Home Energy Magazine Online November/December 1996

Out, Out Dammed Ice!
by Paul Fisette

Ice dams are not the disease, but rather a symptom of a home’s energy sickness. The cure is energy conservation: keep heat from leaking into the attic from the house.

Ice dams need three things to form: snow, heat to melt the snow, and cold to refreeze the  melted snow into solid ice. As little as 1 or 2 inches of snow accumulation  on a roof can cause ice dams to form. Snow on the upper part of the roof  melts, runs down the roof under the blanket of snow to the roof’s edge, and  refreezes into a dam of ice. As more snow melt runs down the roof, it pools  against the ice dam. Eventually, water backs up under the shingles and leaks into the structure.

The reason ice dams form along the roof’s edge, usually above the overhang, is straightforward. Heat and warm air leaking from living space below melts 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 (about R-1 per inch), keeping more heat from the living space in, which further  heats the roof deck. 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 at the very vulnerable roof eaves area. As gutters fill with ice, they often bend and rip away from the house, bringing fascia, fasteners, and downspouts in tow.

Roof leaks wet 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 insulation in the ceiling is not as thick. The lower R-values  become part of a vicious cycle: heat loss-ice dams-leaks-insulation damage-more heat loss! Cellulose insulation is particularly vulnerable to the hazards of wetting.

Water often leaks down within the wall frame, where it wets wall insulation and causes it to sag, leaving un-insulated voids at the top of the wall. Again, energy dollars disappear, but more importantly, moisture gets trapped within the wall cavity between the exterior plywood sheathing and the interior vapor barrier, causing smelly, rotting wall cavities. Structural framing members can decay. Metal fasteners may corrode. Mold and mildew can form on wall surfaces as a result of elevated humidity levels. Both exterior and interior paint blister and peel. And people with allergies suffer.

Peeling wall paint deserves special attention because its cause may be difficult to recognize.  It is unlikely that wall paint will blister or peel when 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 within wall frames. The moisture within the wall cavity eventually wets interior wall coverings and  exterior claddings as it tries to escape (as either liquid or vapor). As a result, interior and exterior walls shed their skin of paint.

Solving the Problem

Check the home carefully when ice dams  form. Investigate the attic, even when there doesn’t appear to be a leak.  Look at the underside of the roof sheathing and roof trim to make sure they  haven’t gotten wet. Check the insulation for dampness. And when leaks inside     the home develop, be prepared. Water penetration pathways are often  difficult to follow. Don’t just patch the roof leak. Make sure that the  roof sheathing hasn’t rotted and that other less obvious problems in the  ceiling or walls haven’t developed. Detail a comprehensive plan to fix the   damage. But more importantly, solve the problem that caused the ice dams to  form.

You can try to block the flow of melt  water into a house by installing a rubber membrane on the roof under the  roof shingles. Or you can craft a real solution: keep the entire roof cold,  and save energy dollars in the process! In most homes this means: block all  air leaks leading to the attic from the house, increase the thickness of  insulation on the attic floor, and install a continuous soffit and ridge  vent system. Be sure the air and insulation barrier you create is   continuous.

Heat loss is often worst just above the  top plate, the continuous horizontal framing where exterior walls and  ceilings are joined. This is partly because there isn’t room in the corner  for adequate insulation. Also, builders are not particularly fussy about  air sealing to prevent the movement of warm air up to the underside of the  roof surface. Air can leak through wire and plumbing penetrations here, or  can come from wall cavities, passing between the small cracks between the  top plate and the drywall.

New houses should include plenty of  ceiling insulation, a continuous air barrier separating the living space  from the underside of the roof, and an effective roof ventilation system.     In both new and retrofitted buildings, insulation should be up to local  standards. In the northern United States, this is usually at least R-38. A  soffit-to-ridge ventilation system is the most effective ventilation scheme  for cooling roof sheathing. Power vents, turbines, roof vents, and gable louvers just aren’t as     good. Both the baffles on the ridge vent and the sun warming up the roof     help drive the air flow out of the ridge vent. Air coming in the soffit  washes the underside of the roof sheathing with a continuous flow of cold air.

Insulation retards conductive heat loss, but a special effort must be made to seal warm indoor air inside. In new  construction, avoid making penetrations through the ceiling whenever  possible. When you can’t avoid making penetrations, or when air tightening     existing homes, use urethane spray foam (in a can), caulk, packed  cellulose, or weatherstripping to seal all ceiling leaks.

Other Options

Sometimes it is not  feasible to treat the cause of the house’s problems, and you must treat the  symptoms. Steeply pitched metal roofs (common in snow country) in a sense     thumb their noses at ice dams. They are slippery enough to shed snow before  it causes an ice problem. However, metal roofs are expensive and they are  no substitute for adequate levels of insulation.

Self-sticking rubberized sheets can go  under roof shingles wherever water could pond against an ice dam: above the  eaves, around chimneys, in valleys, around skylights, and around vent   stacks. If water leaks through the roof covering, the waterproof     underlayment provides a second line of defense.

Sheet-metal ice belts can help, if a shiny 2-ft-wide metal strip along the edge of the roof is acceptable. Ice  or snow belts are used for some patch-and-fix jobs on existing houses. The flashing, installed at the eaves, imitates metal roofing by shedding snow  and ice before it causes a problem. It works-sometimes. The problem with ice belts is that a secondary ice dam often develops on the roof just above  the top edge of the metal strip.

Placing electric heat tape in a zigzag  arrangement on the shingles above the edge of the roof is a poor solution.  I have never seen electrically heated cable actually fix an ice dam  problem. The considerable amount of electricity it takes to prevent ice  formation is expensive, and the heating must be done in anticipation of ice dam conditions, not afterwards. Over time, heat tape embrittles shingles,  creating a fire risk. It’s expensive to install, too, and water can leak   through the cable fasteners. Often the cables create ice dams just above  them. Don’t waste time or money on this retrofit.

The worst of all solutions is shoveling  snow and chipping ice from the edge of a 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.

Paul Fisette is director of Building Materials Technology and Management at the University of Massachusetts, Amherst.