Proper attic insulation should prevent ice dams

Posted March 06, 2009, at 7:31 p.m.
Last modified Feb. 13, 2011, at 10:45 a.m.

After 20 years of doing our radio program, there are certain questions asked that get answered in rote memory mode. They are annual events that we get every season. The past month has kept up with the quota for ice dam questions.

Ice dams usually form on the lower edge of the roof, where there is minimal insulation because of the slope of the roof and there can be heat leakage that comes up from the side walls. All this heat loss causes snow on the roof to melt. As the snow melts, it becomes water. As the water starts to flow down the roof toward the edge, the cold winter air refreezes it into ice. The ice tends to build up in this area and creates a dam near the edge of the roof. The dam can hold back additional water that comes from melting on a sunny day or the occasional rain that occurs in winter.

When the water builds up against the dam, it can leak under the roof shingles and then lets you know you have an ice dam by dripping in your house.

The simple preventative for this problem is to insulate the attic properly. Many people try to solve this problem with better ventilation — the idea being that better ventilation will whisk away the heat loss and keep the attic cool enough to prevent the snow from melting.

This is treating the symptom, not the cause, and usually does not work. The cause is the heat loss and it is usually a simple addition of insulation that will stop the problem.

It is, however, sometimes — make that many times — difficult to insulate properly out at the perimeter of the attic. There just isn’t enough headroom for a normal-sized person to get out there and insulate well. And if you are trying to add insulation out there, it becomes a miserable job to lay in fiberglass properly.

But we must do it right. The right way to do this is to install R-40 or more insulation out there. That is about 13 or 14 inches of fiberglass or cellulose. If you look out there, there is only about 4 to 6 inches of headroom at that edge. And there is even less if you install a ventilation channel to allow some air to move up from the eave.

My suggestion is to use a layer of foam insulation to bump up the insulation value. Polyisocyanurate foam insulation is about R-6 to R-7 per inch, depending on the manufacturer. Six inches of polyiso will get you the insulation value you need.

Of course, you still need to lay in the itchy fiberglass, but it is really worth it.

And you can wash your clothes and take a long, deserved shower after you are done.

You also must be absolute in sealing all air leaks into the attic. And you also must be certain that the rest of the attic is covered with that R-40, which is the minimum insulation level for attics in our climate.

The other way to help mitigate chronic ice dams is to install metal roofing. If there is any pitch on your roof, ice dams will not form since the snow slides off metal roofing rather quickly. There is some metal roofing that will tend to hold snow, so be sure to check with folks who have the same kind of roofing that you might be looking at.

The downside to metal roofing is the snow slides off — and can hit passers-by in the head when they least expect it. But it is only snow and it is fun to watch, unless you are the person getting covered in white stuff. The insulation problem is still there, too.

Some other solutions for ice dams are installing electric heating cables to melt the snow or throwing some form of ice melt on the roof. Both of these options are expensive and should only be used as temporary methods to get you to the point where you can insulate the attic when nicer weather arrives.

If you remember nothing else about ice dams, remember that they are caused by heat loss through your attic and you are paying for this with your heating dollars, in addition to having to repair the ceiling.

Questions for Tom Gocze should be mailed to The Home Page, Bangor Daily News, P.O. Box 1329, Bangor 04402-1329. A library of reference material and a home-project blog are at bangordailynews.com/thehomepage.

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