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Attic air leaks = ice dams

Although many years ago thought of as a problem with roofing or attic  ventilation, ice dams are actually caused by the presence of warm, moist  air in the attic, combined with snow on the roof and the right weather  conditions. Ice dams occur when heat gets into the attic and melts the  underside of the snow on the roof. The melted snow then flows down  the roof surface until it reaches a cold spot (such as the eaves or soffit)  where it forms a frozen dam, behind which more snowmelt and ice pile  up. The ice buildup can back up under the shingles, damaging them and  allowing water to leak to the ceilings and walls below.

What NOT to do:

  • Installing heating cables will shorten the life of your roof and cost you money to operate.
  • Removing the ice with chippers, chemicals, or heat can damage shingles, gutters, and other building components.
  • Adding roof vents—including powered vents—will not eliminate ice dams, and often make the problems worse.
  • Although additional insulation—especially higher density foam on the top plate of exterior walls (see Insulation section, "Attic")—can reduce heat transfer to the roof deck, insulation alone is insufficient.

Infiltration through air leaks can occur in three ways:

  • Wind-driven infiltration happens during coldweather months when the wind blows cold, dry air into a house through cracks and openings and forces warm air out. During warmer weather, the wind blows in warm, humid air, forcing cooler air out.
  • Stack-effect infiltration takes place during the natural process of convection. As warm air rises and escapes through cracks at the top of the house, it pulls cold air into the lower areas.
  • Negative air pressure infiltration starts when appliances use air for combustion or when ventilation fans are operated. Outdoor air enters through any available openings to equalize the pressure inside a home. This can be a problem if adequate fresh air for combustion has not been provided. It can lead to a potentially dangerous carbon monoxide risk as fresh air is pulled down chimneys or vents, interfering with their proper operation.

Typically, air infiltration causes drafts and a chilly feeling near windows and doors and in basements. Adjusting your thermostat will not stop the drafts, but sealing hidden cracks and openings will. By stopping air leaks at their source, you’ll stay warmer in the winter and cooler in the summer, use less fuel, and reduce your utility bills.


Seal wires and fixtures - Electrical wires often enter attics through the tops of walls. Fill the hole around the wire with spray foam or caulking.

Light fixtures that enter the attic space can be sealed with an airtight box made of wood or rigid foam to keep insulation off the fixture. Alternatively, replace with airtight fixtures.

CAUTION: Turn off power to the entire circuit - not just the switch - and keep it off until the caulk or foam has cured completely!

CAUTION: If your wiring consists of old "knob and tube" style wires or wires that are worn or broken, they must be repaired or replaced prior to insulating and sealing.

Where do you start?

Fortunately, air infiltration is one of the easiest forms of energy loss to correct. The process requires only a careful inspection of your home and some inexpensive weather-stripping, caulking, and filler materials. 

Most people know they should caulk and weather-strip around the exterior of their homes to protect it from the elements. However, it is equally important to protect your home from interior air leaks. Moist interior air can enter the walls and ceiling through cracks and holes, and condensation buildup in those locations can damage or destroy insulation, wiring, wood, or other building materials.

Look for air leaks

Before research and building science demonstrated the role air leaks play in energy loss, most people assumed that insulation was enough to stop heat flow through a building. Although insulation slows heat transfer, it is easily compromised by air flow, whether driven by outside wind conditions or convection currents within the building. The only way to stop this air movement—and the associated heat loss—is by eliminating the holes and pathways between the inside of the house and the outside.

The first step to tightening up your home’s envelope is performing a detailed inspection for air leaks. This should be part of every energy assessment, and the inspector should be able to show you locations where air is leaking and needs to be sealed. A good rule of thumb is to seal the high air leaks first; in other words, start by plugging holes and leaks in the attic; then move to the exterior walls, and look for smaller leaks around doors, windows, and electrical switches and outlets; finally check out your basement.

Damages insulation and more

Water vapor carried with the escaping warm air may condense, freeze, and build up in the insulation. When this water builds up, it can soak the insulation (wet insulation has little insulating value), cause plaster and paint to crack and peel, and lead to rot and other structural damage.

The source of ice dams is attic air leaks

Anywhere there is a penetration into the attic space (wires, plumbing vents, light fixtures, chimneys) there is the potential for air leaks. Even homes that are only a few years old may not be properly sealed. To avoid these types of problems and eliminate most ice dams, attic air leaks must be sealed with caulking or spray foam. 


What to do about attic air leaks - Warm, moist air leaking through holes around plumbing vents, electrical wires, chimneys, etc. leads to reduced performance of insulation (right illustration). Foam spray (left illustration) or caulking can seal up the air leak, saving energy and preventing nearly all ice dams.