The movement of humidity in a house is complex, involving two distinct physical processes. Humidity, which is water vapor suspended in the air, moves according to the laws of physics. Its travel is governed primarily by two mechanisms: the slow movement of individual water molecules through materials, known as diffusion, and the much faster movement of air that carries the water vapor along with it, called convection. Understanding these forces is key to effectively managing moisture within a home.
The Movement of Humidity Through Diffusion
Water vapor is a gas, and its molecules naturally spread out until their concentration is uniform throughout a space. This independent movement is called vapor diffusion, driven by a difference in vapor pressure (the concentration of water molecules). Vapor always moves from an area of high concentration to an area of low concentration, regardless of air temperature or bulk air movement. This process happens slowly, molecule by molecule, directly through building materials like drywall, wood, and insulation. For example, moisture diffuses from moist soil, through a concrete basement wall, and into the drier indoor air.
While diffusion is constant, it is generally the less significant moisture transport mechanism compared to airflow, often accounting for only a small percentage of total moisture migration. Materials are rated by their permeance, which measures their resistance to water vapor diffusion. Controlling permeance helps designers manage this specific type of moisture travel. However, even a small amount of moisture diffusing and condensing inside a wall cavity over a long period can lead to damage.
The Role of Temperature in Vertical Humidity Flow
The most substantial movement of humidity occurs when it is carried along by air currents, a process called moisture convection. Warm air holds significantly more water vapor than cold air, so when moisture-laden warm air moves, the humidity travels with it. Since air movement is often driven by temperature differences, this creates a major vertical flow pattern in a home. The primary driver of vertical airflow is the stack effect, which causes a house to act like a chimney. During colder months, heated indoor air rises and escapes through leaks in the upper levels, such as the attic.
This upward current carries moisture from the lower floors to the upper floors and attic, where it can condense upon meeting cold surfaces. For instance, humid air from a damp basement is drawn upward through the living spaces and into the attic, depositing moisture along the way. This air-transported moisture can be up to 100 times greater than the moisture moved by diffusion, making the stack effect the dominant force in vertical humidity movement.
Managing Vertical Moisture Travel in Structures
Controlling vertical moisture travel requires addressing both diffusion and convection using specific building strategies. To manage the bulk movement of moisture through convection, the focus must be on air sealing the entire structure. Sealing gaps and cracks in the building envelope, particularly at the foundation and the attic, eliminates the air pathways that allow the stack effect to pull moist air upward.
Controlling Diffusion
To control vapor diffusion through materials, vapor retarders are used to slow the movement of water molecules through the wall assembly. In cold climates, a vapor retarder is placed on the warm side of the wall assembly to block moisture diffusion before it reaches the cold exterior sheathing. Conversely, in hot, humid climates, the vapor retarder is often placed closer to the exterior, or the wall is designed to dry in both directions.
Reducing Moisture Sources
For high-humidity areas like basements, reducing the source of the moisture is a direct approach. Installing a vapor barrier on the soil in a crawl space or under a slab prevents ground moisture from evaporating and increasing the vapor pressure. Using a dehumidifier in a basement or crawl space directly lowers the indoor concentration of water vapor, which reduces the driving force for both diffusion and convection.