What Are Ice Dams? Ice dams are formed when heat from the inside of a home escapes into the attic and warms the roof decking during the winter. This heat, combined with heat from the sun, can melt snow on the roof. Melting snow on the upper roof and in the valleys then runs down toward the eaves as water. When it reaches the cold eaves and gutters it refreezes. The continual thaw and re-freeze process creates ice dams. The result is water backing up under the roof shingles or behind fascia boards where it can soak through the roof decking or wall sheathing, causing damage to attics, ceilings and walls.
What Causes Ice Dams and Icicles?
Anyone who has lived in a snowy climate has seen ice dams. Thick bands of ice form along the eaves of houses, causing millions of dollars of structural damage every year. Water-stained ceilings, dislodged roof shingles, sagging gutters, peeling paint, and damaged plaster–all are the familiar results of ice dams.
There are many ways to treat the symptoms, but proper air sealing, insulation, and attic venting are the best way to eliminate the problem.
Ice dams form along the roof’s edge, usually above the overhang. Here’s why. Heat and warm air leaking from the living space below melt the snow, which trickles down to the colder edge of the roof (above the eaves) and refreezes. Every inch of snow that accumulates on the roof insulates the roof deck a little more. This keeps more heat in the attic, which in turn makes the roof even warmer and melts more snow. Frigid outdoor temperatures ensure a fast and deep freeze at the eaves. The worst ice dams usually occur when a deep snow is followed by very cold weather.
The Havoc Ice Dams Wreak
Contrary to popular belief, gutters do not cause ice dams. However, gutters do help to concentrate ice and water in the very vulnerable area at the edge of the roof. As gutters fill with ice, they often bend and rip away from the house, bringing fascia, fasteners, and downspouts in tow. Roofs leak on attic insulation. In the short term, wet insulation doesn’t work well. Over the long term, water-soaked insulation remains compressed, so that even after it dries, the R-value is not as high. The lower the R-values, the more heat lost. This sets up a vicious cycle: heat loss-ice dams-roof leaks-insulation damage-more heat loss! Cellulose insulation is particularly vulnerable to the hazards of wetting.
Water often leaks down inside the wall, where it wets wall insulation and causes it to sag, leaving uninsulated voids at the top of the wall. Again, energy dollars disappear, but more importantly, moisture gets trapped in the wall cavity between the exterior plywood sheathing and the interior vapor barrier. Soon you can smell the result. In time, the structural framing members may decay. Metal fasteners may corrode. Mold and mildew may form on the surface of the wall. Exterior and interior paint blisters and peels. As a result, people with allergies suffer.
Peeling paint deserves special attention here because it may be hard to recognize what’s causing it. Wall paint doesn’t usually blister or peel while the ice dams are visible. Paint peels long after the ice–and the roof leak itself–have disappeared. Water from the leak infiltrates wall cavities. It dampens building materials and raises the relative humidity inside the wall. The moisture within the wall cavity tries to escape (as either liquid or vapor) and wets the interior and exterior walls. As a result, the walls shed their skin of paint.
Solving the Problem
The way to stop ice dams from forming is to keep the entire roof cold. In most homes this means blocking all air leaks leading to the attic from the living space below, increasing the thickness of insulation on the attic floor, and installing a continuous soffit and ridge vent system. Be sure that the air and insulation barrier you create is continuous.
Don’t waste time or money placing electric heat tape on the shingles above the edge of the roof. Electrically heated cable rarely, if ever, solves the problem. It takes a lot of electricity to prevent ice formation; and the heating must be done before it gets cold enough for ice dams to form, not afterwards. Over time, heat tape makes shingles brittle. It’s expensive to install, too, and water can leak through the cable fasteners. And often the cables create ice dams just above them.
The worst of all solutions is shoveling snow and chipping ice from the edge of the roof. People attack mounds of snow and roof ice with hammers, shovels, ice picks, homemade snow rakes, crowbars, and chain saws! The theory is obvious. No snow or ice, no leaking water. Unfortunately, this method threatens life, limb, and roof.
Ice Dam Defense
There are three ways to defend against the damage ice dams cause: insulation, ventilation and water-proofing shingle underlayment. All three work together. Insulation keeps heat from escaping from your home’s living space into your attic. Ventilation removes the heat and helps keep the roof deck evenly cool to help prevent snow from melting on the roof. Finally, waterproofing underlayment, such as ice & water seal is laid across the roof before roof shingles are applied. Most ice & water seal underlayment are warranted against leaks from dams that do form on the roof. With existing roofs, waterproofing underlayment is only an option if you remove the existing roofing or are building a new addition. Regardless, increasing the insulation R-value in the attic is always possible and ventilation can usually be added to your attic easily.
An attic insulated to today’s energy standards with fiber glass insulation minimizes heat escape through the ceiling, virtually eliminating the possibility of snow melting and refreezing at the base of the roof. If your home was built before 1980, chances are it needs more attic insulation. The amount of insulation your house should have will vary depending on where you live, how your house is built and many other factors including your lifestyle. Insulation levels are recommended by geographic zones and are stated in R-values. R-value is the resistance to heat flow of a material. The higher the R-value, the greater the insulating power. In the Baltimore metro area R-49 is recommended.
The second thing to look for in your attic is the amount of ventilation that you have. It is important to have ventilation in the attic so any heat lost from the interior of the home is drawn up and out of the attic. Adequate attic ventilation will help the roof deck stay cool. Another benefit of having your attic ventilated is that it allows for moisture that rises into the attic from things such as bathing, cooking and the laundry to escape. Unchecked moisture can promote mold, mildew, and wood rot. There are two common ways to ensure that excess moisture or heat can escape to the outside. One way is to use a power or mechanical ventilation system. The other way is through a natural, or static, ventilation system. A power ventilator is an electric powered fan installed at the roof or gable that runs by a thermostat or humidistat when the attic needs ventilation. Natural or static ventilation systems consist of simple vent or covered openings in your attic. These are typically ridge vents, gable, eave, or roof vents. Many ventilation experts agree that externally baffled ridge vents combined with vented soffits are a very effective method for ventilating an attic. Where older construction doesn’t permit ridge and soffit ventilation, powered fans can be
a good alternative. A properly designed ventilation system must have both intake vents in the soffit or in the eaves at the lower part of the attic, as well as exhaust ventilation, such as ridge vents, high in the attic at or near the ridge. Cooler, dryer outside air typically enters through eave vents near the attic floor, forcing existing moisture-laden or heated air out through vents placed high on the roof or gable. By ensuring proper insulation and ventilation, you will run less risk of the formation of ice dams, and you will substantially reduce the likelihood of damaging your attic components
If you are building a new home, or reroofing an older home, you should also insist that waterproofing shingle underlayment be installed before your roof shingles are applied. As mentioned earlier, it is completely resistant to water and, as such, is a critical last line of defense against leaks, preventing backed up water from getting into your home wherever it is applied. While shingle underlayment does not prevent the formation of ice dams, it will prevent backed up water from getting into the house. Discuss shingle underlayment placement with your builder or contractor. However, as a guide, most manufactures recommend that it be applied:
• Under metal flashing and counter flashing at roof penetrations, sidewalls, etc.
• In areas where roof pitches change, in valleys and around chimneys.
• Along the eaves and at short cornice projections.