How do i ventilate my cathedral ceiling in a Shediac cottage to prevent ice dam damage every winter?

Cathedral ceilings and ice dams are a serious combination in New Brunswick's climate, and Shediac's coastal freeze-thaw cycles make it even worse than inland areas. The good news is there's a proven solution — but it requires getting the ventilation path right from the start.

The core problem with cathedral ceilings is that you have no attic space to buffer heat loss. Warm air escaping through your ceiling heats the roof deck, melts snow, and that meltwater refreezes at the cold eaves — classic ice dam. In Shediac, you get the added challenge of frequent freeze-thaw cycles driven by the Northumberland Strait's moderating influence, which means more melt-refreeze events per winter than you'd see in Fredericton or Edmundston.

The two main approaches are vented and unvented assemblies — and which one works for your cottage depends heavily on your existing roof structure.

A vented cathedral ceiling requires a continuous air channel from soffit to ridge of at least 63mm (2.5 inches) between the top of your insulation and the underside of the roof sheathing. This means installing baffles (rigid foam or cardboard) in each rafter bay before adding insulation, plus a functional ridge vent and continuous soffit vents to create airflow. The National Building Code of Canada 2020 (now in effect for new applications in NB as of May 1, 2025) requires a minimum 1:300 net free vent area ratio. For a retrofit, this often means stripping your existing insulation and starting fresh — not a small job.

An unvented assembly skips the air channel entirely and relies on continuous rigid foam insulation (closed-cell spray foam or rigid polyiso boards) applied either above the roof deck or as a thick layer below the rafters. The key is keeping the dew point outside the assembly so moisture never condenses inside the structure. For an unvented NB cathedral ceiling, you typically need R-20 of impermeable insulation above the sheathing (or spray foam filling the full rafter depth) to avoid condensation problems. This approach is increasingly popular for cottage retrofits because it doesn't require tearing apart the ceiling below.

A third option worth considering for a Shediac cottage is installing a self-adhering ice-and-water shield membrane under the roofing material, extending at least 900mm (3 feet) past the interior wall line at the eaves. This won't stop ice dams from forming, but it prevents the meltwater from penetrating the roof deck when they do. It's a smart addition regardless of which ventilation strategy you choose, and it's code-required in NB for the first 900mm at eaves anyway.

Since your cottage is in Shediac, your building permit would come from RSC 7 (Southeast / Plan360) at (506) 533-3637 — not the City of Moncton. Any structural roof work or changes to the assembly require a permit, and if you're adding insulation that affects the energy performance of the building, the contractor should be aware of the NBC 2020 Tier 2 energy requirements now in effect.

Cost-wise, a cathedral ceiling ventilation retrofit in NB typically runs:

• Baffles + reinsulation (vented approach): $4,000–$9,000 depending on roof area and access


• Closed-cell spray foam (unvented approach): $6,000–$14,000 for a full cathedral ceiling


• Above-deck rigid foam board during a re-roof: $3,500–$7,000 added cost if bundled with a shingle replacement

This is not a DIY project — spray foam requires licensed applicators, and any structural modifications to the roof assembly need a qualified contractor. NB Power also offers rebates through their Total Home Energy Savings Program for insulation upgrades, which could offset a portion of the cost if your cottage is your primary residence or qualifies under the program terms.

Your best next step is to have a building envelope specialist or experienced roofing contractor do a thermal assessment of your current assembly before committing to an approach. Browse qualified contractors in the New Brunswick Construction Network directory to find professionals serving the Shediac and greater southeast NB area.