Ice Dams and Frost in a Cathedral Ceiling After a Ventilation “Fix”

The symptoms point to warm, moist indoor air reaching the cold roof sheathing, not simply to poor shingles. The frost on the gable-end sheathing and ceiling stains near the ridge indicate condensation on cold surfaces. The insulation touching the sheathing at the eaves blocks the soffit intake, so the two small roof vents cannot create balanced ventilation. The 45% indoor relative humidity in winter provides the moisture source. Ice dams form because the upper roof is warmed by heat loss and the eave remains cold, so meltwater refreezes at the overhang.

Evidence to collect:

• Infrared images or temperature readings showing warm areas at the ceiling and cold eaves.
• A visual check for air leakage at penetrations, top plates, and the ridge, and for crushed or missing baffles.
• A moisture meter on sheathing and framing, and inspection for deteriorated poly or wet insulation.
• Verification that the two roof vents are open and that soffit vents are not blocked.

Adding foam without air sealing and ventilation would reduce but may not eliminate the problem, and keeping the interior poly while adding exterior foam can create a double vapour barrier.

The proposal is incomplete. Exterior rigid foam can raise the dew-point temperature and reduce condensation, but it must be thick enough for the climate and must be combined with air sealing. Keeping an impermeable poly layer on the warm side while adding impermeable exterior foam creates a potential double vapour barrier, trapping moisture in the batt insulation or at the sheathing. The poly also has many penetrations and is unlikely to be continuous.

Specify:

• Air-seal all ceiling penetrations, top plates, and discontinuities before any insulation work.
• Remove or carefully evaluate the interior poly; do not leave two impermeable layers without a hygrothermal design. In many cold-climate cathedral ceilings, a properly detailed interior air barrier/vapour retarder is used, but the exterior foam thickness and permeance must be coordinated.
• Install insulation baffles at each rafter bay to maintain a minimum ventilation channel from soffit to ridge, and add a continuous ridge vent sized to the roof area.
• Use enough exterior rigid insulation to keep the sheathing above the dew point in design conditions; verify the ratio with the manufacturer or a building scientist.
• Seal the foam joints and use compatible fasteners and flashing.

A five-year fix requires correcting the air leakage and ventilation, not just covering the deck.

Sequence:

1. Investigate from inside and outside: confirm wet sheathing, insulation condition, and air-leak paths.
2. Remove affected interior finishes or insulation only where required; correct any rotten framing.
3. Air-seal the ceiling plane thoroughly.
4. Install or repair baffles at every rafter bay and ensure the soffit intake is open and unobstructed.
5. Resolve the vapour-barrier strategy; do not leave a compromised poly layer hidden.
6. Install exterior rigid insulation with staggered, sealed joints and proper attachment.
7. Install a continuous ridge vent and verify net free vent area; do not rely on two 100 mm vents.
8. Install underlayment, ice-dam protection at the eaves and valleys, and new shingles.

Before shingles: verify that the ventilation path is continuous and unobstructed, that the sheathing is dry, that the foam is securely attached, and that the eave ice-dam membrane extends to the required distance inside the exterior wall. Also confirm that the added foam thickness does not create a dew-point problem with the chosen interior vapour retarder.