What vapour barrier requirements exist for stucco walls under the Ontario Building Code?

Vapour barrier requirements for stucco walls under the Ontario Building Code are one of the most misunderstood aspects of exterior cladding — and in Ottawa's extreme climate swings from -30°C to +35°C, getting the vapour control layer wrong leads to trapped moisture, mould, and structural rot inside your walls.

OBC Vapour Barrier and Air Barrier Requirements for Stucco

The Ontario Building Code (OBC Part 9, Section 9.25) requires all exterior wall assemblies to include both an air barrier and a vapour barrier. For stucco walls, here's how these requirements apply:

Vapour barrier (vapour retarder):

• Must be installed on the warm side of the insulation — in Ottawa, this means on the interior side of the wall assembly.- Must have a vapour permeance of 60 ng/(Pa·s·m²) or less (approximately 1 US perm or less). Standard 6-mil polyethylene sheeting meets this requirement and is the most common choice in Ottawa residential construction.- Must be continuous and sealed at all joints, penetrations, and transitions to prevent interior moisture from migrating into the wall cavity.

Weather-resistive barrier (WRB) — the exterior side:

This is where stucco walls get tricky. Behind the stucco, you need a weather-resistive barrier that stops bulk water from reaching the sheathing but allows outward drying. The OBC requires:

• A WRB with at least two layers of Grade D building paper (60-minute rated) or an equivalent membrane behind traditional three-coat stucco. Two layers are critical because stucco bonds to and can degrade a single layer over time.- For EIFS (synthetic stucco), the WRB requirements depend on whether it's a barrier or drainage system — drainage EIFS is now standard in Ottawa and requires a dedicated drainage plane membrane.

The critical principle — drying direction:

Ottawa's climate creates a challenging dynamic. In winter, moisture drive is from warm interior to cold exterior. In summer with air conditioning, it can reverse. A properly designed stucco wall assembly must:

• Block vapour from entering the cavity from inside (poly vapour barrier).- Allow any incidental moisture in the cavity to dry outward through the WRB and stucco.- Prevent bulk water (rain) from penetrating inward through the stucco and WRB.

Common mistakes in Ottawa stucco installations:

• Using a non-permeable WRB on the exterior — This traps moisture between the interior vapour barrier and an exterior vapour barrier, creating a "moisture sandwich" that leads to rot. Your exterior WRB should be vapour-permeable (breathable).- Single-layer building paper — Code requires two layers behind stucco for good reason. The alkalinity of wet cement stucco can deteriorate a single layer.- Missing or poorly sealed interior vapour barrier — Especially common in renovations where stucco is added to existing walls without addressing the interior side.

A code-compliant stucco wall assembly in Ottawa — including proper vapour barrier, dual-layer WRB, lath, and three-coat stucco — typically costs $12–$18 per square foot fully installed. Cutting corners on vapour management to save a few dollars per square foot is a false economy when moisture remediation in Ottawa can run $15,000–$40,000 after just a few winters.

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