Ice Dams on a Cathedral-Ceiling Retrofit

No. The symptoms point to a heat-and-moisture path, not simply a missing vent. Frost on the underside of the deck near the ridge means moist indoor air is reaching a cold surface; 42 percent indoor RH at 0 degrees Fahrenheit is a strong moisture load. The batts are pressed into the rafter bays with no baffles, so even if soffit vents were cut, there is no continuous 1-inch intake channel and the ridge vent would have little useful path. In a 2x10 cathedral bay, compressed insulation also reduces R-value and leaves the deck cold. Adding vents without air sealing could move more moist air into the cavity. Ice damming is driven by heat loss through the assembly, not by the absence of a vent alone. The reliable options are: air-seal the ceiling plane, then either create a true vented channel with baffles and balanced intake and exhaust sized for the roof area, or convert to an unvented assembly with closed-cell foam against the deck and no vents. Two layers of shingles and no drip edge also need correction at reroofing. Ventilation alone is not a repair.

First, diagnose rather than sell ventilation. Perform a blower-door test with zonal pressure diagnostics, thermal imaging at a temperature difference of at least 20 degrees Fahrenheit, and moisture mapping of the ceiling and deck with a pin meter. Use a borescope at the hatch and at a removed drywall patch to verify insulation contact, baffle space, and whether a 1-inch vent channel can physically be maintained. Confirm that bath and kitchen exhausts discharge outdoors and are not leaking into the cavity. Second, air-seal top plates, chases, penetrations, and the hatch, and control indoor humidity toward 30 to 40 percent. Third, choose an assembly: vented with baffles, full-depth insulation, and correctly sized soffit and ridge free area, or unvented with closed-cell foam to the deck and an ignition barrier. Fourth, at reroofing, remove the second shingle layer, inspect the tongue-and-groove deck, install ice barrier at the eaves, drip edge, and underlayment. Verification should include a post-work blower-door reduction of at least 30 percent or achievement of a stated cfm50 target, and an infrared scan at a 20-degree temperature difference showing no cold deck surfaces below the dew point plus moisture content at or below 16 percent after 30 days.

The proposal should state that the cause of leakage has not been fully verified until the ceiling or roof cavity is opened; that venting feasibility is unknown because the 2x10 bays may not provide a continuous channel; that two shingle layers and tongue-and-groove decking may require redecking; and that no drip edge is visible. It should identify that ice damming is a heat-loss and indoor-moisture issue and that ventilation alone is not warranted to stop leaks. Concealed conditions include rot, saturated decking, previous repairs, and blocked or missing firestopping. The scope should exclude repair of stained drywall, mold remediation, and humidity-control equipment unless listed. State that verification will be by blower door and infrared scan under adequate weather conditions, not by a promise that leaks will never recur. Clarify that code-required ice barrier and drip edge will be installed at reroofing and that the owner must maintain indoor RH and exhaust ventilation. Include a change-order rate for discovered rot and a limitation that the contractor cannot verify concealed framing without destructive access.