Lesson Learned: Air Leakage

Most readers of a building science magazine will understand that uncontrolled air leakage is not a good thing; air leakage wastes energy and drafts make occupants uncomfortable. The Alberta Building Code requires an air barrier. Less understood is the damage that air leakage can do, how air transports moisture to cause damage and shorten the service life of building elements and ultimately the buildings themselves. This is not a Halloween issue, but it is the season. Here are a couple of the horrors of air leakage:

Looking down from a rooftop of a high humidity facility, we see the outer layer of a custom stucco finish coat intended to mimic terrazzo or a stone-like finish. The problem is that the wall construction is our standard batt-filled steel stud wall with poly vapour barrier, and no reliable air barrier. Because the air barrier is not intact continuously, the HVAC system was adjusted to pressurize the building and avoid infiltration. This creates a constant drive of moist air out through the wall assembly and into the stucco. When the weather is cold, which it is much of our year, the moisture cools and condenses in the stucco. The less permeable outer stucco layer does not allow drainage or drying, so the water freezes faster than it can dry out, the finish coat separates, and boom!

Looking up, we see a damaged soffit. Behind the metal siding is a deep cavity, with exterior insulation over an innovative assembly of interlocking sheet metal sheathing and caulking, intended to act as both an air and vapour barrier and structure. It leaks air, as metal and sealant assemblies normally do. When you add stack effect to HVAC pressure, it leaks a lot. When the weather is cold, which is much of our year, the moisture cools outside of the insulation and freezes, sometimes into a mass behind the cladding. When the sun comes out or the weather warms, the ice is dislodged. The big chunks come crashing through the soffit, deflect off the sloped sill, and land on the roof below, and kaboom!

Luckily, no one was injured, and the envelope has been fixed! Stay tuned for our spring issue, where we will offer the story of repair and renewal. n