Ice dams form when melted snow refreezes at roof edges. Anyone who has lived in cold climates has seen ice dams. We’ve enjoyed the sparkling beauty of ice formations built along roof eaves. However, most of us don’t stop to understand why these ice bands form until they damage our homes.
Three things are required for an ice dam to form: snow, heat to melt the snow and cold to refreeze the melted snow into solid ice. Ice dams can form when as little as 1 or 2 inches of snow accumulates on a roof – if the roof is poorly insulated and air sealed, and the snowfall is followed by several days of sub-freezing temperatures. Ice dams develop as snow on the upper part of the roof melts. Water runs down the roof slope under the blanket of snow and refreezes into a band of ice at the roof’s edge creating a “dam”. Additional snow-melt pools against the dam and eventually leaks into the building through the roof or roof trim.
The reason ice-dams form along the roof’s lower edge, usually above the overhang, is straight-forward. The upper roof surface (toward the ridgeline) is at a temperature that is above freezing. And the lower part of the roof surface (along the eaves) is below freezing. The upper roof surface is located directly above the living space. Heat lost from the house warms this section of the roof, melting snow in this area. During periods of sub-freezing temperature the lower regions of the roof deck remain at sub-freezing ambient temperatures. Roof overhangs are not warmed by indoor heat-loss.
The layer of snow holds indoor heat below the sheathing and could warm the roof sheathing above freezing in the area over the living space. If the inside temperature is 70 degrees and the outside 20 degrees (50 degree different). Snow will probably melt under these conditions. Yet, the temperature of the roof over the unheated overhangs is 20 degrees, the same as the outdoor temperature. The melt water will freeze when it reaches that part of the roof. Deeper snow makes things worse. More insulation makes the situation better.
It’s easy to understand that allowing water to leak into your house is a bad idea. Ice dams cause millions of dollars of damage every year. Much of the damage is apparent. Water-stained ceilings, dislodged roof shingles, sagging ice-filled gutters, peeling paint, and damaged plaster are all easily recognized and usually repaired when weather or budgets permit. But other damage is not as obvious and often goes unchecked.
Ice dams usually develop along roof eaves, above the plate line of exterior walls. Heat lost from homes at this point aggravates snow melting and ice-dam development. There are two reasons for increased heat loss at this point: Rafters on most homes sit directly on top of exterior walls leaving a shallow space for insulation between the top of the wall and underside of the roof sheathing: Low R-value = heat loss! And secondly, builders are not particularly fussy when it comes to air-sealing this point to prevent the movement of warm indoor air up to the underside of the roof surface. Air can leak through wire and plumbing penetrations here. Also warm indoor air can leak from the wall cavities rising upward and passing between the small cracks that exist between the wall top-plate and drywall. Ice dams also form below skylights because they are typically low-R assemblies that melt water that subsequently runs down to a cold section of roof below the skylight.
Roof leaks wet attic insulation. In the short term, wet insulation doesn’t work well. Over the long term, water-soaked insulation is compressed so that even after it dries, the insulation in the ceiling is not as thick. Thinner insulation means lower R-values. It is a vicious cycle. The more heat lost – the more ice dams form – the more it leaks – the more the insulation gets damaged – and so on. As a result you pay more to heat (and cool) your house. Cellulose insulation is hygroscopic and 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 . Energy dollars are again robbed, but more importantly, moisture gets trapped within the wall cavity between the exterior plywood sheathing and interior vapor barrier. The result: 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. Exterior and interior paint blisters and peels. And the well-being of allergy-sensitive individuals is compromised.
Peeling of wall paint deserves special attention here because its cause may be difficult to recognize. It is unlikely that wall paint (interior or exterior) will blister or peel when ice dams are visible. Paint peels long after the ice and all signs of a roof leak have evaporated.
Water from ice dams infiltrate 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 its skin of paint.
So the message here is to check your home carefully when ice dams form. Investigate 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 your home develop, be prepared. Water penetration often follows pathways difficult follow. Don’t just patch the roof leak. Make sure that the roof sheathing hasn’t rotted or that other less obvious problems in your ceiling or walls haven’t developed. And then detail a comprehensive plan to fix the damage. But more importantly, solve the problem.
The damage caused by ice dams can be controlled in 2 ways: Maintain the entire roof surface at ambient outdoor temperatures or build a roof so that it can’t leak into sensitive building materials if an ice dam forms.
Cold roofs make a lot of sense. Here you let the cold outdoor air work for you. Keep the entire roof as cold as the outdoor air and you solve the ice-dam riddle. Look at the roof of an unheated shed or garage, a pile of lumber or an abandoned home. Ice dams don’t form on these structures because there is no uneven melting and freezing!
For new construction it’s easy. Design the house to 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. Insulation retards the conductive flow of heat from the house to the roof surface. An air barrier retards the flow of heated air to the underside of the roof. And a good roof-ventilation system helps keep the roof sheathing cold. In an existing house this approach may be more difficult to follow. Often you are stuck with less than desirable conditions. But let’s look more closely at all the issues that will guide your strategy.
Ventilation: A soffit-to-ridge ventilation system is the most effective ventilation scheme you can use to cool roof sheathing. Power vents, roof vents and gable louvers just aren’t as good. Soffit and ridge vents should run continuously along the length of the house. The exhaust pressure created by the ridge vent sucks cold make-up air into the attic through the soffit vents. A 2-inch space or “air-chute” should be provided between the top of the insulation and the underside of the roof sheathing in all applications. The in-coming “soffit” air washes the underside of the roof sheathing with a continuous flow of cold air
Air Leakage: Insulation retards conductive heat loss, but a special effort must be made to block the flow of warm indoor air into the attic or roof area. Small holes allow significant volumes of warm indoor air to pass into attic spaces. In new construction avoid making penetrations through the ceiling whenever possible. But when you can’t avoid making penetrations or when you need to air-tighten existing homes use urethane spray-foam (in a can), caulking, packed cellulose, or weatherstripping to seal all ceiling leaks like: