Water is the most significant factor in the premature deterioration of our buildings. Excessive moisture accumulation on porous materials can lead to water penetration, freeze-thaw damage, efflorescence, cracking, and façade soiling. Further, water penetration or interstitial condensation may lead to the chemical breakdown of organic materials (e.g. wood), failure of structural systems and fasteners, mould growth and damage to interior finishes and furniture.
There are three main sources of moisture that affect buildings:
- exterior moisture (e.g. rainwater, groundwater, surface runoff and melting snow);
- interior moisture resulting from occupants and occupant use (e.g. perspiration, respiration and activities); and
- built-in moisture (i.e. manufacturing, construction and naturally occurring moisture).
The three strategies to manage water penetration are known as the Three Ds: deflection, drainage, and drying. For example, the bulk of exterior rainwater can be deflected away using overhangs, flashings and drip edges. Any rainwater not deflected can be drained away using drainage layers within the wall assembly, and away from the foundation wall using drains. Water capable of bypassing the two previous strategies must be capable of drying.
Numerous techniques have been employed by designers and builders to deal with bulk water. However, the transport of moisture through air or vapor diffusion is trickier and less understood. Condensation from air leakage and vapor diffusion on cold condensing surfaces within a wall assembly can lead to significant structural and indoor air quality (IAQ) issues that may cost more to repair than to prevent.
Controlling air and vapor flow