Attic Ventilation And Ice Dams Are The Same Problem

Ridges of ice along a gutter and icicles hanging from an eave are usually treated as a weather event. They are a symptom of heat leaving the house through the ceiling.

The mechanism runs from the ceiling to the eave

Warm air escaping into an attic heats the underside of the roof deck. Snow lying on the warmed part melts even while the air outside is well below freezing.

The meltwater runs down toward the eave, which overhangs unheated space and stays cold. There it refreezes into a ridge.

That ridge grows and blocks the path of later meltwater, which backs up beneath the shingles and finds its way into the ceiling below.

Shingles shed water, they do not hold it back

Roofing is lapped so that water running downhill passes over each joint. Standing water behind a dam works its way upward through those laps.

This is why the leak from an ice dam appears in a ceiling that was completely sound during the heaviest rain of the previous summer.

Waterproof membranes at the eave are used to resist exactly this, but they are a second line of defense rather than a cure.

Air sealing comes before insulation

The heat reaching the attic mostly travels with escaping air rather than through the ceiling material. Recessed lights, wiring penetrations, plumbing chases and the tops of interior walls are the usual paths.

Sealing those openings reduces the warm air arriving above, and only then does adding insulation depth deliver what it should.

Insulating over unsealed leaks buries them, which makes the problem harder to find later without changing much about the heat loss.

Ventilation keeps the deck cold

A vented attic works by letting outdoor air enter low at the eaves and leave high at the ridge, carrying away any heat that does get in.

Blocked soffit vents are extremely common, usually because insulation was pushed into the eave, and baffles exist to hold that path open.

Unvented assemblies take the opposite approach, insulating along the roof line so the attic is inside the thermal envelope entirely. Mixing the two strategies is what produces failures.

The exhausts that make it worse

Bathroom fans and dryer ducts terminating inside an attic deliver both heat and moisture directly into the coldest part of the house.

That moisture condenses on the underside of the deck, producing frost that melts into stains that look like a roof leak but are not.

Running every exhaust duct through to the outside, insulated along its length, removes a heat source and a moisture source in the same piece of work.