Preventing Ice Dams
Proper insulation and roof ventilation can stop ice dams from forming, prevent damage and lower energy bills.
I have investigated countless ice-dam problems as a builder, researcher and consultant. Although individual cases look different, and often result in different types of damage, all ice-dam situations have two things in common: They happen because melting snow pools behind dams of ice at the roof’s edge and leaks into the house; also, ice dams and the damage that results from them is avoidable.
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 (of other people’s homes). 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.
Deeper snow and cold temperatures increase the likelihood and size of ice dams. Every inch of snow that accumulates on the roof’s surface insulates the roof deck a little more, trapping more indoor heat beneath the roof deck and warming the roof sheathing. Each inch of snow has an R-value of approximately 0.5 – 1. The worst ice dams occur when deep snow accompanies cold weather. Here is an example to provide a sense of scale: A poorly insulated and unvented R-20 cathedral roof with 10 inches of snow can result with serious ice damming. In this example, 10 inches of snow adds ~between R-5 and R-10 to the roof system resulting in a total roof R of 25 – 30. 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 differential), the temperature of the roof sheathing would be between 5/25 and 10/30 of the way from 20 degrees toward 70 degrees. In other words, the roof should temperature should be between 30 and 37 degrees over the living space. 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. You can do the math for a variety of snow depths and various indoor/outdoor temperature conditions to get a sense of how the variables are related. The trick is to keep the entire roof below freezing if possible. Roof venting helps and is discussed below.
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.
Insulation: Houses in the northern United States should be equipped with ceiling insulation of at least R-38 (about 12 inches of fiberglass or cellulose). The insulation should be continuous and consistently deep. The most notable problem area is located above the exterior wall. Raised-heel trusses or roof-framing details that allow for R-38 above the exterior wall should be used in new construction. In existing structures, where the space between the wall’s top plate and underside of the roof sheathing is restricted, install high R/inch insulating foam (R-6/inch). Be sure to seal the insulation at this point to prevent warm-air leakage from the living space.
Ventilation: A soffit-to-ridge ventilation system is the most effective ventilation scheme you can use to cool roof sheathing. Power vents, turbines, roof vents and gable louvers just aren’t as good. Soffit and ridge vents should run continuously along the length of the house. A baffled ridge vent (like the one sold by Air Vent) is best because it will exhaust attic air regardless of wind direction. 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. CAUTION: Be sure to install insulation baffles above the exterior wall to protect the insulation from the air that blows in through the soffit vents.
Air Leakage: Insulation retards conductive heat loss, but a special effort must be made to block the flow of warm indoor air (convection) 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:
ceiling light fixtures
bathroom exhaust fans
intersection of interior partitions and ceiling
The list of attempted solutions is long. The problem I have with many of these efforts to prevent ice dams is that they don’t deal with the root cause which is heat loss. They merely treat the symptom.
Metal roofs are common in snow country so they must work! Right? Steeply pitched metal roofs in a sense thumb their nose at ice dams. They are slippery enough to shed snow before it causes an ice problem. However, metal roofs are expensive and do not substitute for adequate levels of insulation.
Many people install self-sticking rubberized sheets under roof shingles wherever ponding of water against an ice dam is possible: above the eaves, around chimneys, in valleys, around skylights and around vent stacks. The theory is that if any water leaks through the roof covering, the waterproof underlayment will provide a second line of defense. The material is sold in 3-foot x 75-foot rolls for about $80/roll. These products adhere directly to clean roof decking. Roof shingles are nailed to the deck through the membrane. The membrane is self-healing and seals nail penetrations automatically. W.R.Grace (Ice and Water Shield), Domtar (Eaveshield) and Bird all make competitive products. This band-aid solution is reasonable alternative for many existing structures where real cures are not possible or cost effective. These products also serve as a redundant layer of protection. Sometimes even well-constructed and designed roofs can have ice dams. Deep snow will act to insulate the roof deck making it warm enough to melt snow over the living area. A redundant layer of protection is helpful here.
You might consider using sheet-metal ice belts if you don’t mind the look of a shiny 2-foot-wide metal strip strung along the edge of your roof. I think, ice/snow belts are reasonable choices for some patch and fix jobs on existing houses. This eaves flashing system tries to do what metal roofing does: shed snow & ice before it causes a problem. It works —- sometimes. The problem with ice belts is they don’t work well. Often, a secondary ice dam develops on the roof just above the top edge of the metal strip. Ice/snow belts are sold as 32-inch x 36-inch pieces (with additional fastening hardware) for about $12 per panel.
Contrary to popular belief, gutters do not cause ice dams. However, gutters do help concentrate ice and water at a 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.
And what about those heat tapes? In my entire life, I have never seen a zig-zag arrangement of electrically-heated cable work to fix an ice dam problem. The cable is heated by electric power, so you throw good energy after bad energy (keep in mind that ice dams are a heat-loss problem!). Over time heat tape embrittles shingles, creates a fire risk, are expensive to install and use, and leak water through loose fasteners. In fact take a good look at roofs that are equipped with heat tape. The electric cable creates an ice dam just above it. My advice is don’t waste your time or money here.
Different styles of shoveling snow and chipping ice from the edge of a roof is my favorite of all solutions! People attack mounds of snow and roof ice with hammers, shovels, ice picks, home made snow rakes, crow bars and CHAIN SAWS! Can you believe it? The theory is obvious: no snow or ice, no leaking water. BUT, and it is a big but, more damage is done to life, limb and roof in the process. Having said that, carefully removing snow from roofs with roof snow rakes does help. You can scrape some of the protective mineral surface from asphalt shingles as you remove snow, but removing the insulating snow and potential meltwater does help reduce ice dam potential – just be careful.
Whatever plan you decide to follow, focus on the cause. Ice dams are created by the heat lost from the house. Develop a strategy that is centered around this fact whenever possible. Ventilate, insulate well and block as many air leaks as practical. There are no excuses for new construction. However, cures for existing structures are often elusive and expensive. In some cases you have to treat the symptom. The payback is damage prevented.
Last updated: February 6, 2011