Does stucco perform well in Ottawa's high humidity summer months?

Ottawa summers bring a combination that surprises many homeowners — sustained humidity often above 70% combined with temperatures regularly hitting 30°C to 35°C with humidex values pushing past 40°C. Stucco handles this remarkably well when it's properly installed with the right drainage plane and vapour management behind it.

How Stucco Handles Ottawa's Summer Humidity

Stucco is a cementitious material, which means it's inherently breathable. Unlike vinyl siding that traps moisture behind it, properly applied stucco allows water vapour to pass through the wall assembly in both directions. This is critical during Ottawa's humid July and August stretches when the dew point can remain above 20°C for days at a time.

The key is what's behind the stucco. Ontario Building Code requires a proper weather-resistant barrier (WRB) and drainage cavity behind any cladding system. For stucco in Ottawa, this means a minimum 10mm drainage gap between the stucco and the WRB — typically achieved with a drainage mat or furring strips. This gap allows any condensation that forms on the back side of the stucco during humid periods to drain downward rather than sitting against your sheathing.

Where humidity becomes problematic is with older Ottawa homes — particularly those built before the 1990s — where stucco was applied directly over building paper without a drainage cavity. These direct-applied systems can trap summer moisture between the stucco and the wall sheathing, leading to concealed rot, mould growth, and eventual sheathing failure. If your Ottawa home has older stucco and you notice musty smells or soft spots around window frames during humid months, a moisture assessment is worth the $300 to $600 investment.

Modern stucco systems in Ottawa should include a ventilated rainscreen assembly that allows airflow behind the cladding. This keeps the wall assembly drier during humid periods and also helps with winter moisture management when warm interior air pushes vapour outward through the wall.

One real advantage of stucco in humid Ottawa summers: it doesn't support mould growth on its surface the way wood siding can. The alkaline pH of cured portland cement naturally resists biological growth. If you do see green or dark staining on Ottawa stucco during humid months, it's typically algae rather than mould, and it's growing on accumulated dirt rather than on the stucco itself. A gentle wash with a garden hose and soft brush in early summer handles this — avoid pressure washing, which can damage the finish coat.

Acrylic stucco finishes are slightly more moisture-resistant on the surface than traditional portland cement stucco, but they're also less breathable. For Ottawa's climate, many experienced applicators recommend traditional stucco with a quality acrylic-modified finish coat — you get surface durability without sacrificing the vapour permeability that keeps your wall assembly healthy through our humid summers.

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Looking for experienced contractors? The Ottawa Construction Network connects Ottawa homeowners with qualified professionals:

• Justyn Rook Contracting
• JC Carpentry
• The Granite shop
• DC Stucco & Parging
• Elie The Carpet Guy Inc.

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