How do I prevent moisture problems where my addition meets my existing Ottawa home?

The junction where a new addition meets the existing house is one of the most vulnerable spots for moisture in the whole project. Two structures that may settle differently, move differently with temperature and use different cladding meet at one seam, and Ottawa's big seasonal temperature swings make those materials expand and contract all year. Get this detail wrong and you can end up with water getting in, ice damage and mould inside the wall within a few years.

The most important exterior detail is flashing where the addition roof meets the house wall. Step flashing and counter-flashing need to tuck behind the existing cladding and lap over the new roofing so water is always directed outward and down. Ask your contractor how they will open up the existing cladding to install flashing properly, what materials they use, and how the laps are sealed. Your designer will show the required details on the drawings.

Below grade, the joint where the new foundation meets the existing foundation needs waterproofing that can flex. Because the two foundations may settle slightly differently, a rigid seal can crack. Contractors commonly use a flexible waterproofing membrane that bridges the joint, covered by drainage board. The addition's perimeter drainage also needs somewhere to go, either connected to the existing system or with its own discharge, so ask how they plan to handle it.

Inside the wall at the junction, the goal is to keep warm, moist indoor air from reaching cold surfaces where it would condense. That means a continuous air barrier across the transition, often using membranes, sealants or spray foam to fill the irregular gaps between old and new framing. Ask how the air barrier in the addition will connect to whatever exists in the old wall. Interior joints that will move seasonally can be finished with backer rod and flexible sealant instead of rigid filler.

Soil matters too. Clay and silt soils are frost-susceptible and can heave, so drainage and grading around the junction are part of keeping water away. Make sure the finished grade slopes away from both the old and new walls.

All of this work must meet the Ontario Building Code, and the junction details should appear on your permit drawings. Ask each contractor to explain their flashing, waterproofing and air-sealing details in the quote so you can compare. You can browse addition contractors in the Ottawa Construction Network directory at justynrookcontracting.com and compare quotes.

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