Good fences make good neighbours, but so do walls, if they separate units in multi-family dwellings. In Canada, 60% to 65% of total housing starts are multi-family, and a large proportion of those residents will be owners in condominium arrangements. Of course all residents deserve privacy, quiet, and safety, and thus unit owners’ expectations of separation from their neighbours will be high.

According to Gord Cooke, President of Building Knowledge Canada, builders have to take extraordinary care when designing the walls separating units (also called demising or party walls) if their housing designs are going to appeal to their customers. Fortunately, some straightforward strategies for airtightness in party walls help to address all of the factors in “separation anxiety” between units—noise, odours, and fire safety. Gord sets out these strategies in an article originally published in Spring 2017 in Better Builder Magazine.

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Identify and detail the air barrier

Gord recommends having the architect identify and detail the air barrier for separation walls, rather than having the construction team “cobble it together” after the fact.

Strategies for ensuring airtightness depend on the type of separation wall, he says. In many high-rise projects demising walls are constructed of poured concrete  because they’re intended to carry structural loads. As such, they’re inherently airtight. However, in low-rise projects, wood framed demising walls are most common and the airspaces between double framed walls that create the required fire separation often connects directly with the attic or exterior envelope. These assemblies should be treated as “outside walls” for detailing air barrier purposes, says Gord—except that they’re warm on both sides. Using poly on both sides could make it difficult for moisture to escape, so Gord suggests using a permeable membrane such as Tyvek as the air barrier instead.

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Compartmentalization

Still, that leaves the challenge of detailing the air barrier through floor separations, intersecting walls, and the ceiling, and then the complex intersections, transitions, and penetrations throughout the wall. Each air space should be compartmentalized, ensuring a bubble that limits air leakage. To do this, says Gord, involves using:

• a fully detailed exterior house wrap to transition across the fire-separation wall on exterior walls;
• a thin sheet metal plate between floors and at attics; and
• blocking and caulking between the primary air barrier and any steel or concrete elements at each penetration.

Bumped out walls or chases for mechanical systems present a further challenge, usually best addressed with drywall, thin-ply sheathing, or OSB behind the bump out, then building the bump out in front of that.

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Air test early

Gord suggests creating a mock-up suite in advance, to show the trades involved the acceptable air barrier details. This demonstration should involve an airtightness test.

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Keep learning

Perhaps the most important is to keep learning, as materials and techniques to improve airtightness continue to improve. Building Knowledge Canada can help builders and contractors understand how to make the best use of them. Get in touch to see how we can support your design and business goals.

Read the full article >

_{Originally published in Better Builder Magazine, Issue 21 / Spring 2017}