How Water Moves Through Buildings

Water moves in, on, and through buildings in four ways, which we’ll discuss in order of the quantities involved; this order helps us set water management priorities.

1. Bulk water —rain, runoff, and other flows—is driven primarily by gravity but also by wind and pressure differences. Bulk water on the exterior of a building is managed by moving water down, off, and away from the building. Inside, we manage bulk water by preventing plumbing leaks and condensation.

2. Capillary water moves under tension through porous building materials or through narrow channels between building materials, which act like straws. The primary defenses are capillary breaks in appropriate locations, such as between the foundation and any moisture-sensitive materials (like wood) sitting on it.

3. Air-transported moisture is carried into or out of buildings by air leaks. Air moves through holes in the building envelope, propelled by pressure differences due to wind, the stack effect, or mechanical ventilation. When moisture-laden air hits a cold surface within the building envelope, condensation can occur. For example, humid summer air can be pushed by breezes through utility penetrations in the sheathing. When that air reaches the back side of a cold air-conditioned wall, moisture condenses out of it. We manage air-transported moisture by preventing air leakage with a continuous air barrier.

4. Vapor diffusion is the movement of water as a gas through a material according to differences in relative humidity or vapor pressure. Picture that same hot summer day, with a thunderstorm that wets the exterior bricks. The sun comes out, warms the bricks, and water evaporates through those bricks into the building.

Restricting vapor movement is a double-edged sword: while we may want to control the movement of vapor into a building assembly, we should be much more interested in how the vapor permeability of individual building materials and assemblies allows the movement of vapor out of building assemblies. While building assemblies can get wet by all four forms of water, once water gets in, the main way it can get out is by diffusion.

The vapor drive of water into building assemblies is climate- and season-related: vapor drive is from the inside of heated buildings in the winter and from the outside of cooled buildings during the summer. We need to balance the restriction of this climate- and season-based vapor movement into building assemblies with the allowance for drying of the same assemblies. We do this by conducting a vapor profile analysis or hygrothermal modeling.

Insulation restricts the flow of heat, which in turn reduces the ability of wet building assemblies to dry out. If we use a lot of insulation, we must manage the movement of moisture with equal intensity.