Ice damming, Part 1 - insulation and ventilation
Winter's on its way again, and along with that comes the problem of ice buildup on the roof. That accumulation of ice can lead to ice damming, which in turn gives way to shingle damage, wet insulation, interior damage, and a whole host of other problems that you certainly want to deal with.
Roof icing and ice damming is both a simple and a complicated issue to understand and to deal with, so this year I'm going to approach it in two parts; Part 1, where we'll look at what causes ice damming and why insulation and ventilation are the only two real solutions to the problem, and Part 2, the pros and cons of snow removal, heat tape and other measures.
How ice dams occur
An ice dam begins with snow falling and building up on the roof. If the temperatures remain relatively low, the snow layer won't melt. As more snow falls, additional layers build up. The new layers of snow insulate and protect the previous layers.
Simultaneously, you have the heat on inside your home. Some of that heated air gets lost up into the attic. Once it gets up there, it comes into contact with the underside of the roof.
So now two things are going on with your roof. It has an insulating layer of snow above it, and heat below it. That creates perfect conditions for the snow to begin melting from below. That allows a thin film of water to form below the snow layer, where it then runs down the roof until it reaches the eaves.
Once that water reaches the eaves, it's past the part of the attic where the heat loss from the house is occurring, so now it begins to freeze. A solid dam of ice forms, right above the line of your exterior walls. The longer this cycle continues, the larger the ice dam grows.
Now the real problems start. As water coming down the roof below the snow layer hits the ice dam, it has no where to go, so it begins to back up the roof. Since shingles are overlapped from top to bottom, there's no protection against water coming in from below. The water begins to work its way under the shingles, and then into the house.
Ice dam warning signs
Early signs of an ice dam are a buildup of snow on the roof that doesn't melt off. At the eaves, you can see ice forming under the snow layer. Next, icicles will begin to form, hanging over the edge of the roof, and will continually lengthen.
As the problem worsens, you may begin to see ice coming out between siding boards, or under vents or other exterior wall penetrations. You may see it in soffit or eave vents. If water is running down the wall cavities, you may see ice forming all the way at the bottom of the wall, along the top of the foundation stemwall.
Inside the house, water staining may appear along the joint where an exterior wall meets the ceiling, and it may extend into the room along the ceiling. You may also see water stains around windows, at the bottom of exterior walls, and even dripping out of electrical outlets and switches. These are all indicators that water is working its way into wall cavities.
Insulation and a “cold roof” strategy
As you can see, an ice dam needs three things in order to develop; cold temperatures, a thick snow layer, and heat loss from the house. If you can control any of those three things, and you can prevent the ice dam.
You obviously can't control the outside temperature, so the second two are the only things you can have an impact on. In Part 2 we'll take a closer look at controlling the snow layer, but for now let's focus on the single most important element of preventing an ice dam – stopping the heat loss. The way to do that is through insulation and ventilation.
First, increase your attic insulation to a minimum level of R-38 or even higher. That minimizes the amount of heat being lost into the attic, which lessens the chance of the snow layer melting. Insulation also has the added benefits of keeping your house warmer in the winter, cooler in the summer, and prolonging the life of your roofing, so it's a winning combination all the way around!
Second, you need to have good ventilation. Even with proper insulation, it's inevitable that a certain amount of heat is going to be lost into the attic. Proper ventilation will flush that waste heat out of the attic before it can warm the underside of the snow layer. This is what's known as a “cold roof” strategy.
You need at least 1 square foot of attic ventilation area for every 300 square feet of attic space, so if your home has 1800 square feet of attic space, you should have approximately 6 square feet of attic vents. Split that requirement between high and low vents, so there should be 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, which allows for natural air flow through convection currents.
Finally, make sure that your kitchen range hood, bath fans, and any other ventilation fans are vented all the way out of the attic. That prevents warm moist air from accumulating in 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.