In the context of retrofit work, when installing rigid insulation on the exterior of an existing framed wall in a high-humidity environment (e.g., coastal British Columbia), what key consideration from the National Building Code of Canada (NBC) dictates the placement and type of vapour barrier/air barrier to prevent interstitial condensation?

Question: In the context of retrofit work, when installing rigid insulation on the exterior of an existing framed wall in a high-humidity environment (e.g., coastal British Columbia), what key consideration from the National Building Code of Canada (NBC) dictates the placement and type of vapour barrier/air barrier to prevent interstitial condensation?

Answer options:

• The requirement for a minimum R-value based on climate zone.
• The need for fire-retardant treated sheathing. ✅ The principle that the exterior side of the insulation layer should be less vapour-resistant than the interior side, and the air barrier should be continuous.
• The maximum allowable thickness for exterior cladding.

Correct answer: The principle that the exterior side of the insulation layer should be less vapour-resistant than the interior side, and the air barrier should be continuous.

Explanation: The NBC guidance on moisture control emphasizes that building assemblies should dry to one side. When adding insulation externally, it's critical that the exterior side can 'breathe' more than the interior side to allow trapped moisture to escape, preventing interstitial condensation. A continuous air barrier on the warm side of the insulation is also crucial, but the relative vapour permeance of layers is key for condensation control.