How to Prevent Ice Dams
The picture above shows the classic way to prevent ice dams in the winter time. All you need is adequate ventilation. That's great, but most roofs do not have a simple gable style roof with no valleys, dormers, sidewalls, or hips in them. So what do you do if your roof has a ton of angles or you don't have the ability to provide ventilation to your roof? Simple. Insulate with cellulose.
Cellulose insulation is very simply ground up newspaper with either Borax or sulfides added to make the material fire resistant and, for the most part, pest resistant as well. The average R-value per inch is 3.8 and has excellent air sealing ability along with performance. This is especially important along the edge of the roof because there is usually less space for insulation making it an area for large temperature changes. Most people are worried about the cellulose settling, but the R-values on the bags you buy are shown based off of settled value so the efficiency experienced is experienced the same every year. The first thing you will notice is there is less outside noise. This is just a convenience of insulating with cellulose.
Cellulose reduces ice damming by simply not allowing as much temperature transfer to happen. The entire point of insulation is to eliminate or at least minimize temperature in a conditioned space from varying greatly from one surface to the other. The reason fiberglass is not recommended for this application or any attic, is because it's design is not made for open air spaces such as an attic and has a very poor performance as the temperatures increase over 70 or decrease under 70 degrees Fahrenheit. If you currently have fiberglass insulation installed in your attic, do not worry. Just go over it with at least 4-5 inches of cellulose. Doing this will actually increase the performance of the fiberglass by 20-30%.
The reason cellulose is a great option reducing ice damming, is because it is simple, it's cheap, and any one can do it. All you need is the blower you get for free when you order you bags of cellulose, a buddy to load the hopper in the machine for you, and a standard outlet. The entire premise of blowing insulation into a house is too make sure that the edges of the attic are highly concentrated on. If there are ventilation baffles currently present, then make sure you do not obstruct them with material. If they aren't long enough, you may pick up baffle extensions at your local store.
Blowing an average house attic with cellulose insulation (1500 square feet) costs roughly $400 and takes about 4 hours. The average energy savings from a small project like this is 10-25% depending on the style of house you have. Ranch houses benefit significantly, while 2 story homes get rid of their problem, but see less savings. Either way, this is an easy way to take care of a potentially large problem.
Please keep in mind that if you have excess moisture in your attic or when you poke your head into the attic it is warm, you may have some other issues that need to be solved first. Here is a simple check list before starting an attic insulation project.
1) Do you have can lights? If so, are they rated to have insulation contact them? If not, you need to protect them with build-out boxes for each one.
2) Check the plumbing stacks and chimneys for leaks.
3) Are the ventilation bathroom vent fans vented properly? if not, they need to be vented outside before any insulation is installed.
4) Are there any chases or framing that lead to the basement? If so, they need to be sealed using urethane foam sealant or boarding of the areas.
5) Do you have the opportunity to add ventilation? If you do not have baffles and you have a soffit (overhang around the perimeter of the house) of at least 6 inches, then you should install baffles in every space and drill in soffit vents when the weather permits.
As always, if you are not sure about ice dams, or attic efficiency, then you should call an expert. A good energy consultant will inspect your entire home, go through it with a thermal imaging camera so you can see all of the areas of possible improvement. They may also be able to get you a rebate for your project depending on your state.