Ice Dams

Why are Ice Dams a Problem of Insulation and Ventilation, Not Roofing?

Bob Cowherd

Aqua Barriers has proposed to remove and replace the asphalt shingles on the low-pitch section of the unit 117 roof over the bathroom and walk-in closet installing an ice and water shield layer between the plywood and shingles for $1,850. This will involve replacing (or installing for the first time) any flashing around penetrations and wall intersections.

Two points:

First, the root cause of ice damming is the uneven temperatures of the roof surface due to incomplete insulation and ventilation in the attic space (see <www.thisoldhouse.com/toh/article/0,,1131346,00.html > and attached graphic explanation from This Old House). Pitched asphalt shingle roofs shed water when it is allowed to flow freely (like off a raincoat). When water is contained, as it is behind an ice dam (like a bath tub), any asphalt shingle roof will eventually leak. If the root cause is not addressed and ice dams continue to form, even an excellent installation that I believe Aqua Barriers would give us is no guarantee against water damage. Plan A should always be maintaining a free drainage path along an even temperature plane with well-installed flashing (like a rain coat). Given the historically demonstrated uncertain performance of seams and connections in a complex assembly that moves with dynamic structural and thermal loadings, creating a water proof barrier (bath tub) should only be Plan B.

Second, ice and water shield has proven to be a revolutionary product for situations that previously had no good solution. However, it has also introduced unanticipated water problems because it doesn’t just keep water from getting in, it also stops water vapor from getting out. The overdependence on ice and water shield has generated water problems (condensation, rot, mold) in locations where previously water vapor was allowed to pass over time without obstruction. Specifically, water vapor currently passes through and out of the plywood on the low-pitched section of unit 117 by evaporation through the asphalt shingle roof, and into the ventilated attic space. After the application of ice and water shield to a large area, the plywood will depend entirely on the ventilation channels between the plywood and insulation in each rafter bay—the same ventilation channel needed to prevent ice damming.