Vapor Diffusion

How Vapor Diffusion Works

Imagine a water bottle left in a room. The air inside the bottle is saturated with water vapor (very humid), while the air outside is drier. Over time, water molecules will slowly escape through the plastic of the bottle to the drier air outside. This is a simple analogy for vapor diffusion.

In a building, this process happens through the materials that make up the walls, roofs, and floors. For example, during a cold winter, the air inside a heated house is much warmer and more humid than the cold, dry air outside. This creates a vapor pressure difference. The water vapor inside the house is "pushed" by this pressure difference to migrate through the wall assembly toward the outside.

This movement is slow, but it's constant. Over time, a significant amount of water vapor can pass through the building envelope.

The Role of Vapor Retarders

To control this moisture movement, building scientists use materials called vapor retarders (or vapor barriers). These are materials that are specifically designed to resist the flow of water vapor. They are rated based on their "permeance," which is a measure of how easily water vapor can pass through them.

Vapor retarders are typically installed on the warm side of the insulation. In a cold climate, this is on the interior side of the wall assembly. By placing the vapor retarder here, you prevent the humid indoor air from reaching the colder parts of the wall where the vapor could condense into liquid water.

The Problem with Condensation

If water vapor manages to diffuse through a wall and hits a surface that is below the dew point—the temperature at which the air can no longer hold all of its water vapor—it will condense into liquid water. This is similar to how water droplets form on the outside of a cold soda can on a humid day.

In a wall, this condensation can be a major problem. Liquid water trapped within the wall cavity can lead to:

• Rotting of wood framing

• Deterioration of insulation

• Growth of mold and mildew, which can cause health problems

Vapor Diffusion vs. Air Leakage

It's important to understand that vapor diffusion is a different process from air leakage. Air leakage is the flow of air itself through cracks and gaps in the building envelope. Air leakage can carry a huge amount of water vapor—far more than vapor diffusion—and is the most common cause of moisture problems in buildings.

While vapor diffusion is a slow, molecular-level process, air leakage is like a rushing wind. A single, small hole can let more moisture-laden air through than an entire wall's worth of vapor diffusion. This is why building scientists focus heavily on air sealing as the most critical step in controlling moisture. You must control the air flow before you even worry about vapor diffusion.