Nipping 'ice dams' in the bud

Proper attic insulation, vents are keys to prevention

By Inman News Feed
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Proper attic insulation, vents are keys to prevention

Paul Bianchina
Inman News

As winter's cold weather approaches again, you may be remembering back to those frightening buildups of ice on your roof that you've had to contend with in years past. Known as ice dams, they can present a lot of potential problems for your roofing, attic, insulation and other parts of your home.

The time to deal with an ice dam is before it occurs, not after. So you might want to consider taking some steps to prevent their occurrence before winter really tightens its grip on your home.

How ice dams occur
As snow falls, it builds up on the roof. If the daytime temperatures are low, the snow won't melt, and the layer continues to grow. In the meantime, you've got the heat on inside your home to keep things warm. Some of that heat is inevitably lost into the attic, and once it gets there, it rises until it contacts the underside of the roof.

With an insulating snow layer above and heat below, the snow on the roof begins to melt from the bottom. A film of water is created between the underside of the snow layer and the top of the roofing, and the water runs down the top of the roof, beneath the snow, until it reaches the eaves. Once there, the water is past the end of the attic. Now there's no more heat being lost from the house to keep the water warm, so it freezes, and a solid dam of ice begins to form. The longer the cycle continues, the larger the ice dam grows.

Eventually, water coming down the roof and hitting the dam has nowhere else to go, so it begins to work its way back up the roof. When it gets back to the point where the roof and wall meet -- where the heat loss area is -- it no longer freezes. It continues to work back up the roof as a liquid instead of as ice. Since shingles are overlapped from top to bottom, there's no protection against water coming in from below, so eventually the water gets under the shingles and enters the house.

Prevent the heat loss
You can't stop the snow from falling, and for the most part, you usually can't do all that much about keeping it from piling up on your roof. So the next best option is to keep the heat from getting to it from below, and melting the underside of the snow layer. If you can do that, you can stop the water layer from forming, which will stop the ice buildup.

There are two things you'll want to take into consideration here. The first line of defense is good attic insulation. By increasing your attic insulation to a minimum level of R-38 or even higher, you're going to minimize the amount of heat that's being lost from the house into the attic. That's going to lessen the chance of the snow layer melting, and will have the added benefit of keeping your house warmer. After all, you're paying a lot of money to heat the house, so it only makes sense to keep the heat inside where you can use it, not let it go up into the attic where it's wasted.

Increase attic ventilation
Even with good insulation, a certain amount of heat is still going to be lost into the attic. That's pretty much inevitable. So with that in mind, you want to prevent as much of that heat as possible from reaching the underside of the roof, where it's going to warm up the snow layer. To do that, you want to be sure that you have good levels of ventilation, which will allow the wasted heat to be flushed out of the attic. This is known as a "cold roof."

A good rule of thumb for attic ventilation is a ratio of 1:300. That means 1 square foot of attic ventilation area for every 300 square feet of attic space. So if your home has 1,800 square feet of attic space, you should have approximately 6 square feet of attic vents. Furthermore, that 6 square feet of vent area should be roughly split between high and low vents, meaning you should have about 3 square feet of vents down low in the soffits, and another 3 square feet of vents high in the gables or along the ridge. This high/low placement allows natural convection currents to move air through the attic and flush warm air out through the high vents.

As with the higher levels of attic insulation, increasing your attic ventilation pays other dividends besides helping reduce ice damming. During the hot summer months, it flushes hot air out of the attic, which keeps your house cooler, reduces your air conditioning load, and makes your roofing last longer as well.

Finally, you want to make sure that all of your inside fans are vented all the way out of the attic. That includes the kitchen range hood, as well as the bath fans and any other ventilation fans you might have. That prevents warm moist air from being vented directly into the attic, which not only helps prevent ice damming, it also helps prevent mold in the attic, as well as potential moisture problems to insulation and wood framing.

Remodeling and repair questions? E-mail Paul at All product reviews are based on the author's actual testing of free review samples provided by the manufacturers.

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