What causes ice dams and how to stop them

What you're seeing, and what it means

If you have icicles and your neighbour doesn't, the difference is almost always how much heat is getting into your attic.

Why ice dams form

Snow on a roof melts from underneath when the roof boards are warm. The meltwater runs down until it reaches the eaves, which hang out past the house and stay at outdoor temperature, and it freezes again there. That ridge of ice grows through the winter and holds back the water behind it, which then works its way up under the shingles.

The ice is only the symptom. The cause is warm indoor air getting into the attic. That same heat is also costing you money on every day of the heating season.

The fix, in order

1. 1.

   Seal the air leaks

   Insulation slows heat moving through it, but it does nothing to stop air moving through it, and moving air carries far more heat and moisture. Piling insulation on top of unsealed gaps is the most common way people spend money on ice dams without fixing them.

2. 2.

   Add insulation

   Bring the attic up to the level your climate zone calls for, and pay attention to the edges, where it usually gets thin.

3. 3.

   Open up the ventilation

   Clear or add soffit vents at the eaves and a vent at the ridge, so outside air washes the underside of the roof and keeps it cold. This only works once the first two steps are done.

Where attics leak air

• The attic hatch. Usually the biggest single leak, and usually not insulated. Weatherstrip it and insulate the lid.
• The tops of interior walls. There's a gap along the top of every wall below the attic.
• Plumbing stacks and wiring holes. Seal them with fire-rated foam or caulk suited to the surface.
• Pot lights. Older fixtures that aren't IC-rated act like chimneys. Replace them with airtight IC-rated ones instead of burying them in insulation.
• Bathroom and kitchen fan ducts. Seal them where they pass through the ceiling, and make sure they vent outside and not into the attic.
• Chimney chases and dropped ceilings. Big open cavities that often connect straight into the attic.

You can't see air leaks, and the biggest ones are often where nobody thinks to look, like a dropped ceiling over the kitchen or a chase behind the tub. A blower door test with an infrared camera finds them in an afternoon. Get one done before you pay for insulation.

How much insulation you need

The eaves are where most attics come up short. Where the roof meets the outside wall there's very little height, so the insulation gets thin right where ice dams form. New houses solve this with raised-heel trusses. In an older house, foam or cardboard baffles hold the air channel open so the insulation can be pushed out further without blocking the soffit vents.

Blocked soffit vents are very common, and insulation stuffed into them during an earlier upgrade is usually the reason. Check them while you're up there.

What won't fix it

Heat cables melt a channel through the ice and can stop damage in the short term, but they only treat the symptom. They run all winter and don't last forever. Raking snow off the roof after a storm helps, but it isn't a plan. Neither one does anything about the heat you're losing.

What about...