How do I ensure my addition is properly air-sealed for Ottawa's extreme cold?

Ottawa winters regularly reach well below -20°C, which means a large temperature difference between the inside and outside of your addition for months at a time. That difference turns every gap, crack and poorly sealed penetration into a path for warm, moist indoor air to escape into wall and roof cavities, where it can condense and cause rot, mould and wet insulation. Air sealing is one of the most important details in an addition, and it's worth asking every contractor exactly how they handle it.

The usual approach is a continuous air barrier around the whole envelope: walls, ceiling and floor, with every seam, penetration and transition sealed. Depending on the assembly, the air barrier may be a sealed interior membrane, sealed sheathing, an exterior membrane or a combination. Your designer will specify the assembly and where the air and vapour control layers sit, and your contractor should be able to explain how they keep those layers continuous.

The junction between the addition and the existing house is the hardest part. The old wall's air barrier rarely lines up neatly with the new one, so the transition needs deliberate detailing with membranes, tapes, sealants, gaskets or spray foam. Ask how the contractor plans to connect the two and ask to see that detail before it gets covered.

Insulation works with air sealing, not instead of it. For an addition, the energy rules require a compliance package or the addition table, and the verified Zone 1 addition values are R-60 for a ceiling with an attic and R-19 + 5 ci for above-grade walls. Rim joists, where the floor framing sits on the foundation, are a common leakage point and deserve careful sealing.

Testing is the best way to know whether the sealing worked. A blower door test measures how airtight the house is and can help find leaks while they're still easy to fix. Ask whether your contractor will arrange a test, when in the process it will happen, and what result they're aiming for.

A tighter house also needs planned ventilation. If you have a heat recovery ventilator, ask your HVAC contractor whether it can handle the added space or needs to be upgraded, and ask for a heat-loss calculation to size any new heating or cooling.

Ask for the air-sealing and insulation work to be spelled out in the quote so you can compare contractors. If you have permit questions, Building Code Services can help (buildingpermits@ottawa.ca). You can browse addition contractors in the Ottawa Construction Network directory at justynrookcontracting.com and compare quotes.

---

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

• Justyn Rook Contracting
• JC Carpentry
• Rrenovatios
• Somar Contracting Inc.
• Pure Flow Water Solutions inc.