Ice dams and how to prevent them

Ice dams are a continual source of frustration for many homeowners in colder climates.

Ice dams form on roofs, usually above eaves, when poor roof ventilation causes roof snow to melt. In turn, this snowpack melts, then freezes when temperatures drop. This constant melting and refreezing eventually builds an “ice dam.” The resulting meltwater eventually finds its way up and under the shingles and wreaks havoc on interior walls and ceilings.

In winter, normal household activities such as laundry, dishwasher use, and showering create moisture that can damage insulation, joists, decking, subflooring, and shingles. In summer, heat buildup can cause premature aging and cracking of wood and roofing materials.

The only way to combat these problems is with balanced ventilation. When a ventilation system is balanced, wind blowing over the ridge creates a negative pressure that draws warmer air out of the attic. Replacement air enters through vents under the eaves, bathes the underside of the roof, and exits through the ridge.

Even without wind, the natural convection action of rising hot air maintains a continuous airflow along the underside of the roof. It is a system that works all year round with no moving parts or power consumption.

Ice dam formation can be alleviated by properly ventilated roofs. One of the ways this is accomplished is by installing a roof ridge vent.

How Ridge Vent Works

It allows outside air to flow naturally up and out of the attic.
· Promotes a cooler, drier attic.
Helps prevent moisture from being trapped in insulation, structural lumber, shingles, and roof deck.
Helps prevent rot, mildew, drywall damage, peeling paint and warped siding.
· Provides year-round performance for constant ventilation without energy consumption.

According to the Department of Construction and Technology at the University of Massachusetts:

SOLUTIONS

Damage from ice dams can be controlled in two ways: keep the entire roof surface at outside ambient temperature, or build a roof so it cannot seep into sensitive building materials if an ice dam forms.

Cool ceilings make a lot of sense. Here you let the cold air from outside work for you. Keep the entire roof as cold as the outside air and you’ll solve the ice dam puzzle. Look at the roof of an unheated shed or garage, a pile of lumber, or an abandoned house. No ice dams form on these structures because there is no uneven melting and freezing!

For new construction it is easy. Design the house to include sufficient roof insulation, a continuous air barrier separating the living space from the lower part 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 hot air to the underside of the roof. And a good roof ventilation system helps keep the roof lining cool. In an existing home, this approach may be more difficult to follow. You are often stuck in less than desirable conditions. But let’s take a closer look at all the themes that will guide your strategy.

Insulation: Homes in the northern United States must be equipped with roof insulation of at least R-38 (about 12 inches of fiberglass or cellulose). The insulation must be continuous and consistently deep. The most noticeable problem area is above the exterior wall. In new construction, raised heel trusses or roof framing details that allow R-38 above the exterior wall should be used. In existing structures, where the space between the top wall plate and the bottom of the roof sheathing is restricted, install high R/inch (R-6/inch) insulation foam. Be sure to seal the insulation at this point to prevent hot air from escaping from the living space.

Ventilation: A soffit-to-ridge ventilation system is the most effective ventilation scheme you can use to cool your roof sheathing. Vents, turbines, roof vents, and gable shutters just aren’t that great. Soffit and ridge vents should run continuously the length of the house. A ridge vent with baffles (such as the one sold by Air Vent) is best because it will push air out of the attic regardless of wind direction. Exhaust pressure created by the ridge vent draws cold makeup air into the attic through the soffit vents. A 2-inch gap or “air duct” must be provided between the top of the insulation and the bottom of the roof sheathing in all applications. Incoming “soffit” air washes the underside of the roof sheathing with a continuous flow of cool air. CAUTION: Be sure to install insulation baffles on the exterior wall to protect the insulation from air entering through the soffit vents.

Air Leakage: Insulation retards conductive heat loss, but special effort must be made to block the flow of warm interior air (convection) into the attic or roof area. The small holes allow large volumes of warm indoor air to pass into the attic spaces. In new construction, avoid making penetrations through the roof whenever possible. But when you can’t avoid making penetrations or when you need to seal existing homes, use urethane spray foam (in a can), caulk, packed cellulose, or weather stripping to seal all roof leaks like:

• wire penetrations
• plumbing penetrations
• ceiling lamps
• attic hatches
• chimneys
• bathroom extractor fans
• intersection of interior partitions and ceiling