What type of exterior sheathing is best for Ottawa's humid summers and cold winters?

Ottawa's climate puts real stress on a wall assembly, with deep cold in winter and humid summers. The exterior sheathing for your addition needs to handle that whole range while connecting cleanly with the existing house's walls. A poor choice or poor detailing can lead to moisture damage and mould within a few years.

One option is sheathing with an integrated water-resistive barrier, where the panel facing serves as the weather barrier and the seams are taped. It removes the separate housewrap layer, which can tear or come loose in high winds, and the taped seams can form part of the air barrier. It costs more per sheet than plain panels, but the labour saved on housewrap narrows the gap. Ask for it priced against a conventional assembly.

Plywood sheathing remains a solid performer in a cold, wet climate and tends to handle wetting and drying better than OSB. It holds fasteners well for cladding. If your existing house is sheathed in plywood, matching it in the addition can simplify the junction between old and new walls.

OSB (oriented strand board) is the budget option and is widely used. Its weakness is moisture: OSB can swell at the edges when it gets wet and may not shrink back fully, and that swelling can show through siding. If you choose OSB, make sure it is covered with a good sheathing membrane soon after installation and isn't left exposed through spring rains.

Whatever the panel, the membrane and flashings over it decide how well the wall handles water. Ask how the contractor will lap the membrane, flash windows and doors, and tie the new wall's weather barrier into the existing house.

Energy rules shape the whole assembly. For additions, Ontario's energy requirements can be met with a compliance package or the addition table, which for Ottawa's climate zone calls for above-grade walls of R-19 plus R-5 continuous insulation. That continuous layer, rigid foam or mineral wool board, often goes over the sheathing, so the sheathing, membrane, insulation and cladding attachment all have to be designed together. Foam over the sheathing also affects how the wall dries, which is another reason to treat the wall as a system.

Your permit drawings need to show the wall assembly, and the City's Building Code Services can tell you what the submission must include. Ask your designer to draw the full wall section, then ask contractors to price exactly that assembly so quotes are comparable. You can browse addition contractors in the Ottawa Construction Network directory to start.

---

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

• 613Bins
• RenoMotion Inc.
• Estra Design
• JMY Renovations
• Elie The Carpet Guy Inc.