Ice Damming

The Best Defense Against Ice Dams: Energy Efficient Roofs

Ice dams may be hidden under the snow on sloped roofs with inadequate insulation. Two circumstances combine to make an ice dam. First, snow melts on the part of the roof over the building interior (inside the perimeter of the outside walls) because heat and air that leak from the warm interior raise the roof temperature above the freezing temperature. Second, on the part of the roof covering the overhang (beyond the perimeter of the outside walls), the roof is cold and the runoff refreezes. The refrozen runoff forms a dam, further backing up melt water.

Backed up water can get under the shingles and through the roof deck. It can drip on the ceiling insulation or run down the underside of the deck to the connection between the roof and the walls. It then makes its way into the building in the form of damaging leaks. Melt water under the unmelted snow can decrease the friction between the snow and the roof and cause a snow slide, like an avalanche. Melt water often refreezes as icicles hanging from the gutters or edge of the roof. These icicles eventually break off when they get too heavy. Both snow slides and falling icicles endanger passersby.

Energy efficient roofs minimize problems with ice dams because they keep the entire roof cold. There is little difference in temperature between the part of the roof inside the perimeter of the outside walls and the part covering the overhangs. Thus, melting and refreezing is minimized. Insulating to prevent heat leaks and sealing against air leaks between the inside of the building and the attic are the best ways to achieve a cold roof. Increasing the level of insulation from R-11 to R-38 in a 1000 square foot attic should cost about $500. Accumulated dollar savings for heating and cooling, beyond the cost of the installed insulation, should be more than $1000 in the Washington DC area. Costs to repair damage from a leak caused by an ice dam could easily exceed $5000. Ventilation of the attic may help to achieve a cold roof. Its primary purpose, though, is to prevent moisture from condensing in the attic on the underside of the roof and dripping down into the insulation. This moisture is in any warm air that leaks from the inside of the building. Sealing the air leaks is more effective than increasing the ventilation.

To stop dammed up water from leaking under the shingles use a waterproof membrane under the parts of the roof where ice dams occur and melt water backs up. The membrane is usually placed from the edge of the roof up beyond where the walls intersect the roof. This membrane is installed when old shingles are replaced. If ice damming is a recurrent problem, heaters along the edge of the roof can be used to break up ice dams as they form. But these heaters use a lot of expensive electrical energy; they need to be used whenever it snows until air temperatures are about 45°F.
If ice dams are building up and no heaters are in place, building owners may want to take emergency action. Hammers, hatchets, ice picks or even salt used at the edge of a roof to attack ice dams and icicles do more harm than good and are not recommended. If snow is piling up to the point where the roof seems or sounds vulnerable to collapse, some snow can be removed but safety is the first concern. The object of snow removal from roofs is to reduce the snow load to safe levels, not to clean it off entirely. Regardless, there is danger of damage to the roof surface from using implements such as rakes or shovels. There are hazards to people who climb up to the roof on slippery ladders and stand on them. Walking on an already stressed roof may cause local failures in the structure. Snow removal from a roof is a risky proposition. Use a licensed contractor in most cases.

A well-designed, energy efficient roof should tolerate the loads of once every 50 year events like the recent snowstorms in the northeastern U.S. Nature eventually melts snow from roofs more cheaply and safely than artificial means. Best to let nature take its course and observe deficiencies in a particular roof, such as lack of insulation and unwanted air leakage paths from the interior space to the roof, so corrections can be made before the next emergency.

The U.S. Department of Energy, through its Buildings Technology Center at the Oak Ridge National Laboratory in Tennessee (, is working to provide builders, contractors and owners of buildings with research results to facilitate cost effective, energy efficient roofs and/or attics in every building. Much of the relevant information is collected in an upcoming Attic Handbook. Energy information is available from the Energy Efficiency and Renewable Energy Clearinghouse. The information presented here was prepared by the Buildings Technology Center and has focused on ice dams, how to eliminate their cause, and what to do and not do about ones that appear as a result of the blizzard of ’96 and future snowstorms. This information is summarized from the Attic Handbook and from results of research at the U.S. Army Corps of Engineers’ Cold Regions Research and Engineering Laboratory as well as in the Division of Building Research of the National Research Council of Canada.


Controls air infiltration
Spray Polyurethane Foams have been used as a high-performance insulation material for more than 30 years. People call it air infiltration but, technically, it’s unwanted, uncontrolled air-flow through the house, moving in or out of the structure. For example, in cold weather, the heated air inside the house flows out. Frigid outside air and moisture leaks in  rooms feel drafty. This uncontrolled air disrupts the temperature, air pressure and humidity of the living space, in any weather. The heating and cooling system has to work harder to maintain your familys comfort. Energy is wasted and that causes higher utility bills.

Reduces/eliminates Convection Currents Within The Walls
Intrusion of air into the stud wall cavity causes convection (movement of warm air to the top of the cavity and cold to the bottom). After the insulation is installed, there should not be any air circulation from one side of the stud space to the other. Any cracks or breaks in the system will reduce the effectiveness of the insulation. Unlike other insulations such as fiber, which permit air movement, Comfort Foam brand SPF stops air infiltration and reduces convection.

Minimizes Infestation
Comfort Foam has no nutritional value that would attract insects or other pests.

Minimizes Thermal Bridging
Different components of building construction have differing thermal resistances factors. This can produce cold areas that lead to condensation inside the walls. Comfort Foam covers the bridges on all sides, greatly reducing the transmission of heat or cold. Fully-adhered Comfort Foam replaces the need for adhesives or numerous mechanical fasteners that penetrate the substrate and decrease insulation efficiency.

Controls Condensation
Many homes now are built tighter and with greater insulation than ever before. However, this can mean that moisture from cooking, laundry and showers can be trapped in attics and other places where it can lead to wood rot, peeling paint, and deteriorating roofing and ceiling materials. In turn, that can affect the insulation value of commonly used home insulation materials. Used to insulate, Comfort Foam insulation systems control both heat build-up and condensation, resulting in a cooler attic in warmer environments and a dryer attic in cold climates. For the homeowner, that means comfort, energy conservation and structural stability.