Winter Nemesis: Ice Dams - 7/3/2000 - Home Exterior Roofs Siding

Winter Nemesis: Ice Dams

Ice dams are a common cold weather problem caused by snow melting over heated portions of a building and refreezing at colder portions of the roof, creating a dam. Water produced by subsequent melting then backs up under the shingles, eventually causing damage to insulation, interior finishes, and more. The snow melts due to heat loss into the attic from inadequate insulation, air leakage, and/or inadequate ventilation.

The success of retrofit solutions depends in part on existing construction details and accessibility of the attic. Prevention is easier and more assured in new construction.

SUGGESTED REMEDIES:

• Install more attic insulation (the U.S. DOE recommends R-49 in the coldest climates).
• Install raised-heel trusses to allow full-depth insulation and proper ventilation over exterior walls or, in retrofit situations, install rigid foam insulation between the drywall and other insulation (at least 2’ in from walls and at least 2" thick).
• Insulate vent pipes, ducts, etc., that are in the attic.
• Seal all penetrations into the attic from ceilings or walls.
• Seal ductwork connections with mastic.
• Install continuous soffit-and-ridge vent systems and baffles at lower side of the roof, providing at least a 2" space between insulation and sheathing.

OTHER CONSIDERATIONS:

• Consider metal roofs. They shed snow but have a relatively high first cost (offset by longevity, potential energy savings, and avoidance of ice dam repair costs).
• Do not install ductwork in attics or toward exterior walls where heat loss or air leakage most impacts warming of sheathing.
• Install a rubberized membrane under shingles from lower edge of the roof to at least 12" inside the exterior wall. In colder regions, application 10’ to 12’ up the roof and the full length of valleys may be needed. Local codes may provide minimum requirements for this type of protection.