Ice Damming Defined and How to Avoid It



Here is one of the best articles I’ve ever found to describe what ice dams are and how to avoid this potentially devastating condition.



Home     Energy Magazine Online November/December 1996

Out, Out Dammed Ice!

by Paul Fisette

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



Ice dams cause millions of dollars of     structural damage to houses every year, including water damage from roof     leaks. There are many ways to treat the symptoms, but proper air sealing,     insulation, and attic venting are the best ways to eliminate the problem.

Ice dams form when heat leaking from the living space below melts       snow, which then runs down the roof and refreezes at the edge. Damage can       result not only to gutters and roof coverings, but also to the interior       of the house.

Anyone     who has lived in a snowy climate has seen ice dams. Thick bands of ice form     along the eaves of homes, causing millions of dollars of structural damage     every year. Water-stained ceilings, dislodged roof shingles, sagging     gutters, peeling paint, and damaged plaster are familiar results of ice     dams.

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 snowmelt 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 uninsulated 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.

Figure 1. Ice dams are caused by a combination of snow, sub-freezing       outdoor temperature, and a warm roof over the interior of a building.

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 (see “Roofing and Siding Rehabs Get an Energy Fix,”     p. 25). 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.

Figure 2. With proper air sealing and attic ventilation, the roof can       be kept cold enough to prevent ice dams.

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.


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