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  • Boone Restoration

Ice Dams

How often do you venture into the attic in your home? If you’re like most homeowners, the only time you see the attic is to retrieve and stow your Christmas decorations. Even if you access your attic for more regular storage, you probably don’t spend much time examining the condition of the air in that space. Paying attention to the winter-time temperature of your attic might help prevent water stains on your ceilings.

Imagine this scenario: the winter sky is clear after a snow storm blew through earlier in the week. It’s starting to warm up, but the temperatures are still in the mid-20′s. There is still a thick blanket of snow on all the rooftops. One day, you notice growing water stains on your ceiling. What you are seeing is likely the result of an ice dam.

What causes an ice dam?

An ice dam, in short, is a result of rooftop snow melting and refreezing, forcing its way behind healthy shingles. To begin, imagine the thermal picture in an attic. The attic should be well insulated to keep warm air in the living space from escaping into the attic. Some heat transfer is inevitable, so attics are also ventilated to carry away any heat that does get in. Ideally, the attic space should be the same temperature as the outside air. If it’s not, ice dams are more likely to form. The diagram to the right illustrates how the ice dam causes interior water damage:

  1. Heat trapped in the attic begins to melt the snow on the roof. The attic heat is not sufficient to melt all the snow, but just enough to create a layer of water underneath. The water runs down the roof until it comes to the eave.

  2. At this point, there is no longer heat under the snow, so the water succumbs to freezing temperatures.

  3. As the water continues slowly run down the roof and freeze, it begins to “pile up” and form a dam. As the dam grows, it forces its way under the shingles.

  4. Once the water reaches the inside of the attic (where water should never be), the kind of damage caused depends on the house construction and where the water enters. At the least, it will cause wet insulation. Worse, it may cause water stains inside, or even collapsed ceilings in severe cases.

Ice Dam Prevention

Proper Attic Insulation

The first key is to keep warm air inside the house. Here are several tips for a well-insulated attic:

  • Air sealing: The first priority should be sealing any air gaps. These can be caused by holes in ceilngs and walls for plumbing, electrical, or heating. Unfinished drywall or poor construction can also leave gaps for air to escape to the attic. Insulation is not designed to stop air flow, so it is important to seal these gaps with an airtight barrier before installing more insulation.

  • Insulation: Once the air gaps are sealed, insulation prevents thermal transfer through the drywall and framing. More insulation is generally warmer, but be careful here. Improper insulation can be worse than too little. For example, too much insulation in the eaves closes off the attic vents, which compounds ice dam problems.

Good Ventilation

It is not possible to eliminate all heat from the attic space, so proper ventilation is essential. Careless insulation can cover eave vents, and significant snowfall can bury ridge and other rooftop vents. Ensure all attic vents are clear from obstructions, both inside and outside.


Ice dams are not caused by a fault with the roof, but by improper attic conditions. A well maintained attic is the best defense.

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