Air leakage, mold, and rot — Air leaking through the building assembly can carry a lot of moisture, and as the leaking air cools down, it can deposit that moisture as condensation inside the assembly.
The quantities of moisture can be appreciable: a leak in an exterior wall of just 1 square inch (6.5 cm2) can move as much as 30 quarts (28.4 liters) of water into the assembly over the course of a single heating season in a cold climate.16 This amount of water is enough to grow mold and instigate rot, especially if the assembly is unable to dry out.
Small leaks are important — Airtightness requires diligence to be effective, as the size of a leak is not proportional to the amount of loss that will occur. An analogy is covering half the opening of a hose with your thumb: the amount of water flowing is not reduced by half, instead a similar amount of water comes out, just at a higher pressure. A half-hearted attempt to create an air control layer is not sufficient, and the details are critical.
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Materials for air control layer — Building enclosure materials used for the air control layer must have an air permeance rate no greater than 0.02 L/(s^m2) at a pressure difference of 75 Pascals when tested in accordance with ASTM E 2178. Air barrier materials should meet the requirements of the CAN/ULC S741 Air Barrier Material Specification. Many common building materials can be used as an air control layer. Ideally, materials for the air control layer have as few joints and seams as possible, and are simple to seal at each seam.
Continuity — The weakest zones for air control layers are transitions, joints, and seams between different materials. As air will move through tiny gaps and holes, a plan for ensuring continuity throughout the entire air control layer is critical.
Design to minimize penetrations — Service cavity walls allow for plumbing and wiring to stay on one side of the air barrier, greatly minimizing the number of penetrations.
Durability — As very small punctures in the air control layer can have dramatic negative effects, the air control layer will ideally be durable over the lifespan of the building, and holes or punctures will be easy to see and repair. Thin, sheet-style barriers are often inherently weak and can be damaged during construction or in common use.