Last year I wrote about the problems I had with ice damming (see picture from last winter). I had insulators out and improved the problem marginally, but after being dumped with snow the past two weeks, the dams are back although it's not as bad as last year. I didn't hang my icicle lights this year, but I don't need to - I have the real thing.
Icicles hanging off your gutter may be picturesque, but it's dangerous for your house. Ice damming occurs when the upper part of the roof is warm enough to warm and melt the snow. As it drips down, it hits the lower part over the eaves. This is generally cold, and the water freezes. After several cycles of freezing and thawing, the ice can back up under the shingles, over the fascia board, into the eaves, and even into your house. Ice and water shields - a plastic membrane that gets applied before the shingles - is your first line of defense. I have ice and water shields installed, but last year I still got some water in the eaves. The key is to keep your roof cold.
Generally, you will see this problem on older homes with poorly insulated attics. Heat from within the house rises and warms the roof. However, as new homes go to multiple heating/cooling zones, a furnace is often installed in the attic and the heating ducts runs across the attic floor. Couple that with a trend in modern architecture for interesting roof lines, dormers, and vaulted ceilings, and you have put lots of heat into the attic. This is the problem I am fighting. From the outside, you want to see no snow melting off the roof or for it to do so evenly from top to bottom. See the example of how unevenly the snow melts off of my roof.
I'm taking a multi-faceted attack to this problem. I'm adding insulation to the semi-finished attic floor where currently there is none (just above). Then I'll add more insulation (blown in) to the unfinished attic portion. Then I'll either build a smaller room around the furnace or add the old fashioned attic vents to the peak of the outer walls to get more cold air into the unfinished attic. I can't leave the semi-finished attic space completely cold because of the humidifier unit to the furnace which has a water line that cannot be allowed to freeze. Stay tuned as this develops.