Canadian building code question

What are the building code requirements for attic ventilation in Canada?

The National Building Code requires unobstructed attic vent area of generally 1 in 300 of insulated ceiling area for residential roofs, with provisions for unvented or hybrid assemblies that meet specific moisture-control criteria, all subject to provincial adoption.

Attic ventilation is a basic building-science requirement that prevents moisture accumulation, ice damming, and decking degradation. The code sets a minimum vent ratio, but the design intent — balanced intake and exhaust — is where most permit-stage and inspection issues actually surface.

What to check first

  • Confirm the required minimum vent area against the insulated ceiling area for the applicable code edition.
  • Balance soffit intake and ridge or roof-level exhaust to avoid short-circuiting and moisture build-up.
  • Check whether the design uses a conventional vented attic, an unvented assembly, or a cathedral roof, since each has distinct code paths.

Jurisdiction notes

National baseline

Part 9 of the NBC sets a default vent ratio of approximately 1 in 300 for most residential attic spaces.

Provincial adoption can adjust

Some provinces and municipalities require closer to 1 in 150 in low-slope or high-humidity conditions.

Unvented assemblies are conditional

Unvented attics and cathedral roofs require specific moisture-control detailing and sometimes engineered designs.

Work through it in this order

  1. Calculate the insulated ceiling area and the required net free vent area.
  2. Locate intake vents low (soffit) and exhaust vents high (ridge, gable, or roof) for balanced airflow.
  3. Confirm that insulation, baffles, and air barriers preserve the vent path.
  4. Decide whether any roof areas need an unvented or hybrid assembly with appropriate detailing.
  5. Coordinate vent locations with fire separation and exposure-to-property-line provisions.

Common questions

What is the minimum attic vent area required?

The default ratio in the NBC is about 1 in 300 of insulated ceiling area, balanced between intake and exhaust.

Can you build an unvented attic in Canada?

Yes, but the assembly must meet code conditions for moisture control and is typically subject to additional engineering or detailing review.

Do cathedral ceilings need attic ventilation?

Cathedral and sloped insulated assemblies have their own code path; some require continuous airflow channels, while others can be designed unvented.