approach is to prevent heat from the interior of your home from escaping into the attic and warming areas of the roof deck.

To accomplish this, the first thing you need to do is to make sure the ceiling beneath the attic is air tight. Air seal every seam and hole in the ceiling from either the interior or the attic side.

Try to limit the number of holes you make in your ceiling. For example, you can install track light or ceiling-mounted fixtures instead of recessed can lighting to avoid cutting larger holes in your ceiling.

Another reason to avoid using recessed can lighting fixtures is that they can introduce heat directly into the attic. Especially if they are not airtight, insulation-contact rated, and covered with insulation.

If possible, do not install heat ducts and furnaces in the attic, particularly if you live in an area with significant snow accumulation. It is nearly impossible to air seal and insulate these systems well-enough to prevent heat loss from warming the roof deck, causing snow melt and ice dam formation.


Vented attic needs an air tight floor and smooth air flow

A well-sealed and properly insulated attic is essential to preventing ice dam formation. However, the type and level of insulation you use will depend on whether your attic is vented or unvented.

  • If your attic is vented, make sure the attic floor is well insulated, especially over the top plates.
  • If your home is not designed with raised heel trusses, as shown, you can spray with foam from under the baffle to the attic floor to get full insulation coverage.
  • Leave a minimum of a 2 inch space between the roof deck and the wind baffle to vent the underside of the roof deck. Install vent screens at every rafter bay to provide exterior air to flush away any heat that gets to the roof deck.

If your house has a compact roof assembly with an unvented attic, there are two options for insulating the attic to prevent the formation of ice dams. These options depend on the climate where you live and the weight of the typical snow load on your roof.


Unvented roof with moderate snow (less than 50 pounds/sq.ft.)

If you live in a moderately snowy area, with snow loads less than fifty pounds per square foot, you can choose to leave your attic unvented. However, it will be necessary for you to:

  • Install a fully-adhered air barrier membrane to the top of the roof sheathing that covers your fully-insulated attic.
  • Apply rigid foam insulation equivalent to R-50 on top of the air barrier membrane in two or more layers with horizontal and vertical joints staggered.
  • Install plywood or OSB sheathing over the rigid insulation as a nail base for finishing the roof, and screw down through the insulation into the rafters.


Unvented roof with heavy snow (greater than 50 pounds/sq.ft.)

If you live in a region with snow loads greater than fifty pounds per square foot, the snow itself creates an insulating blanket that can elevate the temperature of the roof deck above freezing, which can cause snow melt and ice dams.

To counteract the thermal effect of the heavier snow load, you will need to add ventilation, by constructing a vent over roof on top of your unvented compact roof.

  • As with the unvented roof procedure, you begin by applying an air barrier membrane to the top of the roof sheathing.
  • Cover with rigid foam insulation, but increase the layers to a minimum R-60, staggering the horizontal and vertical joints as before. Apply roof sheathing and roof membrane over the rigid foam insulation.
  • Leave an inch or so of space for venting between the roofing membrane and the last layer of roof sheathing before finishing the roof.
  • Be sure to vent the fascia to allow exterior air to flow under the over roof to keep it cool.

Upgrading your ceilings, attic, and roof to prevent ice dams will make your home safer and more durable. It also may pay for itself in avoided repair costs for water, mold, and structural damage.


—This video is from the Pacific Northwest National Lab through the Building America Solutions Center. We'd like to thank all of the scientists, builders, and manufacturers of Building America program for doing mind-numbing work sometimes to help us build the best houses we can. 



Kudos to you all.  This is the best ice dam prevention video I have seen and I am a veracious student of all things ice dam. As the owner of the country’s largest ice dam removal and prevention company, The Ice Dam Company, I would like to offer my two cents. We are a large-scale construction firm that engages directly in hundreds of attic insulation, ventilation and roofing projects every year that are aimed specifically at reducing the likelihood if ice dams. We have many years of hard data to support the fact that such projects cost an average of $12,000-$40,000+. Addressing the underlying thermal dynamics of a home is the ideal path for ice dam prevention and is the first recommendation I have for our clients. The reality is, however, that many homeowners simply cannot afford to pursue the aforementioned expensive enhancements to their home. For these people there are only two other options; one is to keep their roof virtually snow free by shoveling the other is to install heat cables. The latter is something I believe you need to address as a viable--albeit less than ideal--alternative to the architectural approaches addressed in your video.


Average heat cable installations run in the $1,000-$2,500 range, making it a very realistic expenditure for homeowners on a limited budget. We have completed over a thousand heat cable installations around the country and it is a very effective approach to ice dam prevention for those who can’t afford the more expensive path.

Heat cable gets a bad rap. It’s ugly, it’s inefficient and it doesn’t address the underlying causes of the problem. Some of that is true but what other options exist for people who don’t care to drop the big bucks? The answer is none. Next, and this is very important, not all heat cable is created equal. Cheap heat cables--usually sold at big box retailers under brands like EasyHeat and Frost King--are called ‘constant wattage’ cable and are unfortunately the most widely installed products. To put it bluntly, these cables are worthless at best. Such cables, which often fail in the first year or two, burn at full capacity 100% of the time. Regardless of outdoor temperatures, they are hot, hot, hot. As we all know, winter temperatures in the northern half of the country vary greatly from day to day, meaning that such cables are super inefficient when temperatures are in the thirties and forties. The cable systems we install are an entirely different beast. We use HeatTapePro, by Radiant Solutions Company. It is a ‘self-regulating’ cable that adjusts its energy demand based on outdoor temperatures to greatly reduce energy consumption. The addition of a thermostat to the system enhances the efficiency even more but it is not necessary. Self-regulating heat cable technologies have been used for over 40 years and were originally developed to keep pipelines from freezing in the arctic. It is a well-proven technology with a robust track record.


Understand that heat cables are not meant to melt all of the snow and ice on the edge of a roof. This is a common misconception among people not in the construction field. Their only function is to create melted pathways through any ice or snow that tries to accumulate. As you eloquently illustrated, ice dams cause damage to homes when the melt water from a higher roof area hits the dam and has nowhere else to go but back up into the roof system and into the home. Heat cables create multiple relief channels that allow that melt water to escape. It’s that simple. Admittedly, there are limited circumstances when heat cables can’t do the job, but those circumstances are exceedingly rare. Cable failure is a problem, but 99% of the ‘failed’ heat cable jobs we have fixed are a function of cheap cable installed by hacks. High quality systems are incredibly effective and reliable.


There are circumstances when heat cables are the only realistic option. We run into these homes everyday in our inspections. Homes sometimes have a series of intersecting roof planes, vaulted ceilings with little to no room for insulation or other variables that make architectural solutions impractical to say the least. In such situations, the installation of a commercial-grade, self-regulating heat cable is a smart choice. While heat cables are not the most attractive addition to a home, most homes these days have darker colored asphalt shingles which helps make heat cables a non-factor in the overall visual impact on the home.  Plus, aesthetics are somewhat irrelevant when you consider that many areas affected by ice dams are not easily seen from the ground.


Here is the bottom line. Ice dam prevention comes down to a lesser of evils. Homeowners can: 1) Spend a lot on an architectural solution, 2) Spend a lot on professional roof shoveling or risk doing it themselves or, 3) Spend a fair amount on heat cables. We are happy to help homeowners with any of the above. Our job is to educate them on all of the available options so they make the best decision based on what they can afford.  I would love to dialogue with you all about this topic and anything else relating to ice dams as I have built a career on both professional prevention and removal of ice dams over the past 20+ years. My email is: [email protected]


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