Preventing ice dams can be complicated

By Tom Gocze, BDN Staff
Posted Nov. 07, 2008, at 6:03 p.m.

The big thing we get asked about all winter long is ice dams. Let’s stop them early this year. Most ice dams are caused by insulation problems in the roof.

OK, next question!

Well, all right, we can look into why this is. If the attic is not insulated well enough, heat leaks out to the roof. Once that happens, snow cover on the roof melts. That melted snow becomes water, and water in winter becomes ice. This is pretty simple.

What makes this issue complicated is the matter of ventilation. Many people feel that if the attic is well ventilated, ice dams will not form. This is because the heat could be whisked away before the snow on the roof can melt.

The problem is that usually heat loss can trump ventilation. I have seen people with ice dam problems spend huge amounts of money trying to ventilate the ice dams away. It usually does not work.

Insulation does not always work. Many times, installing extra fiberglass does not solve the problem. This is because fiberglass is susceptible to wind washing. Air can flow through fiberglass and diminish its insulation value. To make matters worse, there is a lot of ventilation at the eaves, where ice dams form.

One more factor adds to this perfect ice dams storm: The insulation at the eave is the thinnest insulation of the roof.

If we analyze this problem, you can see that all these factors mount up to an almost insurmountable problem, especially when fiberglass is the insulation of choice.

The old-timers used to use flashing on the first 3 feet up from the eaves in order to keep ice from sticking to the shingles. Folks also install electric heater cables in this area to melt the snow. It works, but now we are using electricity to help melt the ice that forms from heat loss in the roof. Yikes!

There are a couple ways to solve this problem. First, you can flash the roof as done in the past, covering the areas that are prone to ice dams. This usually works. You can cover the roof with metal roofing. The coatings on metal roofing help prevent ice from sticking to the roof.

The most logical approach is to insulate the roof properly. Regardless of what insulation is used in the attic, if the eave area is insulated properly, the ice dams will usually go away.

This thin area is difficult to insulate to R-38 with fiberglass, unless it is framed in an odd way. A real simple way around this is to insulate the eave with foam insulation. Foam can fill this area, making it tight and keeping the heat loss to a minimum. Many people cut foam and fit it between the ceiling joists. This needs to be sealed well. An alternative in this area is to insulate with spray foam.

One dilemma is that in an existing house, all the usual fiberglass or cellulose in the problem eave area has to be moved. This is one of the most miserable jobs I have ever had to do insulating. It is better than messing with a septic tank, but it is a lot itchier. And it has to be removed and replaced to solve this problem.

Once you do these eaves, the problem is minimal. Note that if you are in the attic insulating the eaves, you can pick the other low-hanging fruit and insulate the rest of the attic properly and seal all the holes that come up into the attic.

Questions for Tom Gocze should be sent to homefront@bangordailynews.net or 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.

http://bangordailynews.com/2008/11/07/business/preventing-ice-dams-can-be-complicated/ printed on September 16, 2014