Ice Dams on a 1950s Cape Cod: Ventilation and Ice Barrier
A homeowner in St. Paul has a 1,400-square-foot attic floor, a 4:12 roof, two gable vents each 12 inches by 18 inches, no soffit vents, and a bathroom exhaust fan terminating in the attic. Every winter, icicles form at the eaves; in January, water stains appear on the kitchen ceiling. The attic has R-19 kraft-faced batts, compressed at the eaves, and the roof deck is stained. The owner wants the roofer to install ice-and-water shield over the entire roof and new 30-year architectural shingles, saying that will 'seal out the ice.' The existing roof has one layer of 20-year shingles. The roofer knows Minnesota code requires an ice barrier at the eaves and that the soffit is blocked by insulation. The ridge has no vent.
1. Why will full ice-and-water shield not solve the ice dams?
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Ice dams are a heat-loss and ventilation problem, not a shingle problem. Warm attic air leaks through the ceiling, heats the roof deck, melts snow, and the meltwater refreezes at the cold eaves. Ice-and-water shield is a backup membrane; it keeps water out of the deck but does not stop melting or refreezing. Full coverage can also trap moisture in the deck if the attic is not dried. The bath fan discharging into the attic adds warm, moist air. Compressed R-19 at the eaves blocks the path for intake air. The roofer should treat the cause: air-seal the ceiling, insulate to code, add balanced intake and exhaust ventilation, and vent the bath fan outdoors.
2. Calculate required net free vent area at 1/150 and compare with existing; recommend ventilation.
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Attic floor area = 1,400 sq ft. At 1/150, NFA = 1,400/150 = 9.33 sq ft = 1,344 sq in. Existing two gable vents: 2 × (12 × 18) = 432 sq in = 3.0 sq ft. Deficiency = 912 sq in. At 1/300, required = 4.67 sq ft = 672 sq in, still short. Gable vents alone are not balanced intake/exhaust. Recommend continuous soffit intake and ridge exhaust sized near 50/50, at least 1,344 sq in total if using 1/150; clear insulation from soffits with baffles; add a ridge vent only after intake is opened. If the attic has no vapor barrier, 1/150 is safer.
3. Specify ice barrier placement and underlayment for the re-roof.
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Install self-adhering ice barrier at the eaves from the roof edge to at least 24 inches inside the exterior wall line, and in valleys. Minnesota adopts the IRC ice-barrier requirement for areas with a history of ice dams. Use a shingle-compatible membrane; prime the deck if the manufacturer requires it. Cover the remainder with 30-lb felt or synthetic underlayment in shingle-lap order. Install drip edge over the ice barrier at the eave and under the barrier at the rake, per manufacturer. Do not rely on the membrane as a substitute for ventilation. Keep the ice barrier back from the ridge if a ridge vent is cut, and seal penetrations.