Moisture transfer

Moisture transfer in building materials must be controlled because it has significant impacts on occupant health and safety, comfort, building durability, and energy efficiency. Moisture can migrate into a building by four methods: bulk moisture, capillary action, air-transported moisture, and vapor diffusion. Hygrothermal modeling tools can be used to estimate moisture transfer and develop strategies to control it.

Modes of moisture transfer

Moisture can travel through building materials in several ways, each driven by different forces.

The rate and amount of moisture travel depend on several factors:

• Material properties: Permeability (resistance to vapor flow) and porosity (void space) of the material influence how easily moisture passes through.
• Vapor pressure differential: The greater the difference in vapor pressure between inside and outside, the faster the vapor diffusion.
• Temperature: Warmer air holds more moisture vapor, affecting the driving force for diffusion.
• Airflow: Leaks and ventilation can significantly impact the movement of moisture-laden air within the building envelope.

The following modes of moisture transfer are listed in order of the highest to lowest quantity of moisture transfer that can occur, and therefore may be used to prioritize management strategies.

Bulk water movement

Bulk water movement (or, liquid flow) occurs from rain, snow, or flowing groundwater. Three conditions are required to allow bulk water flow into a building:

• A source of water,
• A hole in the building envelope,
• A driving force such as gravity or air pressure

Bulk water is the most damaging to the building components. It can be controlled by directing the water off and away from the building using the following strategies:

• Proper grading of the site,
• Drainage, gutters, downspouts
• Proper sealing of the envelope such as flashing, caulking, and proper door and window installation

Bulk water problems can also occur from plumbing leaks inside the building. Interior water sources can be controlled by condensate collection, sensor-driven shutoff valves, and routine maintenance.

Capillary Action

Capillary action is the ability of water to travel upwards through a porous material, against the pull of gravity. This phenomenon is common in building materials such as concrete and brick. Water can be wicked from water in the ground up through concrete footings and then travel up through the foundation wall.

It can also occur above grade where two materials are placed closely together, providing a channel for capillary action to occur. For example, with overlapping wood siding, capillary action can occur where rainwater flows down the wall, and moisture accumulates at the overlapping edge and then is drawn up behind the siding.

Capillary action can be controlled by providing a capillary break with a material such as plastic, metal, water proofing compounds or other impermeable materials. Additionally, larger air spaces can be layered between materials providing an area too large for capillary action to occur.

Air-Transported Moisture

Moisture vapor in the air can leak into, or out of, buildings via leaks in the building envelope, driven by wind, stack effect, or pressure caused by fans and air handlers. Leaky ductwork can also draw air from humid areas into the occupied spaces. This moist air can condense on any cool surfaces in the building that are below the dew point. The amount of condensation is dependent on air temperatures, relative humidity, and the speed of the air moving across the condensing surface.

Air-transported moisture issues can be controlled by sealing the building against infiltration using a continuous air barrier, sealing the ductwork, and properly balancing the HVAC systems. Interior sources of moisture generation (e.g. in kitchens and bathrooms) should be removed via exhaust fans. Condensation can be controlled by providing adequate heating and air movement.

Vapor Diffusion

Vapor diffusion is the movement of water, in a gaseous state (vapor), through materials, from areas of high concentration to areas of low concentration. The amount of vapor diffusion that occurs is determined by two things: the vapor pressure differential, and the permeability of the material the vapor is passing through.

Materials called vapor retarders are often used to reduce the amount of vapor diffusion. They may be installed in a crawlspace to prevent ground moisture from evaporating and traveling up into the building. In cold climates, during the heating season, the pressure differential drives the vapor from inside the building to the outside so, in this case, the vapor retarder is typically installed on the interior face of the wall studs. In hot climates, during the cooling season, the vapor is driven from outside to the inside so the vapor retarder is typically installed on the outside face of the insulation. Material permeability is rated with a metric called the "perm rating," and a material is considered a vapor retarder if it has a perm rating of 1.0 or less.

Effects of Moisture on Building Occupants

Health

Moisture is a key ingredient for mold and bacteria growth. This can cause asthma, allergic reactions, and more serious health problems. "Sick building syndrome" is often related to mold growth in buildings.

Comfort

Relative humidity (RH) plays a key role in comfort of occupants in a building. According to ASHRAE, comfortable conditions for buildings in the winter is between 68F and 75F with a RH of 30%-60%; in the summer, comfortable conditions are between 72F-78F with a RH of 25%-60%.

Material durability

Uncontrolled moisture can cause problems in buildings such as wet crawlspaces, leaking roofs, or moisture trapped in structural assemblies leading to possible mold growth, material rot, or insect infestation.

Modeling Moisture Flow—Hygrothermal Modeling

Hygrothermal modeling focuses on simulating the movement of heat and moisture (hygro refers to moisture) through building materials under various environmental conditions. It's a valuable tool for architects, engineers, and building professionals to:

• Predict moisture and temperature: The model forecasts how moisture and temperature will fluctuate within a building envelope assembly over time. This helps understand how the envelope responds to external and internal environments.
• Identify potential moisture problems: By simulating different scenarios, the model can predict areas prone to condensation, mold growth, or material decay due to moisture issues.
• Evaluate design solutions: The model allows testing the effectiveness of different building materials, ventilation strategies, and insulation levels in managing moisture within the envelope.

Tthe hygrothermal modeling process consists of the following steps:

1. Define the building assembly: The geometry and materials of the building envelope (walls, roof, etc.) are specified in the software.
2. Material properties: Hygrothermal properties like thermal conductivity, vapor permeability, and moisture capacity are assigned to each material.
3. Boundary conditions: External factors like outdoor temperature, humidity, and indoor climate (heating, cooling) are defined.
4. Simulation: The software calculates heat and moisture transfer through the building envelope based on the defined properties and conditions.
5. Analysis of results: The model outputs moisture content, temperature distribution, and potential risk areas within the envelope. This data is analyzed to identify potential problems.

Hygrothermal modeling can be performed with whole-building energy modeling software, however specialty hygrothermal modeling tools are often used for greater accuracy.

Additional Resources

• WUFI software—a modeling tool that calculates 1D and 2D heat and moisture transport in walls and other mult-layer building components.

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